Commit 6c635cae authored Jan 28, 2021 by Baolin Wang Committed by Jens Axboe Jan 28, 2021

blk-cgroup: Use cond_resched() when destroy blkgs



On !PREEMPT kernel, we can get below softlockup when doing stress
testing with creating and destroying block cgroup repeatly. The
reason is it may take a long time to acquire the queue's lock in
the loop of blkcg_destroy_blkgs(), or the system can accumulate a
huge number of blkgs in pathological cases. We can add a need_resched()
check on each loop and release locks and do cond_resched() if true
to avoid this issue, since the blkcg_destroy_blkgs() is not called
from atomic contexts.

[ 4757.010308] watchdog: BUG: soft lockup - CPU#11 stuck for 94s!
[ 4757.010698] Call trace:
[ 4757.010700]  blkcg_destroy_blkgs+0x68/0x150
[ 4757.010701]  cgwb_release_workfn+0x104/0x158
[ 4757.010702]  process_one_work+0x1bc/0x3f0
[ 4757.010704]  worker_thread+0x164/0x468
[ 4757.010705]  kthread+0x108/0x138

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

parent 8dc932d3

block/blk-cgroup.c

+13 −5

Original line number	Diff line number	Diff line
		@@ -1016,6 +1016,8 @@ static void blkcg_css_offline(struct cgroup_subsys_state *css)
		*/
		void blkcg_destroy_blkgs(struct blkcg *blkcg)
		{
		might_sleep();

		spin_lock_irq(&blkcg->lock);

		while (!hlist_empty(&blkcg->blkg_list)) {
		@@ -1023,14 +1025,20 @@ void blkcg_destroy_blkgs(struct blkcg *blkcg)
		struct blkcg_gq, blkcg_node);
		struct request_queue *q = blkg->q;

		if (spin_trylock(&q->queue_lock)) {
		blkg_destroy(blkg);
		spin_unlock(&q->queue_lock);
		} else {
		if (need_resched() \|\| !spin_trylock(&q->queue_lock)) {
		/*
		* Given that the system can accumulate a huge number
		* of blkgs in pathological cases, check to see if we
		* need to rescheduling to avoid softlockup.
		*/
		spin_unlock_irq(&blkcg->lock);
		cpu_relax();
		cond_resched();
		spin_lock_irq(&blkcg->lock);
		continue;
		}

		blkg_destroy(blkg);
		spin_unlock(&q->queue_lock);
		}

		spin_unlock_irq(&blkcg->lock);