Re: [PATCH v2 5/5] sched/fair: Fix use of find_idlest_group when local group is idlest.
From: Vincent Guittot
Date: Fri Aug 25 2017 - 09:38:50 EST
On 25 August 2017 at 12:16, Brendan Jackman <brendan.jackman@xxxxxxx> 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
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@xxxxxxx>
> Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Cc: Josef Bacik <josef@xxxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Morten Rasmussen <morten.rasmussen@xxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> ---
> 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
>