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authorShaohua Li <shaohua.li@intel.com>2012-02-03 15:37:17 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2012-02-03 16:16:41 -0800
commit3deaa7190a8da38453c4fabd9dec7f66d17fff67 (patch)
tree23c522d5661bf7e73a39718eaf4eb547cf02b086 /mm/filemap.c
parent55ca6140e9bb307efc97a9301a4f501de02a6fd6 (diff)
downloadlinaro-lsk-3deaa7190a8da38453c4fabd9dec7f66d17fff67.tar.gz
readahead: fix pipeline break caused by block plug
Herbert Poetzl reported a performance regression since 2.6.39. The test is a simple dd read, but with big block size. The reason is: T1: ra (A, A+128k), (A+128k, A+256k) T2: lock_page for page A, submit the 256k T3: hit page A+128K, ra (A+256k, A+384). the range isn't submitted because of plug and there isn't any lock_page till we hit page A+256k because all pages from A to A+256k is in memory T4: hit page A+256k, ra (A+384, A+ 512). Because of plug, the range isn't submitted again. T5: lock_page A+256k, so (A+256k, A+512k) will be submitted. The task is waitting for (A+256k, A+512k) finish. There is no request to disk in T3 and T4, so readahead pipeline breaks. We really don't need block plug for generic_file_aio_read() for buffered I/O. The readahead already has plug and has fine grained control when I/O should be submitted. Deleting plug for buffered I/O fixes the regression. One side effect is plug makes the request size 256k, the size is 128k without it. This is because default ra size is 128k and not a reason we need plug here. Vivek said: : We submit some readahead IO to device request queue but because of nested : plug, queue never gets unplugged. When read logic reaches a page which is : not in page cache, it waits for page to be read from the disk : (lock_page_killable()) and that time we flush the plug list. : : So effectively read ahead logic is kind of broken in parts because of : nested plugging. Removing top level plug (generic_file_aio_read()) for : buffered reads, will allow unplugging queue earlier for readahead. Signed-off-by: Shaohua Li <shaohua.li@intel.com> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com> Reported-by: Herbert Poetzl <herbert@13thfloor.at> Tested-by: Eric Dumazet <eric.dumazet@gmail.com> Cc: Christoph Hellwig <hch@infradead.org> Cc: Jens Axboe <axboe@kernel.dk> Cc: Vivek Goyal <vgoyal@redhat.com> Cc: <stable@vger.kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'mm/filemap.c')
-rw-r--r--mm/filemap.c8
1 files changed, 4 insertions, 4 deletions
diff --git a/mm/filemap.c b/mm/filemap.c
index 97f49ed35bd..b66275757c2 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -1400,15 +1400,12 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
unsigned long seg = 0;
size_t count;
loff_t *ppos = &iocb->ki_pos;
- struct blk_plug plug;
count = 0;
retval = generic_segment_checks(iov, &nr_segs, &count, VERIFY_WRITE);
if (retval)
return retval;
- blk_start_plug(&plug);
-
/* coalesce the iovecs and go direct-to-BIO for O_DIRECT */
if (filp->f_flags & O_DIRECT) {
loff_t size;
@@ -1424,8 +1421,12 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
retval = filemap_write_and_wait_range(mapping, pos,
pos + iov_length(iov, nr_segs) - 1);
if (!retval) {
+ struct blk_plug plug;
+
+ blk_start_plug(&plug);
retval = mapping->a_ops->direct_IO(READ, iocb,
iov, pos, nr_segs);
+ blk_finish_plug(&plug);
}
if (retval > 0) {
*ppos = pos + retval;
@@ -1481,7 +1482,6 @@ generic_file_aio_read(struct kiocb *iocb, const struct iovec *iov,
break;
}
out:
- blk_finish_plug(&plug);
return retval;
}
EXPORT_SYMBOL(generic_file_aio_read);