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

    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.



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