From: Ming Lei <ming.lei@redhat.com>
To: John Garry <john.garry@huawei.com>
Cc: Kashyap Desai <kashyap.desai@broadcom.com>,
linux-block@vger.kernel.org, linux-scsi@vger.kernel.org,
"Martin K. Petersen" <martin.petersen@oracle.com>,
Jens Axboe <axboe@kernel.dk>,
Douglas Gilbert <dgilbert@interlog.com>,
Hannes Reinecke <hare@suse.com>
Subject: Re: [bug report] shared tags causes IO hang and performance drop
Date: Tue, 27 Apr 2021 07:59:07 +0800 [thread overview]
Message-ID: <YIdTyyVE5azlYwtO@T590> (raw)
In-Reply-To: <9ad15067-ba7b-a335-ae71-8c4328856b91@huawei.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
next prev parent reply other threads:[~2021-04-26 23:59 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-14 7:50 [bug report] shared tags causes IO hang and performance drop Ming Lei
2021-04-14 10:10 ` John Garry
2021-04-14 10:38 ` Ming Lei
2021-04-14 10:42 ` Kashyap Desai
2021-04-14 11:12 ` Ming Lei
2021-04-14 12:06 ` John Garry
2021-04-15 3:46 ` Ming Lei
2021-04-15 10:41 ` John Garry
2021-04-15 12:18 ` Ming Lei
2021-04-15 15:41 ` John Garry
2021-04-16 0:46 ` Ming Lei
2021-04-16 8:29 ` John Garry
2021-04-16 8:39 ` Ming Lei
2021-04-16 14:59 ` John Garry
2021-04-20 3:06 ` Douglas Gilbert
2021-04-20 3:22 ` Bart Van Assche
2021-04-20 4:54 ` Douglas Gilbert
2021-04-20 6:52 ` Ming Lei
2021-04-20 20:22 ` Douglas Gilbert
2021-04-21 1:40 ` Ming Lei
2021-04-23 8:43 ` John Garry
2021-04-26 10:53 ` John Garry
2021-04-26 14:48 ` Ming Lei
2021-04-26 15:52 ` John Garry
2021-04-26 16:03 ` Ming Lei
2021-04-26 17:02 ` John Garry
2021-04-26 23:59 ` Ming Lei [this message]
2021-04-27 7:52 ` John Garry
2021-04-27 9:11 ` Ming Lei
2021-04-27 9:37 ` John Garry
2021-04-27 9:52 ` Ming Lei
2021-04-27 10:15 ` John Garry
2021-07-07 17:06 ` John Garry
2021-04-14 13:59 ` Kashyap Desai
2021-04-14 17:03 ` Douglas Gilbert
2021-04-14 18:19 ` John Garry
2021-04-14 19:39 ` Douglas Gilbert
2021-04-15 0:58 ` Ming Lei
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