Date:	Fri, 21 Sep 2012 09:25:19 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Joseph Glanville <joseph.glanville@...onvm.com.au>,
	cgroups <cgroups@...r.kernel.org>, 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 Thu, Sep 20, 2012 at 02:08:52PM -0700, 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.

Looks good to me. 

Acked-by: Vivek Goyal <vgoyal@...hat.com>

Given the fact that blkcg throttling is broken on all bio based devices
(dm,md), I would think that we need to send these fixes out in 3.6
instead of pushing these out to 3.7.

Thanks
Vivek

> 
> Thanks.
> 
>  block/blk-core.c  |    7 ++-----
>  block/blk-sysfs.c |    6 ++++++
>  2 files changed, 8 insertions(+), 5 deletions(-)
> 
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -608,8 +608,8 @@ struct request_queue *blk_alloc_queue_no
>  	/*
>  	 * A queue starts its life with bypass turned on to avoid
>  	 * unnecessary bypass on/off overhead and nasty surprises during
> -	 * init.  The initial bypass will be finished at the end of
> -	 * blk_init_allocated_queue().
> +	 * init.  The initial bypass will be finished when the queue is
> +	 * registered by blk_register_queue().
>  	 */
>  	q->bypass_depth = 1;
>  	__set_bit(QUEUE_FLAG_BYPASS, &q->queue_flags);
> @@ -714,9 +714,6 @@ blk_init_allocated_queue(struct request_
>  		return NULL;
>  
>  	blk_queue_congestion_threshold(q);
> -
> -	/* all done, end the initial bypass */
> -	blk_queue_bypass_end(q);
>  	return q;
>  }
>  EXPORT_SYMBOL(blk_init_allocated_queue);
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -527,6 +527,12 @@ int blk_register_queue(struct gendisk *d
>  	if (WARN_ON(!q))
>  		return -ENXIO;
>  
> +	/*
> +	 * Initialization must be complete by now.  Finish the initial
> +	 * bypass from queue allocation.
> +	 */
> +	blk_queue_bypass_end(q);
> +
>  	ret = blk_trace_init_sysfs(dev);
>  	if (ret)
>  		return ret;
--
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