Re: Is it a workqueue related issue in 2.6.37 (Was: Re: [libvirt] blkio cgroup [solved])

[Tejun Heo <tj@kernel.org>]

Hello,

On Fri, Feb 25, 2011 at 09:57:08AM -0500, Vivek Goyal wrote:
> blk_throtl_work() calls generic_make_request() to dispatch some bios and I
> guess blk_throtl_work() has been put to sleep because threre are no request
> descriptors available and CFQ is frozen so no requests descriptors get freed
> hence blk_throtl_work() never finishes.
> 
> Following caught my eye.
> 
>      ksoftirqd/0-3     [000]  1640.983585:   8,16   m   N cfq4810 slice
> expired t=0
>      ksoftirqd/0-3     [000]  1640.983588:   8,16   m   N cfq4810
> sl_used=2 disp=6 charge=2 iops=0 sect=2080
>      ksoftirqd/0-3     [000]  1640.983589:   8,16   m   N cfq4810
> del_from_rr
>      ksoftirqd/0-3     [000]  1640.983591:   8,16   m   N cfq schedule
> dispatch
>             sshd-3125  [004]  1640.983597: workqueue_queue_work: work
> struct=ffff88102c3a3110 function=flush_to_ldisc workqueue=ffff88182c834a00
> req_cpu=4 cpu=4
>             sshd-3125  [004]  1640.983598: workqueue_activate_work: work
> struct ffff88102c3a3110
> 
> CFQ tries to schedule a work and but there is no associated
> "workqueue_queue_work" trace. So it looks like that work never got queued.
> 
> CFQ calls following.
> 
> cfq_log(cfqd, "schedule dispatch");
> kblockd_schedule_work(cfqd->queue, &cfqd->unplug_work);
> 
> We do see "schedule dispatch" message and kblockd_schedule_work() calls
> queue_work(). So what happended here? This is strange. I will put one
> more trace after kblockd_schedule_work() to trace that function returned.

It could be that the unplug work was already queued and in pending
state.  The second queueing request will be ignored then.  So, I think
the problem is that blk_throtl_work() occupies kblockd but requires
another work item (unplug_work) to make forward progress.  In such
cases, forward progress cannot be guaranteed.  Either
blk_throtl_work() or cfq unplug work should use a separate workqueue.

Thanks.

-- 
tejun