Messages in this thread | | | Date | Thu, 29 Sep 2011 20:48:15 -0700 | From | Tejun Heo <> | Subject | Re: [PATCH] sched/kthread: Complain loudly when others violate our flags |
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Hello, Steven.
Sorry about the delay, coming back from a rather long vacation.
On Tue, Sep 27, 2011 at 05:17:34PM -0400, Steven Rostedt wrote: > I looked at the task that it tried to migrate, and it happened to be the > kworker thread! Then I went into kernel/workqueue.c and found this > nonsense: > > if (bind && !on_unbound_cpu) > kthread_bind(worker->task, gcwq->cpu); > else { > worker->task->flags |= PF_THREAD_BOUND; > if (on_unbound_cpu) > worker->flags |= WORKER_UNBOUND; > } > > Nothing but the scheduler and kthread_bind() has the right to set the > PF_THREAD_BOUND flag. Especially when the thread IS NOT BOUNDED!!!!!! > > I don't go around and stick my hand down your pants to play with your > flags! Don't stick your hand in ours and play with our flags! > > WTF is the workqueue code setting the PF_THREAD_BOUND flag manually? > Talk about fragile coupling! You just made this flag meaningless. Don't > do that.
IIRC, this was because there was no way to set PF_THREAD_BOUND once a kthread starts to run and workers can stay active across CPU bring down/up cycle. Per-cpu kthreads need PF_THREAD_BOUND to prevent cpu affinity manipulation by third party for correctness.
> Sorry but I just wasted two whole days because of this nonsense and I'm > not particularly happy about it.
Sorry that it wasted your time and made you unhappy but wouldn't grepping for its usage a logical thing if you wanted to add to what it meant? PF_THREAD_BOUND meaning the task's affinity or cpuset can't be manipulated by third party seems like a valid interpretation.
Simply removing it would allow breaking workqueue from userland by manipulating affinity. How about testing PF_WQ_WORKER in set_cpus_allowed_ptr() (and maybe cpuset, I'm not sure)?
Thanks.
-- tejun
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