提交 be2c6b19 编写于 作者: V Vivek Goyal 提交者: Jens Axboe

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: NJeff Moyer <jmoyer@redhat.com>
Signed-off-by: NVivek Goyal <vgoyal@redhat.com>
Acked-by: NJeff Moyer <jmoyer@redhat.com>
Signed-off-by: NJens Axboe <jaxboe@fusionio.com>
上级 ba5bd520
......@@ -168,7 +168,15 @@ static struct throtl_grp * throtl_find_alloc_tg(struct throtl_data *td,
* 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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