Re: [PATCH v2 1/3] sched: Stop nohz stats when decayed

[Will Deacon <will.deacon@arm.com>]

On Thu, Feb 08, 2018 at 03:00:05PM +0100, Peter Zijlstra wrote:
> On Tue, Feb 06, 2018 at 08:23:05PM +0100, Vincent Guittot wrote:
> > @@ -9222,6 +9248,13 @@ void nohz_balance_enter_idle(int cpu)
> >  	atomic_inc(&nohz.nr_cpus);
> >  
> >  	set_cpu_sd_state_idle(cpu);
> 
> 	/*
> 	 * Ensures that if nohz_idle_balance() fails to observe our
> 	 * @idle_cpus_mask store, it must observe the @has_blocked
> 	 * store.
> 	 */
> 	smp_mb__after_atomic();
> 
> > +
> > +out:
> > +	/*
> > +	 * Each time a cpu enter idle, we assume that it has blocked load and
> > +	 * enable the periodic update of the load of idle cpus
> > +	 */
> > +	WRITE_ONCE(nohz.has_blocked, 1);
> >  }
> 
> 
> 
> > @@ -9374,6 +9407,16 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
> >  
> >  	SCHED_WARN_ON((flags & NOHZ_KICK_MASK) == NOHZ_BALANCE_KICK);
> >  
> > +	/*
> > +	 * We assume there will be no idle load after this update and clear
> > +	 * the has_blocked flag. If a cpu enters idle in the mean time, it will
> > +	 * set the has_blocked flag and trig another update of idle load.
> > +	 * Because a cpu that becomes idle, is added to idle_cpus_mask before
> > +	 * setting the flag, we are sure to not clear the state and not
> > +	 * check the load of an idle cpu.
> > +	 */
> > +	WRITE_ONCE(nohz.has_blocked, 0);
> 
> 	/*
> 	 * Ensures that if we miss the CPU, we must see the has_blocked
> 	 * store from nohz_balance_enter_idle().
> 	 */
> 	smp_mb();
> 
> >  	for_each_cpu(balance_cpu, nohz.idle_cpus_mask) {
> >  		if (balance_cpu == this_cpu || !idle_cpu(balance_cpu))
> >  			continue;
> 
> 
> I _think_, but my brain isn't quite willing to turn on today.
> 
> Without this ordering I think it would be possible to loose has_blocked
> and not observe the CPU either.

I had a quick look at this, and I think you're right. This looks very much
like an 'R'-shaped test, which means it's smp_mb() all round otherwise Power
will go wrong. That also means the smp_mb__after_atomic() in
nohz_balance_enter_idle *cannot* be an smp_wmb(), so you might want a
comment stating that explicitly.

On arm64, release/acquire would work, but that's basically not the case for
anybody else including x86, so let's not go there.

Will