Re: [RFC][PATCH 0/8] sched,idle: need resched polling rework

[Andy Lutomirski <luto@amacapital.net>]

On Tue, Jun 3, 2014 at 7:02 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, Jun 03, 2014 at 12:43:47PM +0200, Peter Zijlstra wrote:
>> We need rq->curr, rq->idle 'sleeps' with polling set and nr clear, but
>> it obviously has no effect setting that if its not actually the current
>> task.
>>
>> Touching rq->curr needs holding rcu_read_lock() though, to make sure the
>> task stays around, still shouldn't be a problem.
>
>> @@ -1581,8 +1604,14 @@ void scheduler_ipi(void)
>>
>>  static void ttwu_queue_remote(struct task_struct *p, int cpu)
>>  {
>> -     if (llist_add(&p->wake_entry, &cpu_rq(cpu)->wake_list))
>> -             smp_send_reschedule(cpu);
>> +     struct rq *rq = cpu_rq(cpu);
>> +
>> +     if (llist_add(&p->wake_entry, &rq->wake_list)) {
>> +             rcu_read_lock();
>> +             if (!set_nr_if_polling(rq->curr))
>> +                     smp_send_reschedule(cpu);
>> +             rcu_read_unlock();
>> +     }
>>  }
>
> Hrmm, I think that is still broken, see how in schedule() we clear NR
> before setting the new ->curr.
>
> So I think I had a loop on rq->curr the last time we talked about this,
> but alternatively we could look at clearing NR after setting a new curr.
>
> I think I once looked at why it was done before, of course I can't
> actually remember the details :/

Wouldn't this be a little simpler and maybe even faster if we just
changed the idle loop to make TIF_POLLING_NRFLAG be a real indication
that the idle task is running and actively polling?  That is, change
the end of cpuidle_idle_loop to:

                preempt_set_need_resched();
                tick_nohz_idle_exit();
                clear_tsk_need_resched(current);
                __current_clr_polling();
                smp_mb__after_clear_bit();
                WARN_ON_ONCE(test_thread_flag(TIF_POLLING_NRFLAG));
                sched_ttwu_pending();
                schedule_preempt_disabled();
                __current_set_polling();

This has the added benefit that the optimistic version of the cmpxchg
loop would be safe again.  I'm about to test this with this variant.
I'll try and send a comprehensible set of patches in a few hours.

Can you remind me what the benefit was of letting polling be set when
the idle thread schedules?  It seems racy to me: it probably prevents
any safe use of the polling bit without holding the rq lock.  I guess
there's some benefit to having polling be set for as long as possible,
but it only helps if there are wakeups in very rapid succession, and
it costs a couple of extra bit ops per idle entry.

-- 
Andy Lutomirski
AMA Capital Management, LLC