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.
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.
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).
> > + }
> > +
> > + /* 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.
> 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?
> > - 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?
--
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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