Re: [PATCH block/for-3.18/core] blk-mq: start q->mq_usage_counter in atomic mode

From: Tejun Heo
Date: Wed Sep 24 2014 - 13:34:01 EST

On Tue, Sep 23, 2014 at 02:08:12AM -0400, Tejun Heo wrote:
> From 83b06f4fc6ca2f7f3d706a168b71c248bdada668 Mon Sep 17 00:00:00 2001
> From: Tejun Heo <tj@xxxxxxxxxx>
> Date: Tue, 23 Sep 2014 01:58:34 -0400
> blk-mq uses percpu_ref for its usage counter which tracks the number
> of in-flight commands and used to synchronously drain the queue on
> freeze. percpu_ref shutdown takes measureable wallclock time as it
> involves a sched RCU grace period. This means that draining a blk-mq
> takes measureable wallclock time. One would think that this shouldn't
> matter as queue shutdown should be a rare event which takes place
> asynchronously w.r.t. userland.

Applied to percpu/for-3.18 on top of the klude, its revert and the
required percpu_ref changes.


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