tree f2ec4322adc9f0607b5a78367bc66d8fa333cf54
parent ba5bd520f679c450fb6efa439618703bd0956daa
author Vivek Goyal <vgoyal@redhat.com> 1295450702 -0700
committer Jens Axboe <jaxboe@fusionio.com> 1295450702 -0700

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>
Acked-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>