Re: [PATCH v2] blk-cgroup: Use cond_resched() when destroy blkgs

[Baolin Wang]

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.