On Wed, Jan 12, 2022 at 08:09:02PM +0530, Chitti Babu Theegala wrote:
> Newly forked threads don't have any useful utilization data yet and
> it's not possible to forecast their impact on energy consumption.
>update_pick_idlest These forkees (though very small, most times) end up waking big
> cores from deep sleep for that very small durations.
>
> Bias all forkees to small cores to prevent waking big cores from deep
> sleep to save power.
This bias might be interesting for some workloads, but what about the
others? (see find_energy_efficient_cpu() comment, which discusses forkees).
>
> Signed-off-by: Chitti Babu Theegala <quic_ctheegal@xxxxxxxxxxx>
> ---
> kernel/sched/fair.c | 16 +++++++++++-----
> 1 file changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6e476f6..d407bbc 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5976,7 +5976,7 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p,
> }
>
> static struct sched_group *
> -find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu);
> +find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu, int sd_flag);
>
> /*
> * find_idlest_group_cpu - find the idlest CPU among the CPUs in the group.
> @@ -6063,7 +6063,7 @@ static inline int find_idlest_cpu(struct sched_domain *sd, struct task_struct *p
> continue;
> }
>
> - group = find_idlest_group(sd, p, cpu);
> + group = find_idlest_group(sd, p, cpu, sd_flag);
> if (!group) {
> sd = sd->child;
> continue;
> @@ -8997,7 +8997,8 @@ static inline void update_sg_wakeup_stats(struct sched_domain *sd,
> static bool update_pick_idlest(struct sched_group *idlest,
> struct sg_lb_stats *idlest_sgs,
> struct sched_group *group,
> - struct sg_lb_stats *sgs)
> + struct sg_lb_stats *sgs,
> + int sd_flag)
> {
> if (sgs->group_type < idlest_sgs->group_type)
> return true;
> @@ -9034,6 +9035,11 @@ static bool update_pick_idlest(struct sched_group *idlest,
> if (idlest_sgs->idle_cpus > sgs->idle_cpus)
> return false;
>
> + /* Select smaller cpu group for newly woken up forkees */
> + if ((sd_flag & SD_BALANCE_FORK) && (idlest_sgs->idle_cpus &&
> + !capacity_greater(idlest->sgc->max_capacity, group->sgc->max_capacity)))
> + return false;
> +
Energy biased placement should probably be applied only when EAS is enabled.
It's especially true here, if all CPUs have the same capacity, capacity_greater
would be always false. So unless I missed something, we wouldn't let the group_util
evaluation happen, would we?
[...]
