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From: Tejun Heo <tj@kernel.org>
To: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Kent Overstreet <kmo@daterainc.com>, Jens Axboe <axboe@kernel.dk>,
	Christoph Hellwig <hch@lst.de>,
	"linux-kernel@vger.kernel.org >> Linux Kernel Mailing List"
	<linux-kernel@vger.kernel.org>,
	linux-s390 <linux-s390@vger.kernel.org>
Subject: Re: blk-mq vs cpu hotplug performance (due to percpu_ref_put performance)
Date: Tue, 28 Oct 2014 16:00:55 -0400	[thread overview]
Message-ID: <20141028200055.GA8205@htj.dyndns.org> (raw)
In-Reply-To: <544FF00B.8050403@de.ibm.com>

Hello,

On Tue, Oct 28, 2014 at 08:35:39PM +0100, Christian Borntraeger wrote:
> when going from 3.17 to 3.18-rc2 cpu hotplug become horrible slow on some KVM guests on s390
> 
> I was able to bisect this to
> 
> commit 9eca80461a45177e456219a9cd944c27675d6512
> ("Revert "blk-mq, percpu_ref: implement a kludge for SCSI blk-mq stall during probe")

That removes the earlier kludge to avoid the RCU delay so RCU
latencies are expected to show up right after; however, the following
patches implement proper fix for the problem and the latencies
shouldn't be visible afterwards.

So, 17497acbdce9 ("blk-mq, percpu_ref: start q->mq_usage_counter in
atomic mode") should remove the latencies again.  It doesn't?

Thanks.

-- 
tejun

  reply	other threads:[~2014-10-28 20:00 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-10-28 19:35 blk-mq vs cpu hotplug performance (due to percpu_ref_put performance) Christian Borntraeger
2014-10-28 20:00 ` Tejun Heo [this message]
2014-10-28 20:20   ` Christian Borntraeger
2014-10-28 20:22     ` Tejun Heo
2014-10-28 20:26       ` Tejun Heo
2014-10-28 20:29       ` Christian Borntraeger
2014-10-28 20:30         ` Tejun Heo
2014-11-04 18:52           ` [PATCH block/for-linus] blk-mq: make mq_queue_reinit_notify() freeze queues in parallel Tejun Heo
2014-11-04 19:46             ` Christian Borntraeger
2014-11-04 21:48             ` Jens Axboe

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