linux-kernel - Re: [PATCH 1/2] block: lift the initial queue bypass mode on blk_register_queue() instead of blk_init_allocated

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Date:	Fri, 21 Sep 2012 15:25:40 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Tejun Heo <tj@...nel.org>
CC:	Joseph Glanville <joseph.glanville@...onvm.com.au>,
	cgroups <cgroups@...r.kernel.org>,
	Vivek Goyal <vgoyal@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] block: lift the initial queue bypass mode on blk_register_queue()
 instead of blk_init_allocated_queue()

On 09/20/2012 11:08 PM, Tejun Heo wrote:
> b82d4b197c ("blkcg: make request_queue bypassing on allocation") made
> request_queues bypassed on allocation to avoid switching on and off
> bypass mode on a queue being initialized.  Some drivers allocate and
> then destroy a lot of queues without fully initializing them and
> incurring bypass latency overhead on each of them could add upto
> significant overhead.
> 
> Unfortunately, blk_init_allocated_queue() is never used by queues of
> bio-based drivers, which means that all bio-based driver queues are in
> bypass mode even after initialization and registration complete
> successfully.
> 
> Due to the limited way request_queues are used by bio drivers, this
> problem is hidden pretty well but it shows up when blk-throttle is
> used in combination with a bio-based driver.  Trying to configure
> (echoing to cgroupfs file) blk-throttle for a bio-based driver hangs
> indefinitely in blkg_conf_prep() waiting for bypass mode to end.
> 
> This patch moves the initial blk_queue_bypass_end() call from
> blk_init_allocated_queue() to blk_register_queue() which is called for
> any userland-visible queues regardless of its type.
> 
> I believe this is correct because I don't think there is any block
> driver which needs or wants working elevator and blk-cgroup on a queue
> which isn't visible to userland.  If there are such users, we need a
> different solution.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Reported-by: Joseph Glanville <joseph.glanville@...onvm.com.au>
> Cc: Vivek Goyal <vgoyal@...hat.com>
> Cc: stable@...r.kernel.org
> ---
> Jens, while these are fixes, I think it isn't extremely urgent and
> routing these through 3.7-rc1 should be enough.

Agree, I'll shove them into for-3.7/core

-- 
Jens Axboe

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