path: root/async.c
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2017-02-21block: move AioContext, QEMUTimer, main-loop to libqemuutilPaolo Bonzini
AioContext is fairly self contained, the only dependency is QEMUTimer but that in turn doesn't need anything else. So move them out of block-obj-y to avoid introducing a dependency from io/ to block-obj-y. main-loop and its dependency iohandler also need to be moved, because later in this series io/ will call iohandler_get_aio_context. [Changed copyright "the QEMU team" to "other QEMU contributors" as suggested by Daniel Berrange and agreed by Paolo. --Stefan] Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Message-id: 20170213135235.12274-2-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2017-01-16async: optimize aio_bh_pollPaolo Bonzini
Avoid entering the slow path of qemu_lockcnt_dec_and_lock if no bottom half has to be deleted. Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Message-id: 20170112180800.21085-11-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2017-01-16aio: make ctx->list_lock a QemuLockCnt, subsuming ctx->walking_bhPaolo Bonzini
This will make it possible to walk the list of bottom halves without holding the AioContext lock---and in turn to call bottom half handlers without holding the lock. Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Message-id: 20170112180800.21085-4-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2017-01-16aio: rename bh_lock to list_lockPaolo Bonzini
This will be used for AioHandlers too. There is going to be little or no contention, so it is better to reuse the same lock. Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Message-id: 20170112180800.21085-2-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2017-01-03aio: self-tune polling timeStefan Hajnoczi
This patch is based on the algorithm for the kvm.ko halt_poll_ns parameter in Linux. The initial polling time is zero. If the event loop is woken up within the maximum polling time it means polling could be effective, so grow polling time. If the event loop is woken up beyond the maximum polling time it means polling is not effective, so shrink polling time. If the event loop makes progress within the current polling time then the sweet spot has been reached. This algorithm adjusts the polling time so it can adapt to variations in workloads. The goal is to reach the sweet spot while also recognizing when polling would hurt more than help. Two new trace events, poll_grow and poll_shrink, are added for observing polling time adjustment. Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Message-id: 20161201192652.9509-13-stefanha@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2017-01-03aio: add polling mode to AioContextStefan Hajnoczi
The AioContext event loop uses ppoll(2) or epoll_wait(2) to monitor file descriptors or until a timer expires. In cases like virtqueues, Linux AIO, and ThreadPool it is technically possible to wait for events via polling (i.e. continuously checking for events without blocking). Polling can be faster than blocking syscalls because file descriptors, the process scheduler, and system calls are bypassed. The main disadvantage to polling is that it increases CPU utilization. In classic polling configuration a full host CPU thread might run at 100% to respond to events as quickly as possible. This patch implements a timeout so we fall back to blocking syscalls if polling detects no activity. After the timeout no CPU cycles are wasted on polling until the next event loop iteration. The run_poll_handlers_begin() and run_poll_handlers_end() trace events are added to aid performance analysis and troubleshooting. If you need to know whether polling mode is being used, trace these events to find out. Note that the AioContext is now re-acquired before disabling notify_me in the non-polling case. This makes the code cleaner since notify_me was enabled outside the non-polling AioContext release region. This change is correct since it's safe to keep notify_me enabled longer (disabling is an optimization) but potentially causes unnecessary event_notifer_set() calls. I think the chance of performance regression is small here. Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Message-id: 20161201192652.9509-4-stefanha@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2017-01-03aio: add AioPollFn and io_poll() interfaceStefan Hajnoczi
The new AioPollFn io_poll() argument to aio_set_fd_handler() and aio_set_event_handler() is used in the next patch. Keep this code change separate due to the number of files it touches. Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Message-id: 20161201192652.9509-3-stefanha@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2017-01-03aio: add flag to skip fds to aio_dispatch()Stefan Hajnoczi
Polling mode will not call ppoll(2)/epoll_wait(2). Therefore we know there are no fds ready and should avoid looping over fd handlers in aio_dispatch(). Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Message-id: 20161201192652.9509-2-stefanha@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2016-10-28aio: convert from RFifoLock to QemuRecMutexPaolo Bonzini
It is simpler and a bit faster, and QEMU does not need the contention callbacks (and thus the fairness) anymore. Reviewed-by: Fam Zheng <famz@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Message-Id: <1477565348-5458-21-git-send-email-pbonzini@redhat.com> Signed-off-by: Fam Zheng <famz@redhat.com>
2016-10-28iothread: release AioContext around aio_pollPaolo Bonzini
This is the first step towards having fine-grained critical sections in dataplane threads, which will resolve lock ordering problems between address_space_* functions (which need the BQL when doing MMIO, even after we complete RCU-based dispatch) and the AioContext. Because AioContext does not use contention callbacks anymore, the unit test has to be changed. Previously applied as a0710f7995f914e3044e5899bd8ff6c43c62f916 and then reverted. Reviewed-by: Fam Zheng <famz@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Message-Id: <1477565348-5458-19-git-send-email-pbonzini@redhat.com> Signed-off-by: Fam Zheng <famz@redhat.com>
2016-10-28block: only call aio_poll on the current thread's AioContextPaolo Bonzini
aio_poll is not thread safe; for example bdrv_drain can hang if the last in-flight I/O operation is completed in the I/O thread after the main thread has checked bs->in_flight. The bug remains latent as long as all of it is called within aio_context_acquire/aio_context_release, but this will change soon. To fix this, if bdrv_drain is called from outside the I/O thread, signal the main AioContext through a dummy bottom half. The event loop then only runs in the I/O thread. Reviewed-by: Fam Zheng <famz@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Message-Id: <1477565348-5458-18-git-send-email-pbonzini@redhat.com> Signed-off-by: Fam Zheng <famz@redhat.com>
2016-10-07async: add aio_bh_schedule_oneshotPaolo Bonzini
qemu_bh_delete is already clearing bh->scheduled at the same time as it's setting bh->deleted. Since it's not using any memory barriers, there is no synchronization going on for bh->deleted, and this makes the bh->deleted checks superfluous in aio_compute_timeout, aio_bh_poll and aio_ctx_check. Just remove them, and put the (bh->scheduled && bh->deleted) combo to work in a new function aio_bh_schedule_oneshot. The new function removes the need to save the QEMUBH pointer between the creation and the execution of the bottom half. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
2016-07-18aio-posix: remove useless parameterCao jin
Parameter **errp of aio_context_setup() is useless, remove it and clean up the related code. Cc: Stefan Hajnoczi <stefanha@redhat.com> Cc: Fam Zheng <famz@redhat.com> Cc: Eric Blake <eblake@redhat.com> Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com> Reviewed-by: Eric Blake <eblake@redhat.com> Message-id: 1468578524-23433-1-git-send-email-caoj.fnst@cn.fujitsu.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2016-07-18aio_ctx_check: follow CODING_STYLECao jin
replace tab with spaces Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com> Message-id: 1468501843-14927-1-git-send-email-caoj.fnst@cn.fujitsu.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2016-07-18linux-aio: share one LinuxAioState within an AioContextPaolo Bonzini
This has better performance because it executes fewer system calls and does not use a bottom half per disk. Originally proposed by Ming Lei. [Changed #include "raw-aio.h" to "block/raw-aio.h" in win32-aio.c to fix build error as reported by Peter Maydell <peter.maydell@linaro.org>. --Stefan] Acked-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Message-id: 1467650000-51385-1-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> squash! linux-aio: share one LinuxAioState within an AioContext
2016-03-22include/qemu/osdep.h: Don't include qapi/error.hMarkus Armbruster
Commit 57cb38b included qapi/error.h into qemu/osdep.h to get the Error typedef. Since then, we've moved to include qemu/osdep.h everywhere. Its file comment explains: "To avoid getting into possible circular include dependencies, this file should not include any other QEMU headers, with the exceptions of config-host.h, compiler.h, os-posix.h and os-win32.h, all of which are doing a similar job to this file and are under similar constraints." qapi/error.h doesn't do a similar job, and it doesn't adhere to similar constraints: it includes qapi-types.h. That's in excess of 100KiB of crap most .c files don't actually need. Add the typedef to qemu/typedefs.h, and include that instead of qapi/error.h. Include qapi/error.h in .c files that need it and don't get it now. Include qapi-types.h in qom/object.h for uint16List. Update scripts/clean-includes accordingly. Update it further to match reality: replace config.h by config-target.h, add sysemu/os-posix.h, sysemu/os-win32.h. Update the list of includes in the qemu/osdep.h comment quoted above similarly. This reduces the number of objects depending on qapi/error.h from "all of them" to less than a third. Unfortunately, the number depending on qapi-types.h shrinks only a little. More work is needed for that one. Signed-off-by: Markus Armbruster <armbru@redhat.com> [Fix compilation without the spice devel packages. - Paolo] Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
2016-02-04all: Clean up includesPeter Maydell
Clean up includes so that osdep.h is included first and headers which it implies are not included manually. This commit was created with scripts/clean-includes. Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Message-id: 1454089805-5470-16-git-send-email-peter.maydell@linaro.org
2015-11-09aio: Introduce aio_context_setupFam Zheng
This is the place to initialize platform specific bits of AioContext. Signed-off-by: Fam Zheng <famz@redhat.com> Message-id: 1446177989-6702-3-git-send-email-famz@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2015-11-06bottom halves: introduce bh call functionPavel Dovgalyuk
This patch introduces aio_bh_call function. It is used to execute bottom halves as callbacks without adding them to the queue. Signed-off-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru> Message-Id: <20150917162450.8676.56980.stgit@PASHA-ISP.def.inno> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>
2015-10-23aio: Add "is_external" flag for event handlersFam Zheng
All callers pass in false, and the real external ones will switch to true in coming patches. Signed-off-by: Fam Zheng <famz@redhat.com> Reviewed-by: Jeff Cody <jcody@redhat.com> Reviewed-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
2015-07-29AioContext: force event loop iteration using BHStefan Hajnoczi
The notify_me optimization introduced in commit eabc97797310 ("AioContext: fix broken ctx->dispatching optimization") skips event_notifier_set() calls when the event loop thread is not blocked in ppoll(2). This optimization causes a deadlock if two aio_context_acquire() calls race. notify_me = 0 during the race so the winning thread can enter ppoll(2) unaware that the other thread is waiting its turn to acquire the AioContext. This patch forces ppoll(2) to return by scheduling a BH instead of calling aio_notify(). The following deadlock with virtio-blk dataplane is fixed: qemu ... -object iothread,id=iothread0 \ -drive if=none,id=drive0,file=test.img,... \ -device virtio-blk-pci,iothread=iothread0,drive=drive0 This command-line results in a hang early on without this patch. Thanks to Paolo Bonzini <pbonzini@redhat.com> for investigating this bug with me. Cc: Christian Borntraeger <borntraeger@de.ibm.com> Cc: Cornelia Huck <cornelia.huck@de.ibm.com> Cc: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Message-id: 1438101249-25166-4-git-send-email-pbonzini@redhat.com Message-Id: <1438014819-18125-3-git-send-email-stefanha@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2015-07-29AioContext: avoid leaking BHs on cleanupStefan Hajnoczi
BHs are freed during aio_bh_poll(). This leads to memory leaks if there is no aio_bh_poll() between qemu_bh_delete() and aio_ctx_finalize(). Suggested-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Message-id: 1438101249-25166-3-git-send-email-pbonzini@redhat.com Message-Id: <1438014819-18125-2-git-send-email-stefanha@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2015-07-22AioContext: optimize clearing the EventNotifierPaolo Bonzini
It is pretty rare for aio_notify to actually set the EventNotifier. It can happen with worker threads such as thread-pool.c's, but otherwise it should never be set thanks to the ctx->notify_me optimization. The previous patch, unfortunately, added an unconditional call to event_notifier_test_and_clear; now add a userspace fast path that avoids the call. Note that it is not possible to do the same with event_notifier_set; it would break, as proved (again) by the included formal model. This patch survived over 3000 reboots on aarch64 KVM. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Tested-by: Richard W.M. Jones <rjones@redhat.com> Message-id: 1437487673-23740-7-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2015-07-22AioContext: fix broken placement of event_notifier_test_and_clearPaolo Bonzini
event_notifier_test_and_clear must be called before processing events. Otherwise, an aio_poll could "eat" the notification before the main I/O thread invokes ppoll(). The main I/O thread then never wakes up. This is an example of what could happen: i/o thread vcpu thread worker thread --------------------------------------------------------------------- lock_iothread notify_me = 1 ... unlock_iothread bh->scheduled = 1 event_notifier_set lock_iothread notify_me = 3 ppoll notify_me = 1 aio_dispatch aio_bh_poll thread_pool_completion_bh bh->scheduled = 1 event_notifier_set node->io_read(node->opaque) event_notifier_test_and_clear ppoll *** hang *** "Tracing" with qemu_clock_get_ns shows pretty much the same behavior as in the previous bug, so there are no new tricks here---just stare more at the code until it is apparent. One could also use a formal model, of course. The included one shows this with three processes: notifier corresponds to a QEMU thread pool worker, temporary_waiter to a VCPU thread that invokes aio_poll(), waiter to the main I/O thread. I would be happy to say that the formal model found the bug for me, but actually I wrote it after the fact. This patch is a bit of a big hammer. The next one optimizes it, with help (this time for real rather than a posteriori :)) from another, similar formal model. Reported-by: Richard W. M. Jones <rjones@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Tested-by: Richard W.M. Jones <rjones@redhat.com> Message-id: 1437487673-23740-6-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2015-07-22AioContext: fix broken ctx->dispatching optimizationPaolo Bonzini
This patch rewrites the ctx->dispatching optimization, which was the cause of some mysterious hangs that could be reproduced on aarch64 KVM only. The hangs were indirectly caused by aio_poll() and in particular by flash memory updates's call to blk_write(), which invokes aio_poll(). Fun stuff: they had an extremely short race window, so much that adding all kind of tracing to either the kernel or QEMU made it go away (a single printf made it half as reproducible). On the plus side, the failure mode (a hang until the next keypress) made it very easy to examine the state of the process with a debugger. And there was a very nice reproducer from Laszlo, which failed pretty often (more than half of the time) on any version of QEMU with a non-debug kernel; it also failed fast, while still in the firmware. So, it could have been worse. For some unknown reason they happened only with virtio-scsi, but that's not important. It's more interesting that they disappeared with io=native, making thread-pool.c a likely suspect for where the bug arose. thread-pool.c is also one of the few places which use bottom halves across threads, by the way. I hope that no other similar bugs exist, but just in case :) I am going to describe how the successful debugging went... Since the likely culprit was the ctx->dispatching optimization, which mostly affects bottom halves, the first observation was that there are two qemu_bh_schedule() invocations in the thread pool: the one in the aio worker and the one in thread_pool_completion_bh. The latter always causes the optimization to trigger, the former may or may not. In order to restrict the possibilities, I introduced new functions qemu_bh_schedule_slow() and qemu_bh_schedule_fast(): /* qemu_bh_schedule_slow: */ ctx = bh->ctx; bh->idle = 0; if (atomic_xchg(&bh->scheduled, 1) == 0) { event_notifier_set(&ctx->notifier); } /* qemu_bh_schedule_fast: */ ctx = bh->ctx; bh->idle = 0; assert(ctx->dispatching); atomic_xchg(&bh->scheduled, 1); Notice how the atomic_xchg is still in qemu_bh_schedule_slow(). This was already debated a few months ago, so I assumed it to be correct. In retrospect this was a very good idea, as you'll see later. Changing thread_pool_completion_bh() to qemu_bh_schedule_fast() didn't trigger the assertion (as expected). Changing the worker's invocation to qemu_bh_schedule_slow() didn't hide the bug (another assumption which luckily held). This already limited heavily the amount of interaction between the threads, hinting that the problematic events must have triggered around thread_pool_completion_bh(). As mentioned early, invoking a debugger to examine the state of a hung process was pretty easy; the iothread was always waiting on a poll(..., -1) system call. Infinite timeouts are much rarer on x86, and this could be the reason why the bug was never observed there. With the buggy sequence more or less resolved to an interaction between thread_pool_completion_bh() and poll(..., -1), my "tracing" strategy was to just add a few qemu_clock_get_ns(QEMU_CLOCK_REALTIME) calls, hoping that the ordering of aio_ctx_prepare(), aio_ctx_dispatch, poll() and qemu_bh_schedule_fast() would provide some hint. The output was: (gdb) p last_prepare $3 = 103885451 (gdb) p last_dispatch $4 = 103876492 (gdb) p last_poll $5 = 115909333 (gdb) p last_schedule $6 = 115925212 Notice how the last call to qemu_poll_ns() came after aio_ctx_dispatch(). This makes little sense unless there is an aio_poll() call involved, and indeed with a slightly different instrumentation you can see that there is one: (gdb) p last_prepare $3 = 107569679 (gdb) p last_dispatch $4 = 107561600 (gdb) p last_aio_poll $5 = 110671400 (gdb) p last_schedule $6 = 110698917 So the scenario becomes clearer: iothread VCPU thread -------------------------------------------------------------------------- aio_ctx_prepare aio_ctx_check qemu_poll_ns(timeout=-1) aio_poll aio_dispatch thread_pool_completion_bh qemu_bh_schedule() At this point bh->scheduled = 1 and the iothread has not been woken up. The solution must be close, but this alone should not be a problem, because the bottom half is only rescheduled to account for rare situations (see commit 3c80ca1, thread-pool: avoid deadlock in nested aio_poll() calls, 2014-07-15). Introducing a third thread---a thread pool worker thread, which also does qemu_bh_schedule()---does bring out the problematic case. The third thread must be awakened *after* the callback is complete and thread_pool_completion_bh has redone the whole loop, explaining the short race window. And then this is what happens: thread pool worker -------------------------------------------------------------------------- <I/O completes> qemu_bh_schedule() Tada, bh->scheduled is already 1, so qemu_bh_schedule() does nothing and the iothread is never woken up. This is where the bh->scheduled optimization comes into play---it is correct, but removing it would have masked the bug. So, what is the bug? Well, the question asked by the ctx->dispatching optimization ("is any active aio_poll dispatching?") was wrong. The right question to ask instead is "is any active aio_poll *not* dispatching", i.e. in the prepare or poll phases? In that case, the aio_poll is sleeping or might go to sleep anytime soon, and the EventNotifier must be invoked to wake it up. In any other case (including if there is *no* active aio_poll at all!) we can just wait for the next prepare phase to pick up the event (e.g. a bottom half); the prepare phase will avoid the blocking and service the bottom half. Expressing the invariant with a logic formula, the broken one looked like: !(exists(thread): in_dispatching(thread)) => !optimize or equivalently: !(exists(thread): in_aio_poll(thread) && in_dispatching(thread)) => !optimize In the correct one, the negation is in a slightly different place: (exists(thread): in_aio_poll(thread) && !in_dispatching(thread)) => !optimize or equivalently: (exists(thread): in_prepare_or_poll(thread)) => !optimize Even if the difference boils down to moving an exclamation mark :) the implementation is quite different. However, I think the new one is simpler to understand. In the old implementation, the "exists" was implemented with a boolean value. This didn't really support well the case of multiple concurrent event loops, but I thought that this was okay: aio_poll holds the AioContext lock so there cannot be concurrent aio_poll invocations, and I was just considering nested event loops. However, aio_poll _could_ indeed be concurrent with the GSource. This is why I came up with the wrong invariant. In the new implementation, "exists" is computed simply by counting how many threads are in the prepare or poll phases. There are some interesting points to consider, but the gist of the idea remains: 1) AioContext can be used through GSource as well; as mentioned in the patch, bit 0 of the counter is reserved for the GSource. 2) the counter need not be updated for a non-blocking aio_poll, because it won't sleep forever anyway. This is just a matter of checking the "blocking" variable. This requires some changes to the win32 implementation, but is otherwise not too complicated. 3) as mentioned above, the new implementation will not call aio_notify when there is *no* active aio_poll at all. The tests have to be adjusted for this change. The calls to aio_notify in async.c are fine; they only want to kick aio_poll out of a blocking wait, but need not do anything if aio_poll is not running. 4) nested aio_poll: these just work with the new implementation; when a nested event loop is invoked, the outer event loop is never in the prepare or poll phases. The outer event loop thus has already decremented the counter. Reported-by: Richard W. M. Jones <rjones@redhat.com> Reported-by: Laszlo Ersek <lersek@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Tested-by: Richard W.M. Jones <rjones@redhat.com> Message-id: 1437487673-23740-5-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2015-06-12Revert "iothread: release iothread around aio_poll"Stefan Hajnoczi
This reverts commit a0710f7995f914e3044e5899bd8ff6c43c62f916. In qemu-devel email message <556DBF87.2020908@de.ibm.com>, Christian Borntraeger writes: Having many guests all with a kernel/ramdisk (via -kernel) and several null block devices will result in hangs. All hanging guests are in partition detection code waiting for an I/O to return so very early maybe even the first I/O. Reverting that commit "fixes" the hangs. Reverting this commit for the 2.4 release. More time is needed to investigate and correct this patch. Reported-by: Christian Borntraeger <borntraeger@de.ibm.com> Suggested-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Fam Zheng <famz@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2015-04-28iothread: release iothread around aio_pollPaolo Bonzini
This is the first step towards having fine-grained critical sections in dataplane threads, which resolves lock ordering problems between address_space_* functions (which need the BQL when doing MMIO, even after we complete RCU-based dispatch) and the AioContext. Because AioContext does not use contention callbacks anymore, the unit test has to be changed. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> Message-id: 1424449612-18215-4-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
2015-04-28aio-posix: move pollfds to thread-local storagePaolo Bonzini
By using thread-local storage, aio_poll can stop using global data during g_poll_ns. This will make it possible to drop callbacks from rfifolock. [Moved npfd = 0 assignment to end of walking_handlers region as suggested by Paolo. This resolves the assert(npfd == 0) assertion failure in pollfds_cleanup(). --Stefan] Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> Message-id: 1424449612-18215-2-git-send-email-pbonzini@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
2015-04-09aio: strengthen memory barriers for bottom half schedulingPaolo Bonzini
There are two problems with memory barriers in async.c. The fix is to use atomic_xchg in order to achieve sequential consistency between the scheduling of a bottom half and the corresponding execution. First, if bh->scheduled is already 1 in qemu_bh_schedule, QEMU does not execute a memory barrier to order any writes needed by the callback before the read of bh->scheduled. If the other side sees req->state as THREAD_ACTIVE, the callback is not invoked and you get deadlock. Second, the memory barrier in aio_bh_poll is too weak. Without this patch, it is possible that bh->scheduled = 0 is not "published" until after the callback has returned. Another thread wants to schedule the bottom half, but it sees bh->scheduled = 1 and does nothing. This causes a lost wakeup. The memory barrier should have been changed to smp_mb() in commit 924fe12 (aio: fix qemu_bh_schedule() bh->ctx race condition, 2014-06-03) together with qemu_bh_schedule()'s. Guess who reviewed that patch? Both of these involve a store and a load, so they are reproducible on x86_64 as well. It is however much easier on aarch64, where the libguestfs test suite triggers the bug fairly easily. Even there the failure can go away or appear depending on compiler optimization level, tracing options, or even kernel debugging options. Paul Leveille however reported how to trigger the problem within 15 minutes on x86_64 as well. His (untested) recipe, reproduced here for reference, is the following: 1) Qcow2 (or 3) is critical – raw files alone seem to avoid the problem. 2) Use “cache=directsync” rather than the default of “cache=none” to make it happen easier. 3) Use a server with a write-back RAID controller to allow for rapid IO rates. 4) Run a random-access load that (mostly) writes chunks to various files on the virtual block device. a. I use ‘diskload.exe c:25’, a Microsoft HCT load generator, on Windows VMs. b. Iometer can probably be configured to generate a similar load. 5) Run multiple VMs in parallel, against the same storage device, to shake the failure out sooner. 6) IvyBridge and Haswell processors for certain; not sure about others. A similar patch survived over 12 hours of testing, where an unpatched QEMU would fail within 15 minutes. This bug is, most likely, also the cause of failures in the libguestfs testsuite on AArch64. Thanks to Laszlo Ersek for initially reporting this bug, to Stefan Hajnoczi for suggesting closer examination of qemu_bh_schedule, and to Paul for providing test input and a prototype patch. Reported-by: Laszlo Ersek <lersek@redhat.com> Reported-by: Paul Leveille <Paul.Leveille@stratus.com> Reported-by: John Snow <jsnow@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Message-id: 1428419779-26062-1-git-send-email-pbonzini@redhat.com Suggested-by: Paul Leveille <Paul.Leveille@stratus.com> Suggested-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2015-01-13block: replace g_new0 with g_new for bottom half allocation.Paolo Bonzini
This saves about 15% of the clock cycles spent on allocation. Using the slice allocator does not add a visible improvement; allocation is faster than malloc, while freeing seems to be slower. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
2015-01-13block: mark AioContext as recursivePaolo Bonzini
AioContext can be accessed recursively, in fact that's what we do with aio_poll. Marking the GSource as recursive avoids that GLib blocks it and unblocks it around every call to aio_dispatch, which is a pretty expensive operation. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
2014-12-10block: Use g_new0() for a bit of extra type checkingMarkus Armbruster
g_new(T, 1) is safer than g_malloc(sizeof(T)), because it returns T * rather than void *, which lets the compiler catch more type errors. Missed in commit 02c4f26. Signed-off-by: Markus Armbruster <armbru@redhat.com> Message-id: 1417697709-13087-1-git-send-email-armbru@redhat.com Signed-off-by: Max Reitz <mreitz@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
2014-09-22async: aio_context_new(): Handle event_notifier_init failureChrysostomos Nanakos
On a system with a low limit of open files the initialization of the event notifier could fail and QEMU exits without printing any error information to the user. The problem can be easily reproduced by enforcing a low limit of open files and start QEMU with enough I/O threads to hit this limit. The same problem raises, without the creation of I/O threads, while QEMU initializes the main event loop by enforcing an even lower limit of open files. This commit adds an error message on failure: # qemu [...] -object iothread,id=iothread0 -object iothread,id=iothread1 qemu: Failed to initialize event notifier: Too many open files in system Signed-off-by: Chrysostomos Nanakos <cnanakos@grnet.gr> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-08-29AioContext: introduce aio_preparePaolo Bonzini
This will be used to implement socket polling on Windows. On Windows, select() and g_poll() are completely different; sockets are polled with select() before calling g_poll, and the g_poll must be nonblocking if select() says a socket is ready. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-08-29AioContext: export and use aio_dispatchPaolo Bonzini
So far, aio_poll's scheme was dispatch/poll/dispatch, where the first dispatch phase was used only in the GSource case in order to avoid a blocking poll. Earlier patches changed it to dispatch/prepare/poll/dispatch, where prepare is aio_compute_timeout. By making aio_dispatch public, we can remove the first dispatch phase altogether, so that both aio_poll and the GSource use the same prepare/poll/dispatch scheme. This patch breaks the invariant that aio_poll(..., true) will not block the first time it returns false. This used to be fundamental for qemu_aio_flush's implementation as "while (qemu_aio_wait()) {}" but no code in QEMU relies on this invariant anymore. The return value of aio_poll() is now comparable with that of g_main_context_iteration. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-08-29AioContext: take bottom halves into account when computing aio_poll timeoutPaolo Bonzini
Right now, QEMU invokes aio_bh_poll before the "poll" phase of aio_poll. It is simpler to do it afterwards and skip the "poll" phase altogether when the OS-dependent parts of AioContext are invoked from GSource. This way, AioContext behaves more similarly when used as a GSource vs. when used as stand-alone. As a start, take bottom halves into account when computing the poll timeout. If a bottom half is ready, do a non-blocking poll. As a side effect, this makes idle bottom halves work with aio_poll; an improvement, but not really an important one since they are deprecated. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2014-07-09AioContext: speed up aio_notifyPaolo Bonzini
In many cases, the call to event_notifier_set in aio_notify is unnecessary. In particular, if we are executing aio_dispatch, or if aio_poll is not blocking, we know that we will soon get to the next loop iteration (if necessary); the thread that hosts the AioContext's event loop does not need any nudging. The patch includes a Promela formal model that shows that this really works and does not need any further complication such as generation counts. It needs a memory barrier though. The generation counts are not needed because any change to ctx->dispatching after the memory barrier is okay for aio_notify. If it changes from zero to one, it is the right thing to skip event_notifier_set. If it changes from one to zero, the event_notifier_set is unnecessary but harmless. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
2014-06-04aio: fix qemu_bh_schedule() bh->ctx race conditionStefan Hajnoczi
qemu_bh_schedule() is supposed to be thread-safe at least the first time it is called. Unfortunately this is not quite true: bh->scheduled = 1; aio_notify(bh->ctx); Since another thread may run the BH callback once it has been scheduled, there is a race condition if the callback frees the BH before aio_notify(bh->ctx) has a chance to run. Reported-by: Stefan Priebe <s.priebe@profihost.ag> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Tested-by: Stefan Priebe <s.priebe@profihost.ag>
2014-03-13aio: add aio_context_acquire() and aio_context_release()Stefan Hajnoczi
It can be useful to run an AioContext from a thread which normally does not "own" the AioContext. For example, request draining can be implemented by acquiring the AioContext and looping aio_poll() until all requests have been completed. The following pattern should work: /* Event loop thread */ while (running) { aio_context_acquire(ctx); aio_poll(ctx, true); aio_context_release(ctx); } /* Another thread */ aio_context_acquire(ctx); bdrv_read(bs, 0x1000, buf, 1); aio_context_release(ctx); This patch implements aio_context_acquire() and aio_context_release(). Note that existing aio_poll() callers do not need to worry about acquiring and releasing - it is only needed when multiple threads will call aio_poll() on the same AioContext. Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2013-08-22aio / timers: aio_ctx_prepare sets timeout from AioContext timersAlex Bligh
Calculate the timeout in aio_ctx_prepare taking into account the timers attached to the AioContext. Alter aio_ctx_check similarly. Signed-off-by: Alex Bligh <alex@alex.org.uk> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2013-08-22aio / timers: Add a notify callback to QEMUTimerListAlex Bligh
Add a notify pointer to QEMUTimerList so it knows what to notify on a timer change. Signed-off-by: Alex Bligh <alex@alex.org.uk> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2013-08-22aio / timers: Add QEMUTimerListGroup to AioContextAlex Bligh
Add a QEMUTimerListGroup each AioContext (meaning a QEMUTimerList associated with each clock is added) and delete it when the AioContext is freed. Signed-off-by: Alex Bligh <alex@alex.org.uk> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2013-08-19aio: drop io_flush argumentStefan Hajnoczi
The .io_flush() handler no longer exists and has no users. Drop the io_flush argument to aio_set_fd_handler() and related functions. The AioFlushEventNotifierHandler and AioFlushHandler typedefs are no longer used and are dropped too. Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2013-07-19QEMUBH: make AioContext's bh re-entrantLiu Ping Fan
BH will be used outside big lock, so introduce lock to protect between the writers, ie, bh's adders and deleter. The lock only affects the writers and bh's callback does not take this extra lock. Note that for the same AioContext, aio_bh_poll() can not run in parallel yet. Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
2013-03-15aio: add a ThreadPool instance to AioContextStefan Hajnoczi
This patch adds a ThreadPool to AioContext. It's possible that some AioContext instances will never use the ThreadPool, so defer creation until aio_get_thread_pool(). The reason why AioContext should have the ThreadPool is because the ThreadPool is bound to a AioContext instance where the work item's callback function is invoked. It doesn't make sense to keep the ThreadPool pointer anywhere other than AioContext. For example, block/raw-posix.c can get its AioContext's ThreadPool and submit work. Special note about headers: I used struct ThreadPool in aio.h because there is a circular dependency if aio.h includes thread-pool.h. Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
2013-02-21aio: convert aio_poll() to g_poll(3)Stefan Hajnoczi
AioHandler already has a GPollFD so we can directly use its events/revents. Add the int pollfds_idx field to AioContext so we can map g_poll(3) results back to AioHandlers. Reuse aio_dispatch() to invoke handlers after g_poll(3). Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Laszlo Ersek <lersek@redhat.com> Message-id: 1361356113-11049-10-git-send-email-stefanha@redhat.com Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
2012-12-19misc: move include files to include/qemu/Paolo Bonzini
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
2012-12-19block: move include files to include/block/Paolo Bonzini
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
2012-12-11aio: Get rid of qemu_aio_flush()Kevin Wolf
There are no remaining users, and new users should probably be using bdrv_drain_all() in the first place. Signed-off-by: Kevin Wolf <kwolf@redhat.com>
2012-11-12aio: fix aio_ctx_prepare with idle bottom halvesPaolo Bonzini
Commit ed2aec4867f0d5f5de496bb765347b5d0cfe113d changed the return value of aio_ctx_prepare from false to true when only idle bottom halves are available. This broke PC old-style DMA, which uses them. Fix this by making aio_ctx_prepare return true only when non-idle bottom halves are scheduled to run. Reported-by: malc <av1474@comtv.ru> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: malc <av1474@comtv.ru>