Re: workqueue code needing preemption disabled

[Tejun Heo]

Hello, Steven.

On Mon, Mar 18, 2013 at 10:36:23AM -0400, Steven Rostedt wrote:
> kernel BUG at kernel/sched/core.c:1731!
> invalid opcode: 0000 [#1] PREEMPT SMP 
> CPU 5 
> Pid: 16637, comm: kworker/5:0 Not tainted 3.6.11-rt30.25.el6rt.x86_64 #1 HP ProLiant DL580 G7
...
> static void try_to_wake_up_local(struct task_struct *p)
> {
>         struct rq *rq = task_rq(p);
> 
>         BUG_ON(rq != this_rq());  <---- bug here

It's the local chain wake-up code used to main concurrency.  ie. when
a worker bound to a CPU schedules out it kicks another worker to take
its place (in concurrency level).

The function is called from inside __schedule() while holding rq->lock
and requires that the target task is on the same rq as the one trying
to wake it up.  When it isn't, the above BUG_ON() triggers.

On non-RT kernel, this usually happens, when I screw up CPU hotplug
code - e.g. enabling concurrency management when all workers are not
rebound to the CPU yet.

> Now in your code you have the comment:
> 
>  * X: During normal operation, modification requires gcwq->lock and
>  *    should be done only from local cpu.  Either disabling preemption
>  *    on local cpu or grabbing gcwq->lock is enough for read access.
>  *    If GCWQ_DISASSOCIATED is set, it's identical to L.
> 
> struct worker has flags marked with X.
> struct worker_pool has flags and idle_list marked with X.

So, the weird 'X' rule is there to guarantee that wq_worker_sleeping()
and try_to_wake_up() can peek the data fields necessary to perform
local wakeup (determining whether and who to wakeup and actuallying
doing it) while holding rq->lock.

> spin_locks in -rt do not disable preemption, nor do they disable irqs,
> but they do disable migration. If there's code that depends on the
> spin_lock disabling preemption, we need to either change the code to not
> require that, or explicitly disable preemption in the critical paths.
> Note, if we explicitly disable preemption, we can not call spin_locks
> within those locations as in -rt a spin_lock can block and schedule.

Maybe I'm confused but I can't really see how the above would be a
problem to workqueue in itself.  Both rq->lock and gcwq->lock are
irq-safe, so spin_lock() not disabling preemption shouldn't be a
problem.  Are CPU hotplug operations involved?

Thanks.

-- 
tejun
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