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authorPaolo Bonzini <pbonzini@redhat.com>2015-07-21 16:07:53 +0200
committerStefan Hajnoczi <stefanha@redhat.com>2015-07-22 12:41:40 +0100
commit05e514b1d4d5bd4209e2c8bbc76ff05c85a235f3 (patch)
tree1768698d814f687fc322e365c672ddd94c14d55b /async.c
parent21a03d17f2edb1e63f7137d97ba355cc6f19d79f (diff)
downloadqemu-arm-05e514b1d4d5bd4209e2c8bbc76ff05c85a235f3.tar.gz
AioContext: optimize clearing the EventNotifier
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>
Diffstat (limited to 'async.c')
-rw-r--r--async.c10
1 files changed, 9 insertions, 1 deletions
diff --git a/async.c b/async.c
index d625e8a803..9a98a74acb 100644
--- a/async.c
+++ b/async.c
@@ -203,7 +203,7 @@ aio_ctx_check(GSource *source)
QEMUBH *bh;
atomic_and(&ctx->notify_me, ~1);
- event_notifier_test_and_clear(&ctx->notifier);
+ aio_notify_accept(ctx);
for (bh = ctx->first_bh; bh; bh = bh->next) {
if (!bh->deleted && bh->scheduled) {
@@ -267,6 +267,14 @@ void aio_notify(AioContext *ctx)
smp_mb();
if (ctx->notify_me) {
event_notifier_set(&ctx->notifier);
+ atomic_mb_set(&ctx->notified, true);
+ }
+}
+
+void aio_notify_accept(AioContext *ctx)
+{
+ if (atomic_xchg(&ctx->notified, false)) {
+ event_notifier_test_and_clear(&ctx->notifier);
}
}