Re: [patch] sched: beef up wake_wide()

[Josef Bacik]

On 07/10/2015 01:19 AM, Mike Galbraith wrote:
> On Thu, 2015-07-09 at 15:26 +0200, Peter Zijlstra wrote:
> > On Wed, Jul 08, 2015 at 08:13:46AM +0200, Mike Galbraith wrote:
> > >   static int wake_wide(struct task_struct *p)
> > >   {
> > > +	unsigned int waker_flips = current->wakee_flips;
> > > +	unsigned int wakee_flips = p->wakee_flips;
> > >   	int factor = this_cpu_read(sd_llc_size);
> > >
> > > +	if (waker_flips < wakee_flips)
> > > +		swap(waker_flips, wakee_flips);
> >
> > This makes the wakee/waker names useless, the end result is more like
> > wakee_flips := client_flips, waker_flips := server_flips.
>
> I settled on master/slave plus hopefully improved comment block.
>
> > > +	if (wakee_flips < factor || waker_flips < wakee_flips * factor)
> > > +		return 0;
> >
> > I don't get the first condition... why would the client ever flip? It
> > only talks to that one server.
>
> (tightening heuristic up a bit by one means or another would be good,
> but "if it ain't broke, don't fix it" applies for this patchlet)
>
> > > @@ -5021,14 +5015,17 @@ select_task_rq_fair(struct task_struct *
> > >   {
> > >   	struct sched_domain *tmp, *affine_sd = NULL, *sd = NULL;
> > >   	int cpu = smp_processor_id();
> > > +	int new_cpu = prev_cpu;
> > >   	int want_affine = 0;
> > >   	int sync = wake_flags & WF_SYNC;
> > >
> > >   	rcu_read_lock();
> > > +	if (sd_flag & SD_BALANCE_WAKE) {
> > > +		want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, tsk_cpus_allowed(p));
> > > +		if (!want_affine)
> > > +			goto select_idle;
> > > +	}
> >
> > So this preserves/makes worse the bug Morten spotted, even without
> > want_affine we should still attempt SD_BALANCE_WAKE if set.
>
> Fixed.  wake_wide() may override want_affine as before, want_affine may
> override other ->flags as before, but a surviving domain selection now
> results in a full balance instead of a select_idle_sibling() call.
>
> sched: beef up wake_wide()
>
> Josef Bacik reported that Facebook sees better performance with their
> 1:N load (1 dispatch/node, N workers/node) when carrying an old patch
> to try very hard to wake to an idle CPU.  While looking at wake_wide(),
> I noticed that it doesn't pay attention to the wakeup of a many partner
> waker, returning 1 only when waking one of its many partners.
>
> Correct that, letting explicit domain flags override the heuristic.
>
> While at it, adjust task_struct bits, we don't need a 64bit counter.
>
> Signed-off-by: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>
> Tested-by: Josef Bacik <jbacik@xxxxxx>

Not quite as awesome but still better than the baseline so we're good. 
Thanks,

Josef

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