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SubjectRe: [RFC PATCH v6 3/4] scheduler: scan idle cpu in cluster for tasks within one LLC
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On 20/04/2021 02:18, Barry Song wrote:

[...]

> @@ -5786,11 +5786,12 @@ static void record_wakee(struct task_struct *p)
> * whatever is irrelevant, spread criteria is apparent partner count exceeds
> * socket size.
> */
> -static int wake_wide(struct task_struct *p)
> +static int wake_wide(struct task_struct *p, int cluster)
> {
> unsigned int master = current->wakee_flips;
> unsigned int slave = p->wakee_flips;
> - int factor = __this_cpu_read(sd_llc_size);
> + int factor = cluster ? __this_cpu_read(sd_cluster_size) :
> + __this_cpu_read(sd_llc_size);

I don't see that the wake_wide() change has any effect here. None of the
sched domains has SD_BALANCE_WAKE set so a wakeup (WF_TTWU) can never
end up in the slow path.
Have you seen a diff when running your `lmbench stream` workload in what
wake_wide() returns when you use `sd cluster size` instead of `sd llc
size` as factor?

I guess for you, wakeups are now subdivided into faster (cluster = 4
CPUs) and fast (llc = 24 CPUs) via sis(), not into fast (sis()) and slow
(find_idlest_cpu()).

>
> if (master < slave)
> swap(master, slave);

[...]

> @@ -6745,6 +6748,12 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> int want_affine = 0;
> /* SD_flags and WF_flags share the first nibble */
> int sd_flag = wake_flags & 0xF;
> + /*
> + * if cpu and prev_cpu share LLC, consider cluster sibling rather
> + * than llc. this is typically true while tasks are bound within
> + * one numa
> + */
> + int cluster = sched_cluster_active() && cpus_share_cache(cpu, prev_cpu, 0);

So you changed from scanning cluster before LLC to scan either cluster
or LLC.

And this is based on whether `this_cpu` and `prev_cpu` are sharing LLC
or not. So you only see an effect when running the workload with
`numactl -N X ...`.

>
> if (wake_flags & WF_TTWU) {
> record_wakee(p);
> @@ -6756,7 +6765,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> new_cpu = prev_cpu;
> }
>
> - want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, p->cpus_ptr);
> + want_affine = !wake_wide(p, cluster) && cpumask_test_cpu(cpu, p->cpus_ptr);
> }
>
> rcu_read_lock();
> @@ -6768,7 +6777,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> if (want_affine && (tmp->flags & SD_WAKE_AFFINE) &&
> cpumask_test_cpu(prev_cpu, sched_domain_span(tmp))) {
> if (cpu != prev_cpu)
> - new_cpu = wake_affine(tmp, p, cpu, prev_cpu, sync);
> + new_cpu = wake_affine(tmp, p, cpu, prev_cpu, sync, cluster);
>
> sd = NULL; /* Prefer wake_affine over balance flags */
> break;
> @@ -6785,7 +6794,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> new_cpu = find_idlest_cpu(sd, p, cpu, prev_cpu, sd_flag);
> } else if (wake_flags & WF_TTWU) { /* XXX always ? */
> /* Fast path */
> - new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
> + new_cpu = select_idle_sibling(p, prev_cpu, new_cpu, cluster);
>
> if (want_affine)
> current->recent_used_cpu = cpu;

[...]

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