Messages in this thread | | | Date | Mon, 8 Dec 2014 21:18:59 +1100 | From | Anton Blanchard <> | Subject | Re: [PATCH] kthread: kthread_bind fails to enforce CPU affinity (fixes kernel BUG at kernel/smpboot.c:134!) |
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Hi Ingo,
> So we cannot call set_task_cpu() because in the normal life time > of a task the ->cpu value gets set on wakeup. So if a task is > blocked right now, and its affinity changes, it ought to get a > correct ->cpu selected on wakeup. The affinity mask and the > current value of ->cpu getting out of sync is thus 'normal'. > > (Check for example how set_cpus_allowed_ptr() works: we first set > the new allowed mask, then do we migrate the task away if > necessary.) > > In the kthread_bind() case this is explicitly assumed: it only > calls do_set_cpus_allowed(). > > But obviously the bug triggers in kernel/smpboot.c, and that > assert shows a real bug - and your patch makes the assert go > away, so the question is, how did the kthread get woken up and > put on a runqueue without its ->cpu getting set?
I started going down this line earlier today, and found things like:
select_task_rq_fair:
if (p->nr_cpus_allowed == 1) return prev_cpu;
I tried returning cpumask_first(tsk_cpus_allowed()) instead, and while I couldn't hit the BUG I did manage to get a scheduler lockup during testing.
At that point I thought the previous task_cpu() was somewhat ingrained in the scheduler and came up with the patch. If not, we could go on a hunt to see what else needs fixing.
Anton
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