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Date: Tue, 28 Oct 2014 16:00:55 -0400
From: Tejun Heo <tj@kernel.org>
Subject: Re: blk-mq vs cpu hotplug performance (due to percpu_ref_put
 performance)
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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>
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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