Messages in this thread | | | Date | Tue, 28 Oct 2014 16:00:55 -0400 | From | Tejun Heo <> | Subject | Re: blk-mq vs cpu hotplug performance (due to percpu_ref_put performance) |
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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
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