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 <t...@kernel.org>
> Reported-by: Joseph Glanville <joseph.glanvi...@orionvm.com.au>
> Cc: Vivek Goyal <vgo...@redhat.com>
> Cc: sta...@vger.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 <vgo...@redhat.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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