Re: workqueue code needing preemption disabled

From: Tejun Heo
Date: Mon Mar 18 2013 - 12:07:07 EST

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?


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