On 21 September 2012 23:25, Jens Axboe [off-list ref] wrote:

On 09/20/2012 11:08 PM, Tejun Heo wrote:

quoted

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 <redacted>
Reported-by: Joseph Glanville <redacted>
Cc: Vivek Goyal <redacted>
Cc: stable-u79uwXL29TY76Z2rM5mHXA@public.gmane.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

Hi,

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)

Joseph.

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