Re: [tip: sched/core] sched/fair: Multi-LLC select_idle_sibling()

[Abel Wu]

On 5/31/23 8:04 PM, tip-bot2 for Peter Zijlstra wrote:
> The following commit has been merged into the sched/core branch of tip:
>
> Commit-ID:     c7dfd6b9122d29d0e9a4587ab470c0564d7f92ab
> Gitweb:        https://git.kernel.org/tip/c7dfd6b9122d29d0e9a4587ab470c0564d7f92ab
> Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> AuthorDate:    Tue, 30 May 2023 13:20:46 +02:00
> Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> CommitterDate: Tue, 30 May 2023 22:46:27 +02:00
>
> sched/fair: Multi-LLC select_idle_sibling()
>
> Tejun reported that when he targets workqueues towards a specific LLC
> on his Zen2 machine with 3 cores / LLC and 4 LLCs in total, he gets
> significant idle time.
>
> This is, of course, because of how select_idle_sibling() will not
> consider anything outside of the local LLC, and since all these tasks
> are short running the periodic idle load balancer is ineffective.
>
> And while it is good to keep work cache local, it is better to not
> have significant idle time. Therefore, have select_idle_sibling() try
> other LLCs inside the same node when the local one comes up empty.
>
> Reported-by: Tejun Heo <tj@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>   kernel/sched/fair.c     | 38 ++++++++++++++++++++++++++++++++++++++
>   kernel/sched/features.h |  1 +
>   2 files changed, 39 insertions(+)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 48b6f0c..0172458 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7028,6 +7028,38 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
>   }
>   
>   /*
> + * For the multiple-LLC per node case, make sure to try the other LLC's if the
> + * local LLC comes up empty.
> + */
> +static int
> +select_idle_node(struct task_struct *p, struct sched_domain *sd, int target)
> +{
> +	struct sched_domain *parent = sd->parent;
> +	struct sched_group *sg;
> +
> +	/* Make sure to not cross nodes. */
> +	if (!parent || parent->flags & SD_NUMA)
> +		return -1;
> +
> +	sg = parent->groups;
> +	do {
> +		int cpu = cpumask_first(sched_group_span(sg));
> +		struct sched_domain *sd_child;
> +
> +		sd_child = per_cpu(sd_llc, cpu);
> +		if (sd_child != sd) {

Since sd_llc is cpu private, I think it should be:

		if (!cpus_share_cache(cpu, target))

> +			int i = select_idle_cpu(p, sd_child, test_idle_cores(cpu), cpu);
> +			if ((unsigned)i < nr_cpumask_bits)
> +				return i;
> +		}
> +
> +		sg = sg->next;
> +	} while (sg != parent->groups);
> +
> +	return -1;
> +}
> +
> +/*
>    * Scan the asym_capacity domain for idle CPUs; pick the first idle one on which
>    * the task fits. If no CPU is big enough, but there are idle ones, try to
>    * maximize capacity.
> @@ -7199,6 +7231,12 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>   	if ((unsigned)i < nr_cpumask_bits)
>   		return i;
>   
> +	if (sched_feat(SIS_NODE)) {
> +		i = select_idle_node(p, sd, target);
> +		if ((unsigned)i < nr_cpumask_bits)
> +			return i;
> +	}
> +
>   	return target;
>   }
>   
> diff --git a/kernel/sched/features.h b/kernel/sched/features.h
> index ee7f23c..9e390eb 100644
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -62,6 +62,7 @@ SCHED_FEAT(TTWU_QUEUE, true)
>    */
>   SCHED_FEAT(SIS_PROP, false)
>   SCHED_FEAT(SIS_UTIL, true)
> +SCHED_FEAT(SIS_NODE, true)
>   
>   /*
>    * Issue a WARN when we do multiple update_rq_clock() calls