Messages in this thread Patch in this message | | | Date | Thu, 23 Jun 2011 17:07:20 +0200 | From | Tejun Heo <> | Subject | Re: [patch 4/4] sched: Distangle worker accounting from rq->lock |
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Hello, again.
So, let's get it correct first.
On Wed, Jun 22, 2011 at 05:52:15PM -0000, Thomas Gleixner wrote: > The worker accounting for cpu bound workers is plugged into the core > scheduler code and the wakeup code. This is not a hard requirement and > can be avoided by keeping track of the state in the workqueue code > itself. > > Keep track of the sleeping state in the worker itself and call the > notifier before entering the core scheduler. There might be false > positives when the task is woken between that call and actually > scheduling, but that's not really different from scheduling and being > woken immediately after switching away. There is also no harm from > updating nr_running when the task returns from scheduling instead of > accounting it in the wakeup code.
I think false positives on schedule() should be safe. As said earlier, the gap between ttwu and actually running is a bit worrisome but it might be nothing, but please at least describe the behavior change.
> Index: linux-2.6/kernel/workqueue.c > =================================================================== > --- linux-2.6.orig/kernel/workqueue.c > +++ linux-2.6/kernel/workqueue.c > @@ -137,6 +137,7 @@ struct worker { > unsigned int flags; /* X: flags */ > int id; /* I: worker id */ > struct work_struct rebind_work; /* L: rebind worker to cpu */ > + int sleeping; /* None */
bool?
> -struct task_struct *wq_worker_sleeping(struct task_struct *task, > - unsigned int cpu) > +void wq_worker_sleeping(struct task_struct *task) > { > - struct worker *worker = kthread_data(task), *to_wakeup = NULL; > - struct global_cwq *gcwq = get_gcwq(cpu); > - atomic_t *nr_running = get_gcwq_nr_running(cpu); > + struct worker *worker = kthread_data(task); > + struct global_cwq *gcwq; > + int cpu; > > if (worker->flags & WORKER_NOT_RUNNING) > - return NULL; > + return;
This doesn't look safe. It can race with trustee_thread() setting WORKER_ROGUE. Let's just grab gcwq->lock on entry to wq_worker_sleeping() for now; then, the schedule() trickery in trustee_thread() can go away too. This also means we can remove the weird sync rules from ->flags and ->idle_list and just use simple gcwq->lock for those, which is pretty nice.
> - /* this can only happen on the local cpu */ > - BUG_ON(cpu != raw_smp_processor_id()); > + if (WARN_ON_ONCE(worker->sleeping)) > + return;
Re-entrance is prevented by the scheduler hook being called only for non-premption schedule(). Maybe it's better to explain that in the function comment?
Hmmm... Also, I think worker->sleeping should be cleared by trustee_thread() when WORKER_ROGUE is set for the worker; otherwise, it can get out of sync and the above WARN_ON_ONCE() will trigger. Thanks.
-- tejun
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