linux-kernel - [PATCH 1/2] sched/fair: Add EAS checks before updating overutilized

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Message-Id: <20240223150707.410417-2-sshegde@linux.ibm.com>
Date: Fri, 23 Feb 2024 20:37:06 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: mingo@...nel.org, peterz@...radead.org, vincent.guittot@...aro.org
Cc: sshegde@...ux.ibm.com, dietmar.eggemann@....com,
        linux-kernel@...r.kernel.org, nysal@...ux.ibm.com,
        aboorvad@...ux.ibm.com, srikar@...ux.vnet.ibm.com, vschneid@...hat.com,
        pierre.gondois@....com, morten.rasmussen@....com, qyousef@...alina.io
Subject: [PATCH 1/2] sched/fair: Add EAS checks before updating overutilized

Overutilized field of root domain is only used for EAS(energy aware scheduler)
to decide whether to do regular load balance or EAS aware load balance. It
is not used if EAS not possible.

Currently enqueue_task_fair and task_tick_fair accesses, sometime updates
this field. In update_sd_lb_stats it is updated often.
Which causes cache contention due to load/store tearing and burns
a lot of cycles. Hence add EAS check before updating this field.
EAS check is optimized at compile time or it is static branch.
Hence it shouldn't cost much.

With the patch, both enqueue_task_fair and newidle_balance don't show
up as hot routines in perf profile.

6.8-rc4:
7.18%  swapper          [kernel.vmlinux]              [k] enqueue_task_fair
6.78%  s                [kernel.vmlinux]              [k] newidle_balance
+patch:
0.14%  swapper          [kernel.vmlinux]              [k] enqueue_task_fair
0.00%  swapper          [kernel.vmlinux]              [k] newidle_balance

While here, Fix updating overutilized as either SG_OVERUTILIZED or 0
instead. Current code can make it 0, 1 or 2. This shouldn't alter the
functionality.

Fixes: 2802bf3cd936 ("sched/fair: Add over-utilization/tipping point indicator")
Signed-off-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
---
 kernel/sched/fair.c | 36 +++++++++++++++++++++++++-----------
 1 file changed, 25 insertions(+), 11 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8e30e2bb77a0..9529d9ef2c5b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6670,15 +6670,30 @@ static inline bool cpu_overutilized(int cpu)
 	return !util_fits_cpu(cpu_util_cfs(cpu), rq_util_min, rq_util_max, cpu);
 }

-static inline void update_overutilized_status(struct rq *rq)
+static inline void update_rd_overutilized_status(struct root_domain *rd,
+						 int status)
 {
-	if (!READ_ONCE(rq->rd->overutilized) && cpu_overutilized(rq->cpu)) {
-		WRITE_ONCE(rq->rd->overutilized, SG_OVERUTILIZED);
-		trace_sched_overutilized_tp(rq->rd, SG_OVERUTILIZED);
+	if (sched_energy_enabled()) {
+		WRITE_ONCE(rd->overutilized, status);
+		trace_sched_overutilized_tp(rd, !!status);
+	}
+}
+
+static inline void check_update_overutilized_status(struct rq *rq)
+{
+	/*
+	 * overutilized field is used for load balancing decisions only
+	 * if energy aware scheduler is being used
+	 */
+	if (sched_energy_enabled()) {
+		if (!READ_ONCE(rq->rd->overutilized) && cpu_overutilized(rq->cpu))
+			update_rd_overutilized_status(rq->rd, SG_OVERUTILIZED);
 	}
 }
 #else
-static inline void update_overutilized_status(struct rq *rq) { }
+static inline void check_update_overutilized_status(struct rq *rq) { }
+static inline void update_rd_overutilized_status(struct root_domain *rd,
+						 bool status) { }
 #endif

 /* Runqueue only has SCHED_IDLE tasks enqueued */
@@ -6779,7 +6794,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
 	 * and the following generally works well enough in practice.
 	 */
 	if (!task_new)
-		update_overutilized_status(rq);
+		check_update_overutilized_status(rq);

 enqueue_throttle:
 	assert_list_leaf_cfs_rq(rq);
@@ -10613,13 +10628,12 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
 		WRITE_ONCE(rd->overload, sg_status & SG_OVERLOAD);

 		/* Update over-utilization (tipping point, U >= 0) indicator */
-		WRITE_ONCE(rd->overutilized, sg_status & SG_OVERUTILIZED);
-		trace_sched_overutilized_tp(rd, sg_status & SG_OVERUTILIZED);
+		update_rd_overutilized_status(rd,
+				(sg_status & SG_OVERUTILIZED) ? SG_OVERUTILIZED : 0);
 	} else if (sg_status & SG_OVERUTILIZED) {
 		struct root_domain *rd = env->dst_rq->rd;

-		WRITE_ONCE(rd->overutilized, SG_OVERUTILIZED);
-		trace_sched_overutilized_tp(rd, SG_OVERUTILIZED);
+		update_rd_overutilized_status(rd, SG_OVERUTILIZED);
 	}

 	update_idle_cpu_scan(env, sum_util);
@@ -12625,7 +12639,7 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
 		task_tick_numa(rq, curr);

 	update_misfit_status(curr, rq);
-	update_overutilized_status(task_rq(curr));
+	check_update_overutilized_status(task_rq(curr));

 	task_tick_core(rq, curr);
 }
--
2.39.3