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blkcg: implement REQ_CGROUP_PUNT

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When a shared kthread needs to issue a bio for a cgroup, doing so
synchronously can lead to priority inversions as the kthread can be
trapped waiting for that cgroup.  This patch implements
REQ_CGROUP_PUNT flag which makes submit_bio() punt the actual issuing
to a dedicated per-blkcg work item to avoid such priority inversions.

This will be used to fix priority inversions in btrfs compression and
should be generally useful as we grow filesystem support for
comprehensive IO control.

Cc: Chris Mason <clm@fb.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Tejun Heo 2019-06-27 13:39:52 -07:00 • committed by Jens Axboe
parent 653c45c6b9
commit d3f77dfdc7
6 changed files with 94 additions and 6 deletions
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17
include/linux/writeback.h
View file

@ -78,6 +78,8 @@ struct writeback_control {
*/
unsigned no_cgroup_owner:1;
unsigned punt_to_cgroup:1; /* cgrp punting, see __REQ_CGROUP_PUNT */
#ifdef CONFIG_CGROUP_WRITEBACK
struct bdi_writeback *wb; /* wb this writeback is issued under */
struct inode *inode; /* inode being written out */
@ -94,12 +96,17 @@ struct writeback_control {
static inline int wbc_to_write_flags(struct writeback_control *wbc)
{
if (wbc->sync_mode == WB_SYNC_ALL)
return REQ_SYNC;
else if (wbc->for_kupdate || wbc->for_background)
return REQ_BACKGROUND;
int flags = 0;
return 0;
if (wbc->punt_to_cgroup)
flags = REQ_CGROUP_PUNT;
if (wbc->sync_mode == WB_SYNC_ALL)
flags |= REQ_SYNC;
else if (wbc->for_kupdate || wbc->for_background)
flags |= REQ_BACKGROUND;
return flags;
}
static inline struct cgroup_subsys_state *