Re: [PATCH RFC] reduce runqueue lock contention

[Frank Rowand <frank.rowand@am.sony.com>]

On 12/13/10 19:42, Mike Galbraith wrote:
> On Mon, 2010-12-13 at 18:41 -0800, Frank Rowand wrote:
>> On 06/21/10 06:04, Peter Zijlstra wrote:
>>> On Mon, 2010-06-21 at 12:54 +0200, Peter Zijlstra wrote:
>>>>> It looses the ttwu task_running() check, as I must admit I'm not quite
>>>>> sure what it does.. Ingo?
>>>
>>> I think I figured out what its for, its for when p is prev in schedule()
>>> after deactivate_task(), we have to call activate_task() it again, but
>>> we cannot migrate the task because the CPU its on is still referencing
>>> it.
>>
>> I have not been able to make sense of the task_running() check in
>> try_to_wake_up(), even with that clue.  The try_to_wake_up() code in
>> question is:
>>
>>         rq = task_rq_lock(p, &flags);
>>         if (!(p->state & state))
>>                 goto out;
>>
>>         if (p->se.on_rq)
>>                 goto out_running;
>>
>>         cpu = task_cpu(p);
>>         orig_cpu = cpu;
>>
>> #ifdef CONFIG_SMP
>>         if (unlikely(task_running(rq, p)))
>>                 goto out_activate;
>>
>>
>> The relevent code in schedule() executes with the rq lock held (many
>> lines left out to emphasize the key lines):

Additional lines added here to show the function call that Mike pointed out:

>>
>>         raw_spin_lock_irq(&rq->lock);
>>
>>         if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
>>
>>                        deactivate_task(rq, prev, DEQUEUE_SLEEP);

           }
           pre_schedule(rq, prev);
           if (unlikely(!rq->nr_running))
                   idle_balance(cpu, rq);

>>
>>         if (likely(prev != next)) {
>>                 rq->curr = next;
>>                 context_switch(rq, prev, next); /* unlocks the rq */
>>         } else
>>                 raw_spin_unlock_irq(&rq->lock);
>>
>>
>> If (p->se.on_rq) can becomes false due to deactivate_task()
>> then task_running() will also become false while the rq lock is still
>> held (either via "rq->curr = next" or via context_switch() updating
>> p->oncpu -- which one matters depends on #ifdef __ARCH_WANT_UNLOCKED_CTXSW).
>>
>> I  haven't been able to find any case where task_running() can be true
>> when (p->se.on_rq) is false, while the rq lock is not being held.  Thus
>> I don't think the branch to out_activate will ever be taken.
>>
>> What am I missing, or is the task_running() test not needed?
> 
> Say the last runnable task passes through schedule(), is deactivated.
> We hit idle_balance(), which drops/retakes rq->lock _before_ the task
> schedules off.  ttwu() can acquire rq->lock in that window, p->se.on_rq
> is false, p->state is true, as is task_current(rq, p).
> 
> We have to check whether the task is still current, but not enqueued,
> lest the wakeup be a noop, and the wakee possibly then sleep forever.

Thanks Mike, that's just the cluebat I needed!

And for the lkml archives, in case anyone has this question again in the
future, with Mike's clue in hand I found another case in this window where
the rq->lock can be dropped then reacquired.  Just before idle_balance()
is called, pre_schedule() is called:

pre_schedule()
   prev->sched_class->pre_schedule(rq, prev)
   [pre_schedule_rt()]
      pull_rt_task(rq)
      pull_rt_task[this_rq]
         for_each_cpu(cpu, this_rq->rd->rto_mask)
            double_lock_balance(this_rq, src_rq)
               raw_spin_unlock(&this_rq->lock)        <-----
               double_rq_lock(this_rq, busiest)

-Frank