Re: [bug report] worker watchdog timeout in dispatch loop for null_blk

[Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>]

On Mar 10, 2022 / 18:00, Ming Lei wrote:
> On Thu, Mar 10, 2022 at 09:16:50AM +0000, Shinichiro Kawasaki wrote:
> > This issue does not look critical, but let me share it to ask comments for fix.
> > 
> > When fio command with 40 jobs [1] is run for a null_blk device with memory
> > backing and mq-deadline scheduler, kernel reports a BUG message [2]. The
> > workqueue watchdog reports that kblockd blk_mq_run_work_fn keeps on running
> > more than 30 seconds and other work can not run. The 40 fio jobs keep on
> > creating many read requests to a single null_blk device, then the every time
> > the mq_run task calls __blk_mq_do_dispatch_sched(), it returns ret == 1 which
> > means more than one request was dispatched. Hence, the while loop in
> > blk_mq_do_dispatch_sched() does not break.
> > 
> > static int blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
> > {
> >         int ret;
> > 
> >         do {
> >                ret = __blk_mq_do_dispatch_sched(hctx);
> >         } while (ret == 1);
> > 
> >         return ret;
> > }
> > 
> > The BUG message was observed when I ran blktests block/005 with various
> > conditions on a system with 40 CPUs. It was observed with kernel version
> > v5.16-rc1 through v5.17-rc7. The trigger commit was 0a593fbbc245 ("null_blk:
> > poll queue support"). This commit added blk_mq_ops.map_queues callback. I
> > guess it changed dispatch behavior for null_blk devices and triggered the
> > BUG message.
> 
> It is one blk-mq soft lockup issue in dispatch side, and shouldn't be related
> with 0a593fbbc245.
> 
> If queueing requests is faster than dispatching, the issue will be triggered
> sooner or later, especially easy to trigger in SQ device. I am sure it can
> be triggered on scsi debug, even saw such report on ahci.

Thank you for the comments. Then this is the real problem.

> 
> > 
> > I'm not so sure if we really need to fix this issue. It does not seem the real
> > world problem since it is observed only with null_blk. The real block devices
> > have slower IO operation then the dispatch should stop sooner when the hardware
> > queue gets full. Also the 40 jobs for single device is not realistic workload.
> > 
> > Having said that, it does not feel right that other works are pended during
> > dispatch for null_blk devices. To avoid the BUG message, I can think of two
> > fix approaches. First one is to break the while loop in blk_mq_do_dispatch_sched
> > using a loop counter [3] (or jiffies timeout check).
> 
> This way could work, but the queue need to be re-run after breaking
> caused by max dispatch number. cond_resched() might be the simplest way,
> but it can't be used here because of rcu/srcu read lock.

As far as I understand, blk_mq_run_work_fn() should return after the loop break
to yield the worker to other works. How about to call
blk_mq_delay_run_hw_queue() at the loop break? Does this re-run the dispatch?


diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 55488ba978232..faa29448a72a0 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -178,13 +178,19 @@ static int __blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
 	return !!dispatched;
 }
 
+#define MQ_DISPATCH_MAX 0x10000
+
 static int blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
 {
 	int ret;
+	unsigned int count = MQ_DISPATCH_MAX;
 
 	do {
 		ret = __blk_mq_do_dispatch_sched(hctx);
-	} while (ret == 1);
+	} while (ret == 1 && count--);
+
+	if (ret == 1 && !count)
+		blk_mq_delay_run_hw_queue(hctx, 0);
 
 	return ret;
 }

-- 
Best Regards,
Shin'ichiro Kawasaki