Re: [PATCH 1/2] block: lift the initial queue bypass mode onblk_register_queue() instead of blk_init_allocated_queue()

From: Joseph Glanville
Date: Tue Oct 16 2012 - 06:00:23 EST

On 21 September 2012 23:25, Jens Axboe <axboe@xxxxxxxxx> wrote:
> 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@xxxxxxxxxx>
>> Reported-by: Joseph Glanville <joseph.glanville@xxxxxxxxxxxxxx>
>> Cc: Vivek Goyal <vgoyal@xxxxxxxxxx>
>> Cc: stable@xxxxxxxxxxxxxxx
>> ---
>> 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


Has this patch been marked for stable?
This is still currently broken on 3.6.2 (and I would assume other
stable kernels since 3.5)


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