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From: Shrikanth Hegde <sshegde@linux.ibm.com>
To: mingo@kernel.org, peterz@infradead.org, vincent.guittot@linaro.org
Cc: sshegde@linux.ibm.com, yu.c.chen@intel.com, dietmar.eggemann@arm.com,
        linux-kernel@vger.kernel.org, nysal@linux.ibm.com,
        aboorvad@linux.ibm.com, srikar@linux.vnet.ibm.com, vschneid@redhat.com,
        pierre.gondois@arm.com, morten.rasmussen@arm.com, qyousef@layalina.io
Subject: [PATCH v3 1/2] sched/fair: Add EAS checks before updating overutilized
Date: Thu, 29 Feb 2024 16:10:09 +0530
Message-Id: <20240229104010.747411-2-sshegde@linux.ibm.com>
In-Reply-To: <20240229104010.747411-1-sshegde@linux.ibm.com>
References: <20240229104010.747411-1-sshegde@linux.ibm.com>
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Overutilized field of root domain is only used for EAS(energy aware scheduler)
to decide whether to do load balance or not. 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. This causes cache
contention due to true sharing and burns a lot of cycles. overload and
overutilized are part of the same cacheline. Updating it often invalidates
the cacheline. That causes access  to overload to slow down due to
false sharing. Hence add EAS check before accessing/updating this field.
EAS check is optimized at compile time or it is a 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

Minor change: trace_sched_overutilized_tp expect that second argument to
be bool. So do a int to bool conversion for that.

Fixes: 2802bf3cd936 ("sched/fair: Add over-utilization/tipping point indicator")
Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
---
 kernel/sched/fair.c | 47 ++++++++++++++++++++++++++++-----------------
 1 file changed, 29 insertions(+), 18 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6a16129f9a5c..1f7d62b7c26f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6670,15 +6670,29 @@ 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 set_rd_overutilized_status(struct root_domain *rd,
+					      unsigned 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);
-	}
+	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())
+		return;
+
+	if (!READ_ONCE(rq->rd->overutilized) && cpu_overutilized(rq->cpu))
+		set_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 set_rd_overutilized_status(struct root_domain *rd,
+					      unsigned int status) { }
 #endif

 /* Runqueue only has SCHED_IDLE tasks enqueued */
@@ -6779,7 +6793,7 @@ 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);
@@ -9902,7 +9916,7 @@ static inline void update_sg_lb_stats(struct lb_env *env,
 		if (nr_running > 1)
 			*sg_status |= SG_OVERLOAD;

-		if (cpu_overutilized(i))
+		if (sched_energy_enabled() && cpu_overutilized(i))
 			*sg_status |= SG_OVERUTILIZED;

 #ifdef CONFIG_NUMA_BALANCING
@@ -10596,19 +10610,16 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
 		env->fbq_type = fbq_classify_group(&sds->busiest_stat);

 	if (!env->sd->parent) {
-		struct root_domain *rd = env->dst_rq->rd;
-
 		/* update overload indicator if we are at root domain */
-		WRITE_ONCE(rd->overload, sg_status & SG_OVERLOAD);
+		WRITE_ONCE(env->dst_rq->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);
+		if (sched_energy_enabled()) {
+			set_rd_overutilized_status(env->dst_rq->rd,
+						   sg_status & SG_OVERUTILIZED);
+		}
 	} 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);
+		set_rd_overutilized_status(env->dst_rq->rd, SG_OVERUTILIZED);
 	}

 	update_idle_cpu_scan(env, sum_util);
@@ -12609,7 +12620,7 @@ 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