Re: [RFC] Extend mwait idle to optimize away IPIs when possible

[Yong Zhang <yong.zhang0@gmail.com>]

On Wed, Feb 08, 2012 at 03:28:45PM -0800, Venki Pallipadi wrote:
> On Tue, Feb 7, 2012 at 10:51 PM, Yong Zhang <yong.zhang0@gmail.com> wrote:
> > On Mon, Feb 06, 2012 at 12:42:13PM -0800, Venkatesh Pallipadi wrote:
> >> smp_call_function_single and ttwu_queue_remote sends unconditional IPI
> >> to target CPU. However, if the target CPU is in mwait based idle, we can
> >> do IPI-less wakeups using the magical powers of monitor-mwait.
> >> Doing this has certain advantages:
> >
> > Actually I'm trying to do the similar thing on MIPS.
> >
> > The difference is that I want task_is_polling() to do something. The basic
> > idea is:
> >
> >> + ? ? ? ? ? ? ? ? ? ? if (ipi_pending()) {
> >> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? clear_ipi_pending();
> >> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? local_bh_disable();
> >> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? local_irq_disable();
> >> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? generic_smp_call_function_single_interrupt();
> >> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? scheduler_wakeup_self_check();
> >> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? local_irq_enable();
> >> + ? ? ? ? ? ? ? ? ? ? ? ? ? ? local_bh_enable();
> >
> > I let cpu_idle() check if there is anything to do as your above code.
> >
> > And task_is_polling() handle the others with below patch:
> > ---
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 5255c9d..09f633d 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -527,15 +527,16 @@ void resched_task(struct task_struct *p)
> > ? ? ? ? ? ? ? ?smp_send_reschedule(cpu);
> > ?}
> >
> > -void resched_cpu(int cpu)
> > +int resched_cpu(int cpu)
> > ?{
> > ? ? ? ?struct rq *rq = cpu_rq(cpu);
> > ? ? ? ?unsigned long flags;
> >
> > ? ? ? ?if (!raw_spin_trylock_irqsave(&rq->lock, flags))
> > - ? ? ? ? ? ? ? return;
> > + ? ? ? ? ? ? ? return 0;
> > ? ? ? ?resched_task(cpu_curr(cpu));
> > ? ? ? ?raw_spin_unlock_irqrestore(&rq->lock, flags);
> > + ? ? ? return 1;
> > ?}
> 

I assume we are talking about 'return from idle' but seems I don't
make it clear.

> Two points -
> rq->lock: I tried something similar first. One hurdle with checking
> task_is_polling() is that you need rq->lock to check it. And adding
> lock+unlock (without wait) in wakeup path ended up being no net gain
> compared to IPI. And when we actually end up spinning on that lock,
> thats going to add overhead in the common path. That is the reason I
> switched to atomic compare exchange and moving any wait onto the
> target CPU coming out of idle.

I see. But actually we will not spinning on that lock because we
use 'trylock' in resched_cpu(). And you are right there is indeed a
little overhead (resched_task()) if we hold the lock but it can be
tolerated IMHO.

BTW, mind showing you test case thus we can collect some common data?

> 
> resched_task: ttwu_queue_remote() does not imply that the remote CPU
> will do a resched. Today there is a IPI and IPI handler calls onto
> check_preempt_wakeup() and if the current task has higher precedence
> than the waking up task, then there will be just an activation of new
> task and no resched. Using resched_task above breaks
> check_preempt_wakeup() and always calls a resched on remote CPU after
> the IPI, which would be change in behavior.

Yeah, if the remote cpu is not idle, mine will change the behavior; but
if the remote cpu is idle, it will always rescheduled, right?

So maybe we could introduce resched_idle_cpu() to make things more clear:

int resched_idle_cpu(int cpu) 
{
        struct rq *rq = cpu_rq(cpu);
        unsigned long flags;
        int ret = 0; 

        if (!raw_spin_trylock_irqsave(&rq->lock, flags))
                goto out; 
        if (!idle_cpu(cpu))
                goto out_unlock;
        resched_task(cpu_curr(cpu));
                ret = 1; 
out_unlock:
        raw_spin_unlock_irqrestore(&rq->lock, flags);
out:
        return ret; 
}

Thanks,
Yong

> 
> Thanks,
> Venki
> 
> >
> > ?#ifdef CONFIG_NO_HZ
> > @@ -1484,7 +1485,8 @@ 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))
> > + ? ? ? if (llist_add(&p->wake_entry, &cpu_rq(cpu)->wake_list) &&
> > + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? !resched_cpu(cpu))
> > ? ? ? ? ? ? ? ?smp_send_reschedule(cpu);
> > ?}
> >
> > Thought?
> >
> > Thanks,
> > Yong
> --
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