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Commit 76af79f393ad562077f79627a4c719219ef09ee8

Authored by Shaohua Li
Committed by Greg Kroah-Hartman
1 parent 61339713b4

readahead: fix pipeline break caused by block plug 

commit 3deaa7190a8da38453c4fabd9dec7f66d17fff67 upstream.

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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Showing 1 changed file with 4 additions and 4 deletions Side-by-side Diff

... ... @@ -1400,15 +1400,12 @@
1400 1400 unsigned long seg = 0;
1401 1401 size_t count;
1402 1402 loff_t *ppos = &iocb->ki_pos;
1403   - struct blk_plug plug;
1404 1403  
1405 1404 count = 0;
1406 1405 retval = generic_segment_checks(iov, &nr_segs, &count, VERIFY_WRITE);
1407 1406 if (retval)
1408 1407 return retval;
1409 1408  
1410   - blk_start_plug(&plug);
1411   -
1412 1409 /* coalesce the iovecs and go direct-to-BIO for O_DIRECT */
1413 1410 if (filp->f_flags & O_DIRECT) {
1414 1411 loff_t size;
1415 1412  
... ... @@ -1424,8 +1421,12 @@
1424 1421 retval = filemap_write_and_wait_range(mapping, pos,
1425 1422 pos + iov_length(iov, nr_segs) - 1);
1426 1423 if (!retval) {
  1424 + struct blk_plug plug;
  1425 +
  1426 + blk_start_plug(&plug);
1427 1427 retval = mapping->a_ops->direct_IO(READ, iocb,
1428 1428 iov, pos, nr_segs);
  1429 + blk_finish_plug(&plug);
1429 1430 }
1430 1431 if (retval > 0) {
1431 1432 *ppos = pos + retval;
... ... @@ -1481,7 +1482,6 @@
1481 1482 break;
1482 1483 }
1483 1484 out:
1484   - blk_finish_plug(&plug);
1485 1485 return retval;
1486 1486 }
1487 1487 EXPORT_SYMBOL(generic_file_aio_read);