linux-kernel - RE: [PATCH RFT] blk-mq: optimize queue tag busy iter for shared

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Date:   Wed, 22 Dec 2021 16:50:53 +0530
From:   Kashyap Desai <kashyap.desai@...adcom.com>
To:     John Garry <john.garry@...wei.com>, axboe@...nel.dk
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        ming.lei@...hat.com,
        Sathya Prakash Veerichetty <sathya.prakash@...adcom.com>
Subject: RE: [PATCH RFT] blk-mq: optimize queue tag busy iter for shared_tags

>
> But your change seems effectively the same as in
> https://lore.kernel.org/all/1638794990-137490-4-git-send-email-
> john.garry@...wei.com/,
> which is now merged in Jens' 5.17 queue:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux-
> block.git/commit/?h=for-
> 5.17/block&id=fea9f92f1748083cb82049ed503be30c3d3a9b69

John -

Yes, above is the same changes I was looking for. I did very basic mistake.
I applied your above commit while doing megaraid_sas testing.
 While I move to mpi3mr testing, I did not apply your patch set. We can drop
request of this RFT since I tested above series and it serve the same
purpose.

Kashyap

>
> > While doing additional testing for [1], I noticed some performance
> > issue.
> > Along with the performance issue, I noticed CPU lockup as well. Lockup
> > trace -
> >
> > _raw_spin_lock_irqsave+0x42/0x50
> >   blk_mq_find_and_get_req+0x20/0xa0
> >   bt_iter+0x2d/0x80
> >   blk_mq_queue_tag_busy_iter+0x1aa/0x2f0
> >   ? blk_mq_complete_request+0x30/0x30
> >   ? blk_mq_complete_request+0x30/0x30
> >   ? __schedule+0x360/0x850
> >   blk_mq_timeout_work+0x5e/0x120
> >   process_one_work+0x1a8/0x380
> >   worker_thread+0x30/0x380
> >   ? wq_calc_node_cpumask.isra.30+0x100/0x100
> >   kthread+0x167/0x190
> >   ? set_kthread_struct+0x40/0x40
> >   ret_from_fork+0x22/0x30
> >
> > It is a generic performance issue if driver use " shost->host_tagset =
> > 1".
> > In fact, I found that [1] is useful to fix performance issue and
> > provided this additional patch.
> >
> > I changed my setup to have 64 scsi_devices (earlier I just kept 16 or
> > 24 drives, so did not noticed this issue). Performance/cpu lockup
> > issue is not due to [1].
> > More number of scsi device, hardware context per host and high queue
> > depth will increase the chances of lockup and performance drop.
> >
> > Do you think, it is good to have changes in 5.16 + stable ?
> > I don't know if this  patch will create any side effect. Can you
> > review and let me know your feedback. ?
> >
>
> Can you test my merged change again for this scenario?
>
> I will also note that I mentioned previously that
> blk_mq_queue_tag_busy_iter() was not optimum for shared sbitmap, i.e.
> before shared tags, but no one said performance was bad for shared
> sbitmap.
>
> Thanks,
> John

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