Re: [PATCH v2 09/22] sched/fair: Use IPC class score to select a busiest runqueue

[Ricardo Neri]

On Thu, Dec 08, 2022 at 08:51:03AM +0000, Ionela Voinescu wrote:
> Hi,
> 
> On Monday 28 Nov 2022 at 05:20:47 (-0800), Ricardo Neri wrote:
> > For two runqueues of equal priority and equal number of running of tasks,
> > select the one whose current task would have the highest IPC class score
> > if placed on the destination CPU.
> > 
> [..]
> > +static int ipcc_score_delta(struct task_struct *p, int alt_cpu)
> > +{
> > +	unsigned long ipcc = p->ipcc;
> > +
> > +	if (!sched_ipcc_enabled())
> > +		return INT_MIN;
> > +
> > +	return arch_get_ipcc_score(ipcc, alt_cpu) -
> > +	       arch_get_ipcc_score(ipcc, task_cpu(p));
> 
> Nit: arch_get_ipcc_score() return values are never checked for error.

Fair point. I will handle error values.

> 
> > +}
> > +
> >  #else /* CONFIG_IPC_CLASSES */
> >  static void update_sg_lb_ipcc_stats(struct sg_lb_ipcc_stats *sgcs,
> >  				    struct rq *rq)
> > @@ -9258,6 +9276,11 @@ static bool sched_asym_ipcc_pick(struct sched_group *a,
> >  	return false;
> >  }
> >  
> > +static int ipcc_score_delta(struct task_struct *p, int alt_cpu)
> > +{
> > +	return INT_MIN;
> > +}
> > +
> >  #endif /* CONFIG_IPC_CLASSES */
> >  
> >  /**
> > @@ -10419,8 +10442,8 @@ static struct rq *find_busiest_queue(struct lb_env *env,
> >  {
> >  	struct rq *busiest = NULL, *rq;
> >  	unsigned long busiest_util = 0, busiest_load = 0, busiest_capacity = 1;
> > +	int i, busiest_ipcc_delta = INT_MIN;
> >  	unsigned int busiest_nr = 0;
> > -	int i;
> >  
> >  	for_each_cpu_and(i, sched_group_span(group), env->cpus) {
> >  		unsigned long capacity, load, util;
> > @@ -10526,8 +10549,37 @@ static struct rq *find_busiest_queue(struct lb_env *env,
> >  
> >  		case migrate_task:
> >  			if (busiest_nr < nr_running) {
> > +				struct task_struct *curr;
> > +
> >  				busiest_nr = nr_running;
> >  				busiest = rq;
> > +
> > +				/*
> > +				 * Remember the IPC score delta of busiest::curr.
> > +				 * We may need it to break a tie with other queues
> > +				 * with equal nr_running.
> > +				 */
> > +				curr = rcu_dereference(busiest->curr);
> > +				busiest_ipcc_delta = ipcc_score_delta(curr,
> > +								      env->dst_cpu);
> > +			/*
> > +			 * If rq and busiest have the same number of running
> > +			 * tasks, pick rq if doing so would give rq::curr a
> > +			 * bigger IPC boost on dst_cpu.
> > +			 */
> > +			} else if (sched_ipcc_enabled() &&
> > +				   busiest_nr == nr_running) {
> > +				struct task_struct *curr;
> > +				int delta;
> > +
> > +				curr = rcu_dereference(rq->curr);
> > +				delta = ipcc_score_delta(curr, env->dst_cpu);
> > +
> > +				if (busiest_ipcc_delta < delta) {
> > +					busiest_ipcc_delta = delta;
> > +					busiest_nr = nr_running;
> > +					busiest = rq;
> > +				}
> >  			}
> >  			break;
> >  
> 
> While in the commit message you describe this as breaking a tie for
> asym_packing,

Are you referring to the overall series or this specific patch? I checked
commit message and I do not see references to asym_packing.

> the code here does not only affect asym_packing. If
> another architecture would have sched_ipcc_enabled() it would use this
> as generic policy, and that might not be desired.

Indeed, the patchset implements support to use IPCC classes for asym_packing,
but it is not limited to it.

It is true that I don't check here for asym_packing, but it should not be a
problem, IMO. I compare two runqueues with equal nr_running, either runqueue
is a good choice. This tie breaker is an overall improvement, no?

Thanks and BR,
Ricardo