Re: [RFC PATCH 2/2] sched/fair: skip the cache hot CPU in select_idle_cpu()

[Chen Yu]

Hi Gautham,

Sorry for late reply,

On 2023-09-15 at 20:48:14 +0530, Gautham R. Shenoy wrote:
> Hello Chen Yu,
> 
> On Thu, Sep 14, 2023 at 08:09:26PM +0800, Chen Yu wrote:
> [..snip..]
> 
> > > 
> > > So despite "reserving" the CPU for p1, which is likely to have its
> > > data still hot in the case, we would have scheduled p1', thus
> > > defeating the whole purpose of reservation.
> > > 
> > 
> > I see. So you mean, although we reserve the CPU for the wakee,
> > the wakee might not choose its previous CPU, which is against our
> > goal.
> 
> 
> Yes, but only because some other task could have run on the previous
> CPU. That other task could be something that was woken up on that CPU
> due to:
> 
> 1) wake-affine choosing that CPU 
> 2) newidle-balance pulling the other task on that CPU
> 3) !wake-affine && that CPU was also the other task's previous CPU
> 
> It could also be due to this wakee task being woken up on the waker
> CPU due to wake-affine.
> 
> > 
> > The reason to prevent the wakee choosing its previous CPU could be:
> > 1. wake_affine() choose the waker's CPU rather the wakee's previous CPU, or
> > 2. the wakee's CPU has already been taken by someone else, via newidle_balance().
> >
> 
> 
> > For 1, I think Prateek has expressed the concern. One mitigation method could be
> > that, we give penalty to that wakee, if it decides not to choose its previous CPU:
> 
> We would be penalizing the task for something that the scheduler
> decides :-)
> 
> As you point out below, in the presence of the WF_SYNC flag,
> wake_affine_idle() prefer the waker CPU over the previous CPU when
> they are on different LLCs and when the waker is the only task.
> 
> This strategy makes sense for two reasons:
> 
> 1) The wakee may be consuming the data produced by the waker.
> 2) Since the wakeup will happen on the local CPU, there is no risk of
>    task-stacking, exactly what your SIS_CURRENT patchset was
>    attempting.
> 
> But this strategy would also result in increased task-migration. Which
> both Mattieu and you have found is not so beneficial for workloads
> such as hackbench. Is it only because task's data is still hot in the
> previous CPU's cache ? Or is there more to it ?
> 
> 
> It would be good to confirm if this is why lower migration is better
> for these kinds of workloads.
>

Right. According to the previous hackbench test for shared runqueue, higher
migration brings higher DSB miss rate [1]. I'll collect some statistics with
this patch applied to confirm.

https://lore.kernel.org/lkml/ZO7e5YaS71cXVxQN@chenyu5-mobl2/
> > 
> > "
> > new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
> > if (new_cpu != prev_cpu)
> > 	p->burst_sleep_avg >>= 1;
> > So the duration of reservation could be shrinked.
> > "
> > 
> > For 2, maybe inhit the newidle balance, something in my mind:
> > 
> > 
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -12022,6 +12022,7 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> >  	u64 t0, t1, curr_cost = 0;
> >  	struct sched_domain *sd;
> >  	int pulled_task = 0;
> > +	bool cache_hot = false;
> >  
> >  	update_misfit_status(NULL, this_rq);
> >  
> > @@ -12055,8 +12056,19 @@ static int newidle_balance(struct rq *this_rq, struct rq_flags *rf)
> >  	rcu_read_lock();
> >  	sd = rcu_dereference_check_sched_domain(this_rq->sd);
> >  
> > +	if (sched_feat(SIS_CACHE)) {
> > +		s64 delta = this_rq->cache_hot_timeout - sched_clock_cpu(this_cpu);
> > +
> > +		/*
> > +		 * If a short time later, a short sleeping task will be woken up
> > +		 * on this idle CPU, do not launch the newidle balance.
> > +		 */
> > +		if (delta > 0 && delta < this_rq->max_idle_balance_cost)
> > +			cache_hot = true;
> > +	}
> > +
> >  	if (!READ_ONCE(this_rq->rd->overload) ||
> > -	    (sd && this_rq->avg_idle < sd->max_newidle_lb_cost)) {
> > +	    (sd && this_rq->avg_idle < sd->max_newidle_lb_cost) || cache_hot) {
> 
> >  
> >  		if (sd)
> >  			update_next_balance(sd, &next_balance);
> 
> If the benefit that the workload obtains is really due to the data
> being hot near its previous CPU, then this seems a sensible thing to
> do.
> 
> It would be good to confirm this. Let me get some IBS data for
> hackbench which is the workload which likes a sticky wakeup.
> 

I'll collect the statistics too. Thanks for your time.

thanks,
Chenyu