Re: [RFC PATCHC 3/3] sched/fair: use the idle state info to choose the idlest cpu

[Nicolas Pitre]

On Fri, 4 Apr 2014, Rafael J. Wysocki wrote:

> On Tuesday, April 01, 2014 11:05:49 PM Nicolas Pitre wrote:
> > On Fri, 28 Mar 2014, Daniel Lezcano wrote:
> > 
> > > As we know in which idle state the cpu is, we can investigate the following:
> > > 
> > > 1. when did the cpu entered the idle state ? the longer the cpu is idle, the
> > > deeper it is idle
> > > 2. what exit latency is ? the greater the exit latency is, the deeper it is
> > > 
> > > With both information, when all cpus are idle, we can choose the idlest cpu.
> > > 
> > > When one cpu is not idle, the old check against weighted load applies.
> > > 
> > > Signed-off-by: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> > 
> > There seems to be some problems with the implementation.
> > 
> > > @@ -4336,20 +4337,53 @@ static int
> > >  find_idlest_cpu(struct sched_group *group, struct task_struct *p, int this_cpu)
> > >  {
> > >  	unsigned long load, min_load = ULONG_MAX;
> > > -	int idlest = -1;
> > > +	unsigned int min_exit_latency = UINT_MAX;
> > > +	u64 idle_stamp, min_idle_stamp = ULONG_MAX;
> > 
> > I don't think you really meant to assign an u64 variable with ULONG_MAX.
> > You probably want ULLONG_MAX here.  And probably not in fact (more 
> > later).
> > 
> > > +
> > > +	struct rq *rq;
> > > +	struct cpuidle_power *power;
> > > +
> > > +	int cpu_idle = -1;
> > > +	int cpu_busy = -1;
> > >  	int i;
> > >  
> > >  	/* Traverse only the allowed CPUs */
> > >  	for_each_cpu_and(i, sched_group_cpus(group), tsk_cpus_allowed(p)) {
> > > -		load = weighted_cpuload(i);
> > >  
> > > -		if (load < min_load || (load == min_load && i == this_cpu)) {
> > > -			min_load = load;
> > > -			idlest = i;
> > > +		if (idle_cpu(i)) {
> > > +
> > > +			rq = cpu_rq(i);
> > > +			power = rq->power;
> > > +			idle_stamp = rq->idle_stamp;
> > > +
> > > +			/* The cpu is idle since a shorter time */
> > > +			if (idle_stamp < min_idle_stamp) {
> > > +				min_idle_stamp = idle_stamp;
> > > +				cpu_idle = i;
> > > +				continue;
> > 
> > Don't you want the highest time stamp in order to select the most 
> > recently idled CPU?  Favoring the CPU which has been idle the longest 
> > makes little sense.
> 
> It may make sense if the hardware can auto-promote CPUs to deeper C-states.

If so the promotion will happen over time, no?  What I'm saying here is 
that those CPUs which have been idle longer should not be favored when 
it is time to select a CPU for a task to run. More recently idled CPUs 
are more likely to be in a shallower C-state.

> Something like that happens with package C-states that are only entered when
> all cores have entered a particular core C-state already.  In that case the
> probability of the core being in a deeper state grows with time.

Exactly what I'm saying.

Also here it is worth remembering that the scheduling domains should 
represent those packages that share common C-states at a higher level.  
The scheduler can then be told not to balance across domains if it 
doesn't need to in order to favor the conditions for those package 
C-states to be used.  That's what the task packing patch series is 
about, independently of this one.

> That said I would just drop this heuristics for the time being.  If auto-promotion
> is disregarded, it doesn't really matter how much time the given CPU has been idle
> except for one case: When the target residency of its idle state hasn't been
> reached yet, waking up the CPU may be a mistake (depending on how deep the state
> actually is, but for the majority of drivers in the tree we don't have any measure
> of that).

There is one reason for considering the time a CPU has been idle, 
assuming equivalent C-state, and that is cache snooping.  The longer a 
CPU is idle, the more likely its cache content will have been claimed 
and migrated by other CPUs.  Of course that doesn't make much difference 
for deeper C-states where the cache isn't preserved, but it is probably 
simpler and cheaper to apply this heuristic in all cases.

> > > +			}
> > > +
> > > +			/* The cpu is idle but the exit_latency is shorter */
> > > +			if (power && power->exit_latency < min_exit_latency) {
> > > +				min_exit_latency = power->exit_latency;
> > > +				cpu_idle = i;
> > > +				continue;
> > > +			}
> > 
> > I think this is wrong.  This gives priority to CPUs which have been idle 
> > for a (longer... although this should have been) shorter period of time 
> > over those with a shallower idle state.  I think this should rather be:
> > 
> > 	if (power && power->exit_latency < min_exit_latency) {
> > 		min_exit_latency = power->exit_latency;
> > 		latest_idle_stamp = idle_stamp;
> > 	       	cpu_idle = i;
> > 	} else if ((!power || power->exit_latency == min_exit_latency) &&
> > 		   idle_stamp > latest_idle_stamp) {
> > 		latest_idle_stamp = idle_stamp;
> > 		cpu_idle = i;
> > 	}
> > 
> > So the CPU with the shallowest idle state is selected in priority, and 
> > if many CPUs are in the same state then the time stamp is used to 
> > select the most recent one.
> 
> Again, if auto-promotion is disregarded, it doesn't really matter which of them
> is woken up.

If it doesn't matter then it doesn't hurt.  But in some cases it 
matters.

> > Whenever a shallower idle state is found then the latest_idle_stamp is reset for 
> > that state even if it is further in the past.
> > 
> > > +		} else {
> > > +
> > > +			load = weighted_cpuload(i);
> > > +
> > > +			if (load < min_load ||
> > > +			    (load == min_load && i == this_cpu)) {
> > > +				min_load = load;
> > > +				cpu_busy = i;
> > > +				continue;
> > > +			}
> > >  		}
> > 
> > I think this is wrong to do an if-else based on idle_cpu() here.  What 
> > if a CPU is heavily loaded, but for some reason it happens to be idle at 
> > this very moment?  With your patch it could be selected as an idle CPU 
> > while it would be discarded as being too busy otherwise.
> 
> But see below ->
> 
> > It is important to determine both cpu_busy and cpu_idle for all CPUs.
> > 
> > And cpu_busy is a bad name for this.  Something like least_loaded would 
> > be more self explanatory.  Same thing for cpu_idle which could be 
> > clearer if named shalloest_idle.
> 
> shallowest_idle?

Something that means the CPU with the shallowest C-state.  Using 
"cpu_idle" for this variable doesn't cut it.

> > > -	return idlest;
> > > +	/* Busy cpus are considered less idle than idle cpus ;) */
> > > +	return cpu_busy != -1 ? cpu_busy : cpu_idle;
> > 
> > And finally it is a policy decision whether or not we want to return 
> > least_loaded over shallowest_idle e.g do we pack tasks on non idle CPUs 
> > first or not.  That in itself needs more investigation.  To keep the 
> > existing policy unchanged for now the above condition should have its 
> > variables swapped.
> 
> Which means that once we've find the first idle CPU, it is not useful to
> continue computing least_loaded, because we will return the idle one anyway,
> right?

Good point.  Currently, that should be the case.

Eventually we'll want to put new tasks on lightly loaded CPUs instead of 
waking up a fully idle CPU in order to favor deeper C-states. But that 
requires a patch series of its own just to determine how loaded a CPU is 
and how much work it can still accommodate before being oversubscribed, 
etc.


Nicolas
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