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

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Feb 25, 2011 at 10:11:13AM -0500, Vivek Goyal wrote:
> On Fri, Feb 25, 2011 at 04:03:29PM +0100, Tejun Heo wrote:
> > 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.
> 
> Ok, that would make sense. So blk_throtl_work() can not finish as CFQ
> is not making progress and no request descriptors are being freed and
> unplug_work() is not being called because blk_throtl_work() has not finished.
> So that's cyclic dependency and I should use a separate work queue for
> queueing throttle related work. I will write a patch.

The only thing unexplained is why same problem does not happen in 2.6.38-rc
kernels.

Thanks
Vivek