On Friday, April 04, 2014 12:56:52 PM Nicolas Pitre wrote:
> 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.lezc...@linaro.org>
> > >
> > > 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.
Right, I got that the other way around by mistake.
> 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.
Yes, that sounds like it might be a reason, but I'd like to see numbers
confirming that to be honest.
> > > > + }
> > > > +
> > > > + /* 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.
Yes, that was about the typo above only. :-)
> > > > - 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.
Wouldn't we need power consumption numbers for that realistically?
Rafael
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