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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