[Adding tglx to the cc]
On Mon, Jun 23, 2014 at 8:25 PM, Tejun Heo <tj@xxxxxxxxxx> wrote:
Hello,
It's more complex than using flush_work() but there's nothing
On Tue, Jun 24, 2014 at 01:02:40PM +1000, Dave Chinner wrote:
> start_flush_work() is effectively a special queue_work()
> implementation, so if if it's not safe to call complete() from the
> workqueue as the above patch implies then this code has the same
> problem.
>
> Tejun - is this "do it yourself completion" a known issue w.r.t.
> workqueues? I can't find any documentation that says "don't do
> that" so...?
fundamentally wrong with it. A work item is completely unlinked
before its execution starts. It's safe to free the work item once its
work function started, whether through kfree() or returning.
One difference between flush_work() and manual completion would be
that if the work item gets requeued, flush_work() would wait for the
queued one to finish but given the work item is one-shot this doesn't
make any difference.
I can see no reason why manual completion would behave differently
from flush_work() in this case.
I went looking for a short trace in my original log to show the problem, and instead found evidence of the second problem. I still like the shorter flush_work call, but that's not my call.
I did find this comment in the process_one_work function. Sounds like this could be better documented.
/*
* It is permissible to free the struct work_struct from
* inside the function that is called from it, this we need to
* take into account for lockdep too. To avoid bogus "held
* lock freed" warnings as well as problems when looking into
* work->lockdep_map, make a copy and use that here.
*/
> As I understand it, what then happens is that the workqueue codeYes, as long as the workqueue is under its @max_active limit and has
> grabs another kworker thread and runs the next work item in it's
> queue. IOWs, work items can block, but doing that does not prevent
> execution of other work items queued on other work queues or even on
> the same work queue. Tejun, did I get that correct?
access to an existing kworker or can create a new one, it'll start
executing the next work item immediately; however, the guaranteed
level of concurrency is 1 even for WQ_RECLAIM workqueues. IOW, the
work items queued on a workqueue must be able to make forward progress
with single work item if the work items are being depended upon for
memory reclaim.
I mentioned this to Dave when I initially started this thread, but I'm running a RT patched kernel. I don't see forwards progress. The workqueue is only used in 1 spot (xfs_alloc_wq), and has WQ_MEM_RECLAIM set. I started with a 10,000,000 line trace and pulled out the entries which referenced the workqueue and pool leading up to the lockup.
scp-4110 [001] ....1.3 101.184640: workqueue_queue_work: work struct=ffff8803c782d900 function=xfs_bmapi_allocate_worker workqueue=ffff88040c9f5a00 pool=ffff88042dae3fc0 req_cpu=512 cpu=1
scp-4110 [001] ....1.3 101.184641: workqueue_activate_work: work struct ffff8803c782d900
kworker/1:1-89 [001] ....1.1 101.184883: workqueue_nr_running: pool=ffff88042dae3fc0 nr_running=1
kworker/1:1-89 [001] ....1.. 101.184885: workqueue_execute_start: work struct ffff8803c782d900: function xfs_bmapi_allocate_worker
irq/44-ahci-275 [001] ....1.5 101.185086: workqueue_queue_work: work struct=ffff8800ae3f01e0 function=xfs_end_io workqueue=ffff88040b459a00 pool=ffff88042dae3fc0 req_cpu=512 cpu=1
irq/44-ahci-275 [001] ....1.5 101.185088: workqueue_activate_work: work struct ffff8800ae3f01e0
scp-4110 [001] ....1.. 101.187911: xfs_ilock: dev 8:5 ino 0xf9e flags ILOCK_EXCL caller xfs_iomap_write_allocate
scp-4110 [001] ....1.3 101.187914: workqueue_queue_work: work struct=ffff8803c782d900 function=xfs_bmapi_allocate_worker workqueue=ffff88040c9f5a00 pool=ffff88042dae3fc0 req_cpu=512 cpu=1
scp-4110 [001] ....1.3 101.187915: workqueue_activate_work: work struct ffff8803c782d900
scp-4110 [001] ....1.4 101.187926: workqueue_queue_work: work struct=ffff88040a6a01c8 function=blk_delay_work workqueue=ffff88040c9f4a00 pool=ffff88042dae44c0 req_cpu=512 cpu=1
scp-4110 [001] ....1.4 101.187926: workqueue_activate_work: work struct ffff88040a6a01c8
kworker/1:1-89 [001] ....1.. 101.187998: workqueue_execute_end: work struct ffff8803c782d900
kworker/1:1-89 [001] ....1.. 101.188000: workqueue_execute_start: work struct ffff8800ae3f01e0: function xfs_end_io
kworker/1:1-89 [001] ....1.. 101.188001: xfs_ilock: dev 8:5 ino 0xf9e flags ILOCK_EXCL caller xfs_setfilesize
The last work never runs. Hangcheck triggers shortly after.
I spent some more time debugging, and I am seeing that tsk_is_pi_blocked is returning 1 in sched_submit_work (kernel/sched/core.c). It looks like sched_submit_work is not detecting that the worker task is blocked on a mutex.
This looks very RT related right now. I see 2 problems from my reading (and experimentation). The first is that the second worker isn't getting started because tsk_is_pi_blocked is reporting that the task isn't blocked on a mutex. The second is that even if another worker needs to be scheduled because the original worker is blocked on a mutex, we need the pool lock to schedule another worker. The pool lock can be acquired by any CPU, and is a spin_lock. If we end up on the slow path for the pool lock, we hit BUG_ON(rt_mutex_real_waiter(task->pi_blocked_on)) in task_blocks_on_rt_mutex in rtmutex.c. I'm not sure how to deal with either problem.
Hopefully I've got all my facts right... Debugging kernel code is a whole new world from userspace code.
Thanks!
Austin
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