Re: [PATCH] cpu hotplug, sched: Introduce cpu_active_map and redo sched domain managment (take 2)

From: Dmitry Adamushko
Date: Wed Jul 16 2008 - 17:55:41 EST

2008/7/16 Max Krasnyansky <maxk@xxxxxxxxxxxx>:
> Dmitry Adamushko wrote:
>> 2008/7/15 Max Krasnyansky <maxk@xxxxxxxxxxxx>:
>>> From: Max Krasnyanskiy <maxk@xxxxxxxxxxxx>
>>> Addressed Ingo's comments and merged on top of latest Linus's tree.
>> a few remarks:
>> (1) in __migrate_task(), a test for !cpu_active(dest_cpu) should be
>> done after double_rq_lock() [ or add the second one ]
>> migration_thread() calls __migrate_task() with disabled interrupts (no
>> rq-locks held), i.e. if we merely rely on rq-locks for
>> synchronization, this can race with cpu_down(dest_cpu).
>> [ assume, the test was done in __migration_task() and it's about to
>> take double_lock()... and at this time, down_cpu(dest_cpu) starts and
>> completes on another CPU ]
>> note, we may still take the rq-lock for a "dead" cpu in this case and
>> then only do a check (remark: in fact, not with stop_machine() in
>> place _but_ I consider that we don't make any assumptions on its
>> behavior);
> Hmm, as you suggested I added synchronize_sched() after clearing the active
> bit (see below). Is that not nought enough ? I mean you mentioned that
> stop_machine syncs things up, I assume synchronize_sched does too.

Yes, sorry for the noise here.

* synchronize_sched - block until all CPUs have exited any non-preemptive
* kernel code sequences

so "any non-preemptive" sections, not just the ones with run-queue
locks being held.

>> (2) it's worth to take a look at the use of any_online_cpu():
>> many places are ok, because there will be an additional check against
>> cpu_active_mask later on, but e.g.
>> set_cpus_allowed_ptr() ->
>> migrate_task(p, any_online_cpu(mask), ...) ->
>> migrate_task(p, dest_cpu)
>> doesn't seem to have any verifications wrt cpu_active_map.
> How about we just introduce any_active_cpu() and replace all the usages of
> any_online_cpu() in the scheduler ?

I think, at least for places related to task placement (like
migrate_task(..., any_online_cpu()) it would make sense,

> Max

Best regards,
Dmitry Adamushko
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