linux-kernel - Re: [PATCH] sched: fix clear NOHZ_BALANCE

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Date:	Tue, 4 Jun 2013 13:15:10 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] sched: fix clear NOHZ_BALANCE_KICK

On Tue, Jun 04, 2013 at 12:26:22PM +0200, Frederic Weisbecker wrote:
> On Tue, Jun 04, 2013 at 11:36:11AM +0200, Peter Zijlstra wrote:
> > 
> > The best I can seem to come up with is something like the below; but I think
> > its ghastly. Surely we can do something saner with that bit.
> > 
> > Having to clear it at 3 different places is just wrong.
> 
> We could clear the flag early in scheduler_ipi() and set some
> specific value in rq->idle_balance that tells we want nohz idle
> balancing from the softirq, something like this untested:

Yeah, I suppose something like that is a little better.. a few nits
though:

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 58453b8..330136b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -630,15 +630,14 @@ void wake_up_nohz_cpu(int cpu)
>  		wake_up_idle_cpu(cpu);
>  }
>  
> -static inline bool got_nohz_idle_kick(void)
> +static inline bool got_nohz_idle_kick(int cpu)
>  {
> -	int cpu = smp_processor_id();
> -	return idle_cpu(cpu) && test_bit(NOHZ_BALANCE_KICK, nohz_flags(cpu));
> +	return test_and_clear_bit(NOHZ_BALANCE_KICK, nohz_flags(cpu));
>  }
>  
>  #else /* CONFIG_NO_HZ_COMMON */
>  
> -static inline bool got_nohz_idle_kick(void)
> +static inline bool got_nohz_idle_kick(int cpu)
>  {
>  	return false;
>  }
> @@ -1393,8 +1392,12 @@ static void sched_ttwu_pending(void)
>  
>  void scheduler_ipi(void)
>  {
> -	if (llist_empty(&this_rq()->wake_list) && !got_nohz_idle_kick()
> -	    && !tick_nohz_full_cpu(smp_processor_id()))
> +	int cpu = smp_processor_id();
> +	bool idle_kick = got_nohz_idle_kick(cpu);

This puts an unconditional atomic instruction in the IPI path. 
 if (test) clear();
is lots cheaper, esp. since most IPIs won't have this flag set.

> +
> +	if (!(idle_kick && idle_cpu(cpu))
> +	    && llist_empty(&this_rq()->wake_list)
> +	    && !tick_nohz_full_cpu(cpu)

What's with this weird operator first split style?

>  		return;
>  
>  	/*
  
> +enum idle_balance_type {
> +	IDLE_BALANCE = 1,
> +	IDLE_NOHZ_BALANCE = 2,
> +};

You might want to update the rq->idle_balance assignment in
scheduler_tick() to make sure it uses the right value (it does now, but
there's nothing stopping people from changing the values).
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