Re: [PATCH V2 5/5] blk-mq: support concurrent queue quiesce/unquiesce

[Ming Lei <ming.lei@redhat.com>]

On Thu, Sep 30, 2021 at 08:56:29AM -0700, Bart Van Assche wrote:
> On 9/30/21 5:56 AM, Ming Lei wrote:
> > Turns out that blk_mq_freeze_queue() isn't stronger[1] than
> > blk_mq_quiesce_queue() because dispatch may still be in-progress after
> > queue is frozen, and in several cases, such as switching io scheduler,
> > updating nr_requests & wbt latency, we still need to quiesce queue as a
> > supplement of freezing queue.
> 
> Is there agreement about this? If not, how about leaving out the above from the
> patch description?

Yeah, actually the code has been merged, please see the related
functions: elevator_switch(), queue_wb_lat_store() and
blk_mq_update_nr_requests().

> 
> > As we need to extend uses of blk_mq_quiesce_queue(), it is inevitable
> > for us to need support nested quiesce, especially we can't let
> > unquiesce happen when there is quiesce originated from other contexts.
> > 
> > This patch introduces q->mq_quiesce_depth to deal concurrent quiesce,
> > and we only unquiesce queue when it is the last/outer-most one of all
> > contexts.
> > 
> > One kernel panic issue has been reported[2] when running stress test on
> > dm-mpath's updating nr_requests and suspending queue, and the similar
> > issue should exist on almost all drivers which use quiesce/unquiesce.
> > 
> > [1] https://marc.info/?l=linux-block&m=150993988115872&w=2
> > [2] https://listman.redhat.com/archives/dm-devel/2021-September/msg00189.html
> 
> Please share the call stack of the kernel oops fixed by [2] since that
> call stack is not in the patch description.

OK, it is something like the following:

[  145.453672] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.12.0-2.fc30 04/01/2014
[  145.454104] RIP: 0010:dm_softirq_done+0x46/0x220 [dm_mod]
[  145.454536] Code: 85 ed 0f 84 40 01 00 00 44 0f b6 b7 70 01 00 00 4c 8b a5 18 01 00 00 45 89 f5 f6 47 1d 04 75 57 49 8b 7c 24 08 48 85 ff 74 4d <48> 8b 47 08 48 8b 40 58 48 85 c0 74 40 49 8d 4c 24 50 44 89 f2 48
[  145.455423] RSP: 0000:ffffa88600003ef8 EFLAGS: 00010282
[  145.455865] RAX: ffffffffc03fbd10 RBX: ffff979144c00010 RCX: dead000000000200
[  145.456321] RDX: ffffa88600003f30 RSI: ffff979144c00068 RDI: ffffa88600d01040
[  145.456764] RBP: ffff979150eb7990 R08: ffff9791bbc27de0 R09: 0000000000000100
[  145.457205] R10: 0000000000000068 R11: 000000000000004c R12: ffff979144c00138
[  145.457647] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000010
[  145.458080] FS:  00007f57e5d13180(0000) GS:ffff9791bbc00000(0000) knlGS:0000000000000000
[  145.458516] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  145.458945] CR2: ffffa88600d01048 CR3: 0000000106cf8003 CR4: 0000000000370ef0
[  145.459382] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  145.459815] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  145.460250] Call Trace:
[  145.460779]  <IRQ>
[  145.461453]  blk_done_softirq+0xa1/0xd0
[  145.462138]  __do_softirq+0xd7/0x2d6
[  145.462814]  irq_exit+0xf7/0x100
[  145.463480]  do_IRQ+0x7f/0xd0
[  145.464131]  common_interrupt+0xf/0xf
[  145.464797]  </IRQ>

> 
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 21bf4c3f0825..10f8a3d4e3a1 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -209,7 +209,12 @@ EXPORT_SYMBOL_GPL(blk_mq_unfreeze_queue);
> >    */
> >   void blk_mq_quiesce_queue_nowait(struct request_queue *q)
> >   {
> > -	blk_queue_flag_set(QUEUE_FLAG_QUIESCED, q);
> > +	unsigned long flags;
> > +
> > +	spin_lock_irqsave(&q->queue_lock, flags);
> > +	if (!q->quiesce_depth++)
> > +		blk_queue_flag_set(QUEUE_FLAG_QUIESCED, q);
> > +	spin_unlock_irqrestore(&q->queue_lock, flags);
> >   }
> >   EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue_nowait);
> 
> Consider using == 0 instead of ! to check whether or not quiesce_depth is
> zero to improve code readability.

Fine.

> 
> > @@ -250,10 +255,19 @@ EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue);
> >    */
> >   void blk_mq_unquiesce_queue(struct request_queue *q)
> >   {
> > -	blk_queue_flag_clear(QUEUE_FLAG_QUIESCED, q);
> > +	unsigned long flags;
> > +	bool run_queue = false;
> > +
> > +	spin_lock_irqsave(&q->queue_lock, flags);
> > +	if (q->quiesce_depth > 0 && !--q->quiesce_depth) {
> > +		blk_queue_flag_clear(QUEUE_FLAG_QUIESCED, q);
> > +		run_queue = true;
> > +	}
> > +	spin_unlock_irqrestore(&q->queue_lock, flags);
> >   	/* dispatch requests which are inserted during quiescing */
> > -	blk_mq_run_hw_queues(q, true);
> > +	if (run_queue)
> > +		blk_mq_run_hw_queues(q, true);
> >   }
> 
> So calling with blk_mq_unquiesce_queue() q->quiesce_depth <= 0 is ignored
> quietly? How about triggering a kernel warning for that condition?

OK.


Thanks, 
Ming