Re: [RFC] sched/fair: avoid redundant updates for unchanged default slices

[Chengfeng Lin]

Hi,

The inline patch in my previous message lost its tab indentation when
sent. I have attached the original patch to preserve the formatting. It
passes git apply --check against v7.2.

The code and test results are unchanged. Sorry for the formatting issue.

Thanks,
Chengfeng
diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -491,6 +491,39 @@ req_priv:
 	return 0;
 }
 
+static bool fair_params_changed(struct task_struct *p,
+				const struct sched_attr *attr)
+{
+	if (attr->sched_nice != task_nice(p))
+		return true;
+
+	if (attr->sched_runtime == p->se.slice)
+		return false;
+
+	/* Preserve the existing path for PI-boosted and sched_ext tasks. */
+	if (p->sched_class != &fair_sched_class)
+		return true;
+
+#ifdef CONFIG_UCLAMP_TASK
+	/*
+	 * Unchanged fair parameters can still require an implicit clamp reset,
+	 * for example after a task was PI-boosted into the RT class.
+	 */
+	if ((!p->uclamp_req[UCLAMP_MIN].user_defined &&
+	     p->uclamp_req[UCLAMP_MIN].value != uclamp_none(UCLAMP_MIN)) ||
+	    (!p->uclamp_req[UCLAMP_MAX].user_defined &&
+	     p->uclamp_req[UCLAMP_MAX].value != uclamp_none(UCLAMP_MAX)))
+		return true;
+#endif
+
+	/*
+	 * A zero runtime requests the default slice, whereas se.slice stores
+	 * its actual duration. Keep the update if the mode or default changed.
+	 */
+	return attr->sched_runtime || p->se.custom_slice ||
+		p->se.slice != READ_ONCE(sysctl_sched_base_slice);
+}
+
 int __sched_setscheduler(struct task_struct *p,
 			 const struct sched_attr *attr,
 			 bool user, bool pi)
@@ -588,9 +621,7 @@ recheck:
 	 * but store a possible modification of reset_on_fork.
 	 */
 	if (unlikely(policy == p->policy)) {
-		if (fair_policy(policy) &&
-		    (attr->sched_nice != task_nice(p) ||
-		     (attr->sched_runtime != p->se.slice)))
+		if (fair_policy(policy) && fair_params_changed(p, attr))
 			goto change;
 		if (rt_policy(policy) && attr->sched_priority != p->rt_priority)
 			goto change;