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    Subject[PATCH] blkio-throttle: Avoid calling blkiocg_lookup_group() for root group
    o Jeff Moyer was doing some testing on a RAM backed disk and
    blkiocg_lookup_group() showed up high overhead after memcpy(). Similarly
    somebody else reported that blkiocg_lookup_group() is eating 6% extra
    cpu. Though looking at the code I can't think why the overhead of
    this function is so high. One thing is that it is called with very high
    frequency (once for every IO).

    o For lot of folks blkio controller will be compiled in but they might
    not have actually created cgroups. Hence optimize the case of root
    cgroup where we can avoid calling blkiocg_lookup_group() if IO is happening
    in root group (common case).

    Reported-by: Jeff Moyer <jmoyer@redhat.com>
    Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
    ---
    block/blk-throttle.c | 10 +++++++++-
    1 file changed, 9 insertions(+), 1 deletion(-)

    Index: linux-2.6/block/blk-throttle.c
    ===================================================================
    --- linux-2.6.orig/block/blk-throttle.c 2011-01-17 16:23:37.041280712 -0500
    +++ linux-2.6/block/blk-throttle.c 2011-01-17 16:36:09.362940975 -0500
    @@ -168,7 +168,15 @@ static struct throtl_grp * throtl_find_a
    * tree of blkg (instead of traversing through hash list all
    * the time.
    */
    - tg = tg_of_blkg(blkiocg_lookup_group(blkcg, key));
    +
    + /*
    + * This is the common case when there are no blkio cgroups.
    + * Avoid lookup in this case
    + */
    + if (blkcg == &blkio_root_cgroup)
    + tg = &td->root_tg;
    + else
    + tg = tg_of_blkg(blkiocg_lookup_group(blkcg, key));

    /* Fill in device details for root group */
    if (tg && !tg->blkg.dev && bdi->dev && dev_name(bdi->dev)) {


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