Re: [PATCHSET v1 wq/for-6.5] workqueue: Improve unbound workqueue execution locality

[Peter Zijlstra]

On Thu, May 25, 2023 at 03:12:45PM -1000, Tejun Heo wrote:

> CONCLUSION
> ==========
> 
> With the SIS_NODE enabled, there's no downside to CACHE. It always
> outperforms or matches SYSTEM. It's possible that the overhead of searching
> further for an idle CPU is more pronounced on bigger machines but most
> likely so will be the gains. This looks like a no brainer improvement to me.

OK, looking at it again, I think it can be done a little simpler, but it
should be mostly the same.

I'll go queue the below in sched/core, we'll see if anything comes up
negative.

---
Subject: sched/fair: Multi-LLC select_idle_sibling()
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Tue May 30 13:20:46 CEST 2023

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(+)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7028,6 +7028,38 @@ static int select_idle_cpu(struct task_s
 }
 
 /*
+ * 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) {
+			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 ta
 	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;
 }
 
--- 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