Re: [PATCH v2 5/5] sched/fair: Fix use of find_idlest_group when local group is idlest.

[Brendan Jackman <brendan.jackman@arm.com>]

On Fri, Aug 25 2017 at 13:38, Vincent Guittot wrote:
> On 25 August 2017 at 12:16, Brendan Jackman <brendan.jackman@arm.com> wrote:
>> find_idlest_group currently returns NULL when the local group is
>> idlest. The caller then continues the find_idlest_group search at a
>> lower level of the current CPU's sched_domain hierarchy. However
>> find_idlest_group_cpu is not consulted and, crucially, @new_cpu is
>> not updated. This means the search is pointless and we return
>> @prev_cpu from select_task_rq_fair.
>>
>> This patch makes find_idlest_group return the idlest group, and never
>> NULL, and then has the caller unconditionally continue to consult
>> find_idlest_group_cpu and update @new_cpu.
>>
>>  * * *
>>
>> An alternative, simpler, fix for this would be to initialize @new_cpu
>> to @cpu instead of @prev_cpu, at the beginning of the new
>> find_idlest_cpu. However this would not fix the fact that
>
> Yes this is simpler
>
>> find_idlest_group_cpu goes unused, and we miss out on the bias toward
>> a shallower-idle CPU, unless find_idlest_group picks a non-local
>> group.
>
> I'm not sure to catch why it's not enough.
> If no cpu of sched_domain span is allowed, we continue to return prev_cpu
> But if at least 1 cpu is allowed, the default new_cpu is cpu in case
> it is really the idlest and no other group will be returned by
> find_idlest_group
> Otherwise, find_idlest_group will finally return another group than
> the local one when running with sd->child

This patch isn't about affinity but just about which group is idlest.

Consider a 4 CPU system with 2 shared caches where CPU0's sched_domains
look like:

DIE: [0  1][2  3]
 MC: [0][1]

Suppose CPUs 0 and 1 are equally idle wrt load and util, but CPU1 is in
a shallower idle state. Suppose all CPUs are allowed.

Assuming @cpu is 0, on the first iteration, find_idlest_group (fig) will
currently return NULL meaning [0 1], then on the second iteration it
will return NULL meaning [0].

With this patch, on the first iteration fig will return [0 1], then
find_idlest_group_cpu (figc) will return 1. Then on the second
iteration, fig should return [1] (because it is biased towards the local
group i.e. [@cpu] i.e. [1]). This is better because CPU1 is in a
shallower idle state.

So basically we're currently missing out on the biasing in figc, unless
we at some point return a non-local group.

Does that make sense?

>
> Is there sched_domain topology where the sched_group of lowest level
> is not with only 1 cpu ?
>
>>
>> If that alternative solution was preferred, then some code could be
>> removed in recognition of the fact that when find_idlest_group_cpu
>> was called, @cpu would never be a member of @group (because @group
>> would be a non-local group, and since @sd is derived from @cpu, @cpu
>> would always be in @sd's local group).
>>
>> Signed-off-by: Brendan Jackman <brendan.jackman@arm.com>
>> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
>> Cc: Vincent Guittot <vincent.guittot@linaro.org>
>> Cc: Josef Bacik <josef@toxicpanda.com>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: Morten Rasmussen <morten.rasmussen@arm.com>
>> Cc: Peter Zijlstra <peterz@infradead.org>
>> ---
>>  kernel/sched/fair.c | 29 ++++++++++-------------------
>>  1 file changed, 10 insertions(+), 19 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 26080917ff8d..93e2746d3c30 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -5384,11 +5384,10 @@ static unsigned long capacity_spare_wake(int cpu, struct task_struct *p)
>>   * Assumes p is allowed on at least one CPU in sd.
>>   */
>>  static struct sched_group *
>> -find_idlest_group(struct sched_domain *sd, struct task_struct *p,
>> -                 int this_cpu, int sd_flag)
>> +find_idlest_group(struct sched_domain *sd, struct task_struct *p, int sd_flag)
>>  {
>> -       struct sched_group *idlest = NULL, *group = sd->groups;
>> -       struct sched_group *most_spare_sg = NULL;
>> +       struct sched_group *group = sd->groups, *local_group = sd->groups;
>> +       struct sched_group *idlest = NULL, *most_spare_sg = NULL;
>>         unsigned long min_runnable_load = ULONG_MAX;
>>         unsigned long this_runnable_load = ULONG_MAX;
>>         unsigned long min_avg_load = ULONG_MAX, this_avg_load = ULONG_MAX;
>> @@ -5404,7 +5403,6 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
>>         do {
>>                 unsigned long load, avg_load, runnable_load;
>>                 unsigned long spare_cap, max_spare_cap;
>> -               int local_group;
>>                 int i;
>>
>>                 /* Skip over this group if it has no CPUs allowed */
>> @@ -5412,9 +5410,6 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
>>                                         &p->cpus_allowed))
>>                         continue;
>>
>> -               local_group = cpumask_test_cpu(this_cpu,
>> -                                              sched_group_span(group));
>> -
>>                 /*
>>                  * Tally up the load of all CPUs in the group and find
>>                  * the group containing the CPU with most spare capacity.
>> @@ -5425,7 +5420,7 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
>>
>>                 for_each_cpu(i, sched_group_span(group)) {
>>                         /* Bias balancing toward cpus of our domain */
>> -                       if (local_group)
>> +                       if (group == local_group)
>>                                 load = source_load(i, load_idx);
>>                         else
>>                                 load = target_load(i, load_idx);
>> @@ -5446,7 +5441,7 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
>>                 runnable_load = (runnable_load * SCHED_CAPACITY_SCALE) /
>>                                         group->sgc->capacity;
>>
>> -               if (local_group) {
>> +               if (group == local_group) {
>>                         this_runnable_load = runnable_load;
>>                         this_avg_load = avg_load;
>>                         this_spare = max_spare_cap;
>> @@ -5492,21 +5487,21 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p,
>>
>>         if (this_spare > task_util(p) / 2 &&
>>             imbalance_scale*this_spare > 100*most_spare)
>> -               return NULL;
>> +               return local_group;
>>
>>         if (most_spare > task_util(p) / 2)
>>                 return most_spare_sg;
>>
>>  skip_spare:
>>         if (!idlest)
>> -               return NULL;
>> +               return local_group;
>>
>>         if (min_runnable_load > (this_runnable_load + imbalance))
>> -               return NULL;
>> +               return local_group;
>>
>>         if ((this_runnable_load < (min_runnable_load + imbalance)) &&
>>              (100*this_avg_load < imbalance_scale*min_avg_load))
>> -               return NULL;
>> +               return local_group;
>>
>>         return idlest;
>>  }
>> @@ -5582,11 +5577,7 @@ static inline int find_idlest_cpu(struct sched_domain *sd, struct task_struct *p
>>                         continue;
>>                 }
>>
>> -               group = find_idlest_group(sd, p, cpu, sd_flag);
>> -               if (!group) {
>> -                       sd = sd->child;
>> -                       continue;
>> -               }
>> +               group = find_idlest_group(sd, p, sd_flag);
>>
>>                 new_cpu = find_idlest_group_cpu(group, p, cpu);
>>                 if (new_cpu == cpu) {
>> --
>> 2.14.1
>>