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

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Date:	Tue, 3 Jun 2014 09:52:22 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>, nicolas.pitre@...aro.org,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 0/8] sched,idle: need resched polling rework

On Tue, Jun 3, 2014 at 9:19 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, Jun 03, 2014 at 09:05:03AM -0700, Andy Lutomirski wrote:
>> On Tue, Jun 3, 2014 at 7:02 AM, Peter Zijlstra <peterz@...radead.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?
>
> Hysterical raisins, I don't think there's an actual reason, so yes, that
> might be the best option indeed.
>
>> 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.
>
> So you could cheat and set it in pick_next_task_idle() and clear in
> put_prev_task_idle(), that way the entire idle loop, when running has it
> set.
>

Isn't that a little late for sched_ttwu_pending?  I guess it could be
okay, but I'm hesitant to muck around with the scheduler innards that
much.  I don't see anything that'll break, though.

I'm seriously confused by the polling situation, though.
TIF_NRFLAG_POLLING is defined for a whole bunch of architectures, but
I can't find any actual polling idle code outside cpuidle and x86.
For example, arm defines TIF_POLLING_NRFLAG, but I can't find anything
that clears the polling bit in the arm code.  Am I missing something
obvious here?  Is the whole point of this so that a wakeup that
happens right after the idle task is scheduled but before it goes idle
cancels idle and avoids an IPI?  This seems like a lot of complexity
to avoid IPIs in a hopefully extremely narrow window.

This is totally backwards for x86, but it seems to do the right thing
for other architectures.  I'm not convinced it does any good, though.

--Andy

> And then there was the idle injection loop crap trainwreck, which I
> should send patches for..



-- 
Andy Lutomirski
AMA Capital Management, LLC
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