block: skip blkcg walk in blk_cgroup_congested() when nothing throttled

blk_cgroup_congested() walks the current task's blkcg ancestor chain on
every readahead decision and, once swap is in use, on every anonymous and
shmem folio allocation.  The answer is almost always "no", but finding that
out costs two loads per level on two cold cache lines, plus an out-of-line
kthread_blkcg() and an RCU read-side pair.  On a fleet profile of hosts
running containers with 5-10 level hierarchies it costs about as much as
all of mutex_lock(), 99.4% of it under __folio_throttle_swaprate().

Gate the walk on a global count of blkcgs with a non-zero
congestion_count.  The counter only moves on the 0 <-> 1 transitions of
each blkcg's congestion_count, so the extra atomic stays in the throttle
arm/disarm paths and never appears in steady state. When something is
throttled the counter is non-zero and the walk runs as before.

Signed-off-by: Usama Arif <usama.arif@linux.dev>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://patch.msgid.link/20260814165712.510132-4-usama.arif@linux.dev
Signed-off-by: Jens Axboe <axboe@kernel.dk>

0 files changed, 0 insertions, 0 deletions