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SubjectRe: [PATCH v2] blk-cgroup: Use cond_resched() when destroy blkgs
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在 2021/1/28 11:41, Jens Axboe 写道:
> On 1/27/21 8:22 PM, Baolin Wang wrote:
>> 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
>
> Kind of ugly with the two clauses for dropping the blkcg lock, one
> being a cpu_relax() and the other a resched. How about something
> like this:
>
>
> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
> index 031114d454a6..4221a1539391 100644
> --- a/block/blk-cgroup.c
> +++ b/block/blk-cgroup.c
> @@ -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);
>

Looks better to me. Do I need resend with your suggestion? Thanks.

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