linux-kernel - Re: blk-mq vs cpu hotplug performance (due to percpu_ref

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

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
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