Re: [bug report] shared tags causes IO hang and performance drop

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

On Mon, Apr 26, 2021 at 06:02:31PM +0100, John Garry wrote:
> On 26/04/2021 17:03, Ming Lei wrote:
> > > For both hostwide and non-hostwide tags, we have standalone sched tags and
> > > request pool per hctx when q->nr_hw_queues > 1.
> > driver tags is shared for hostwide tags.
> > 
> > > > That is why you observe that scheduler tag exhaustion
> > > > is easy to trigger in case of non-hostwide tags.
> > > > 
> > > > I'd suggest to add one per-request-queue sched tags, and make all hctxs
> > > > sharing it, just like what you did for driver tag.
> > > > 
> > > That sounds reasonable.
> > > 
> > > But I don't see how this is related to hostwide tags specifically, but
> > > rather just having q->nr_hw_queues > 1, which NVMe PCI and some other SCSI
> > > MQ HBAs have (without using hostwide tags).
> > Before hostwide tags, the whole scheduler queue depth should be 256.
> > After hostwide tags, the whole scheduler queue depth becomes 256 *
> > nr_hw_queues. But the driver tag queue depth is_not_  changed.
> 
> Fine.
> 
> > 
> > More requests come and are tried to dispatch to LLD and can't succeed
> > because of limited driver tag depth, and CPU utilization could be increased.
> 
> Right, maybe this is a problem.
> 
> I quickly added some debug, and see that
> __blk_mq_get_driver_tag()->__sbitmap_queue_get() fails ~7% for hostwide tags
> and 3% for non-hostwide tags.
> 
> Having it fail at all for non-hostwide tags seems a bit dubious... here's
> the code for deciding the rq sched tag depth:
> 
> q->nr_requests = 2 * min(q->tags_set->queue_depth [128], BLK_DEV_MAX_RQ
> [128])
> 
> So we get 256 for our test scenario, which is appreciably bigger than
> q->tags_set->queue_depth, so the failures make sense.
> 
> Anyway, I'll look at adding code for a per-request queue sched tags to see
> if it helps. But I would plan to continue to use a per hctx sched request
> pool.

Why not switch to per hctx sched request pool?

Thanks,
Ming