Re: [PATCH 7/8] Btrfs: use REQ_CGROUP_PUNT for worker thread submitted bios

On Thu, Jun 13, 2019 at 05:33:49PM -0700, Tejun Heo wrote:

From: Chris Mason <clm@fb.com>

Async CRCs and compression submit IO through helper threads, which
means they have IO priority inversions when cgroup IO controllers are
in use.

This flags all of the writes submitted by btrfs helper threads as
REQ_CGROUP_PUNT.  submit_bio() will punt these to dedicated per-blkcg
work items to avoid the priority inversion.

For the compression code, we take a reference on the wbc's blkg css and
pass it down to the async workers.

For the async crcs, the bio already has the correct css, we just need to
tell the block layer to use REQ_CGROUP_PUNT.

Signed-off-by: Chris Mason <clm@fb.com>
Modified-and-reviewed-by: Tejun Heo [off-list ref]
---

Reviewed-by: Josef Bacik <josef@toxicpanda.com>

Thanks,

Josef