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    Subject[PATCH 07/17] writeback: add trace event for balance_dirty_pages()
    Here is an interesting test to verify the theory with balance_dirty_pages()
    tracing. On a partition that can do ~60MB/s, a sparse file is created and
    4 rsync tasks with different write bandwidth started:

    dd if=/dev/zero of=/mnt/1T bs=1M count=1 seek=1024000
    echo 1 > /debug/tracing/events/writeback/balance_dirty_pages/enable
    rsync localhost:/mnt/1T /mnt/a --bwlimit 10000&
    rsync localhost:/mnt/1T /mnt/A --bwlimit 10000&
    rsync localhost:/mnt/1T /mnt/b --bwlimit 20000&
    rsync localhost:/mnt/1T /mnt/c --bwlimit 30000&
    (btw, is there a dd that can do --bwlimit?
    it's a bit twisted to use rsync, or lftp net:limit-rate)

    Trace outputs within 0.1 second, grouped by tasks:

    rsync-3824 [004] 15002.076447: balance_dirty_pages: bdi=btrfs-2 weight=15% thresh=130876 gap=5340 dirtied=192 pause=20 bw=34855512

    rsync-3822 [003] 15002.091701: balance_dirty_pages: bdi=btrfs-2 weight=15% thresh=130777 gap=5113 dirtied=192 pause=20 bw=33419085

    rsync-3821 [006] 15002.004667: balance_dirty_pages: bdi=btrfs-2 weight=30% thresh=129570 gap=3714 dirtied=64 pause=8 bw=24541625
    rsync-3821 [006] 15002.012654: balance_dirty_pages: bdi=btrfs-2 weight=30% thresh=129589 gap=3733 dirtied=64 pause=8 bw=24651878
    rsync-3821 [006] 15002.021838: balance_dirty_pages: bdi=btrfs-2 weight=30% thresh=129604 gap=3748 dirtied=64 pause=8 bw=24768628
    rsync-3821 [004] 15002.091193: balance_dirty_pages: bdi=btrfs-2 weight=29% thresh=129583 gap=3983 dirtied=64 pause=8 bw=26274370
    rsync-3821 [004] 15002.102729: balance_dirty_pages: bdi=btrfs-2 weight=29% thresh=129594 gap=3802 dirtied=64 pause=8 bw=25015422
    rsync-3821 [000] 15002.109252: balance_dirty_pages: bdi=btrfs-2 weight=29% thresh=129619 gap=3827 dirtied=64 pause=8 bw=25179342

    rsync-3823 [002] 15002.009029: balance_dirty_pages: bdi=btrfs-2 weight=39% thresh=128762 gap=2842 dirtied=64 pause=12 bw=18885024
    rsync-3823 [002] 15002.021598: balance_dirty_pages: bdi=btrfs-2 weight=39% thresh=128813 gap=3021 dirtied=64 pause=12 bw=20088241
    rsync-3823 [003] 15002.032973: balance_dirty_pages: bdi=btrfs-2 weight=39% thresh=128805 gap=2885 dirtied=64 pause=12 bw=19146453
    rsync-3823 [003] 15002.048800: balance_dirty_pages: bdi=btrfs-2 weight=39% thresh=128823 gap=2967 dirtied=64 pause=12 bw=19673334
    rsync-3823 [003] 15002.060728: balance_dirty_pages: bdi=btrfs-2 weight=39% thresh=128821 gap=3221 dirtied=64 pause=12 bw=21362280
    rsync-3823 [000] 15002.073152: balance_dirty_pages: bdi=btrfs-2 weight=39% thresh=128825 gap=3225 dirtied=64 pause=12 bw=21385010
    rsync-3823 [005] 15002.090111: balance_dirty_pages: bdi=btrfs-2 weight=39% thresh=128782 gap=3214 dirtied=64 pause=12 bw=21333266
    rsync-3823 [004] 15002.102520: balance_dirty_pages: bdi=btrfs-2 weight=39% thresh=128764 gap=3036 dirtied=64 pause=12 bw=20104559

    The data vividly show that
    - the heaviest writer is throttled a bit (weight=39%)
    - the lighter writers run at full speed (weight=15%,15%,30%)
    rsync is smart enough to compensate for the in-kernel pause time

    Don't be confused by the 'bw=' field. It does not take user space
    think time into account.

    Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    ---
    include/trace/events/writeback.h | 47 +++++++++++++++++++++++++++--
    mm/page-writeback.c | 5 +++
    2 files changed, 49 insertions(+), 3 deletions(-)
    --- linux-next.orig/include/trace/events/writeback.h 2010-09-07 23:16:28.000000000 +0800
    +++ linux-next/include/trace/events/writeback.h 2010-09-07 23:20:17.000000000 +0800
    @@ -148,11 +148,52 @@ DEFINE_EVENT(wbc_class, name, \
    DEFINE_WBC_EVENT(wbc_writeback_start);
    DEFINE_WBC_EVENT(wbc_writeback_written);
    DEFINE_WBC_EVENT(wbc_writeback_wait);
    -DEFINE_WBC_EVENT(wbc_balance_dirty_start);
    -DEFINE_WBC_EVENT(wbc_balance_dirty_written);
    -DEFINE_WBC_EVENT(wbc_balance_dirty_wait);
    DEFINE_WBC_EVENT(wbc_writepage);

    +TRACE_EVENT(balance_dirty_pages,
    +
    + TP_PROTO(struct backing_dev_info *bdi,
    + unsigned long task_weight,
    + unsigned long bdi_thresh,
    + unsigned long gap,
    + unsigned long bw,
    + unsigned long pages_dirtied,
    + unsigned long pause),
    +
    + TP_ARGS(bdi, task_weight, bdi_thresh, gap, bw, pages_dirtied, pause),
    +
    + TP_STRUCT__entry(
    + __array(char, bdi, 32)
    + __field(unsigned long, task_weight)
    + __field(unsigned long, bdi_thresh)
    + __field(unsigned long, gap)
    + __field(unsigned long, bw)
    + __field(unsigned long, pages_dirtied)
    + __field(unsigned long, pause)
    + ),
    +
    + TP_fast_assign(
    + strlcpy(__entry->bdi, dev_name(bdi->dev), 32);
    + __entry->task_weight = task_weight;
    + __entry->bdi_thresh = bdi_thresh;
    + __entry->gap = gap;
    + __entry->bw = bw;
    + __entry->pages_dirtied = pages_dirtied;
    + __entry->pause = pause * 1000 / HZ;
    + ),
    +
    + TP_printk("bdi=%s weight=%lu%% "
    + "thresh=%lu gap=%lu dirtied=%lu pause=%lu bw=%lu",
    + __entry->bdi,
    + __entry->task_weight,
    + __entry->bdi_thresh,
    + __entry->gap,
    + __entry->pages_dirtied,
    + __entry->pause,
    + __entry->bw
    + )
    +);
    +
    #endif /* _TRACE_WRITEBACK_H */

    /* This part must be outside protection */
    --- linux-next.orig/mm/page-writeback.c 2010-09-07 23:20:02.000000000 +0800
    +++ linux-next/mm/page-writeback.c 2010-09-07 23:21:58.000000000 +0800
    @@ -536,6 +536,11 @@ static void balance_dirty_pages(struct a
    } else if (pause < HZ/100)
    current->nr_dirtied_pause++;

    + trace_balance_dirty_pages(bdi,
    + 100 * numerator / denominator,
    + bdi_thresh, gap, bw,
    + pages_dirtied, pause);
    +
    __set_current_state(TASK_INTERRUPTIBLE);
    io_schedule_timeout(pause);




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