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    SubjectRe: [PATCH] fix readahead pipeline break caused by block plug
    On Tue, Jan 31, 2012 at 09:48:42AM +0100, Eric Dumazet wrote:
    > Le mardi 31 janvier 2012 à 15:59 +0800, Shaohua Li a écrit :
    >> 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.

    >> Signed-off-by: Shaohua Li <shaohua.li@intel.com>
    >> Tested-by: Herbert Poetzl <herbert@13thfloor.at>
    >> Tested-by: Eric Dumazet <eric.dumazet@gmail.com>

    > Hmm, this is not exactly the patch I tested from Wu Fengguang

    > I'll test this one before adding my "Tested-by: ..."

    second that

    >> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>

    >> diff --git a/mm/filemap.c b/mm/filemap.c
    >> index 97f49ed..b662757 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);

    > This part was not on the tested patch yesterday.

    yep, will test this one tonight ...

    >> 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);

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