linux-kernel - [RFC PATCH 07/19] sched/fair: Account idle cpus instead of busy cpus in sd->shared

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Message-ID: <20250904041516.3046-8-kprateek.nayak@amd.com>
Date: Thu, 4 Sep 2025 04:15:03 +0000
From: K Prateek Nayak <kprateek.nayak@....com>
To: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot
	<vincent.guittot@...aro.org>, Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Frederic Weisbecker <frederic@...nel.org>, Thomas Gleixner
	<tglx@...utronix.de>, <linux-kernel@...r.kernel.org>
CC: Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt
	<rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman
	<mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>, K Prateek Nayak
	<kprateek.nayak@....com>, "Gautham R. Shenoy" <gautham.shenoy@....com>,
	Swapnil Sapkal <swapnil.sapkal@....com>
Subject: [RFC PATCH 07/19] sched/fair: Account idle cpus instead of busy cpus in sd->shared

Switch to keeping track of "sd->shared->nr_idle_cpus" instead of
"nr_busy_cpus". Since previous commit corrected the "sd->nohz_idle"
state during sched domain rebuild, the nr_idle_cpus will reflect the
correct number of idle CPUs.

The idle CPUs accounting will be used for nohz idle balance in the
subsequent commits.

Races are possible during hotplug / cpuset where "nr_idle_cpus" might
be incorrectly accounted if the CPU enters exits out of nohz idle state
between the read of "rq->nohz_tick_stopped" and the subsequent update of
"sd->nohz_idle" in the hotplug path but these inaccuracies are transient
and will be corrected when the CPU enters idle or receives a tick.

  CPU0 (hotplug)                                    CPU1 (exits nohz idle)
  ==============                                    ======================
  online()
    if (rq->nohz_tick_stopped) /* True */
      ...                                          rq->nohz_tick_stopped = 0
      ...                                          set_cpu_sd_state_busy()
      ...
      set_cpu_sd_state_idle()

These situations are rare and should not have any long-term effect on
the nohz idle balancing since there isn't a case where a nohz idle CPU
is not set on the mask - either the hotplug thread sees that
"rq->nohz_tick_stopped" is set or the CPU going idle sees the updated
sched_domain hierarchy.

After the conversion, all the bits that use "nr_idle_cpus" are already
guarded behind CONFIG_NO_HZ_COMMON which makes it convenient to put the
declaration behind CONFIG_NO_HZ_COMMON as well.

Signed-off-by: K Prateek Nayak <kprateek.nayak@....com>
---
 include/linux/sched/topology.h |  4 +++-
 kernel/sched/fair.c            | 10 +++++-----
 kernel/sched/topology.c        |  1 -
 3 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index d816911de435..2f0d8ecea427 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -65,7 +65,9 @@ struct sched_group;
 
 struct sched_domain_shared {
 	atomic_t	ref;
-	atomic_t	nr_busy_cpus;
+#ifdef CONFIG_NO_HZ_COMMON
+	atomic_t	nr_idle_cpus;
+#endif
 	int		has_idle_cores;
 	int		nr_idle_scan;
 };
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 61e1b4deb3e8..dee0ded7f40d 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12429,7 +12429,7 @@ static void nohz_balancer_kick(struct rq *rq)
 		 * the others are - so just get a NOHZ balance going if it looks
 		 * like this LLC domain has tasks we could move.
 		 */
-		nr_busy = atomic_read(&sds->nr_busy_cpus);
+		nr_busy = per_cpu(sd_llc_size, cpu) - atomic_read(&sds->nr_idle_cpus);
 		if (nr_busy > 1) {
 			flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK;
 			goto unlock;
@@ -12458,7 +12458,7 @@ static void set_cpu_sd_state_busy(int cpu)
 	if (!xchg(&sd->nohz_idle, 0))
 		return;
 
-	atomic_inc(&sd->shared->nr_busy_cpus);
+	atomic_dec(&sd->shared->nr_idle_cpus);
 }
 
 void nohz_balance_exit_idle(struct rq *rq)
@@ -12488,7 +12488,7 @@ static void set_cpu_sd_state_idle(int cpu)
 	if (xchg(&sd->nohz_idle, 1))
 		return;
 
-	atomic_dec(&sd->shared->nr_busy_cpus);
+	atomic_inc(&sd->shared->nr_idle_cpus);
 }
 
 static void cpu_sd_exit_nohz_balance(struct rq *rq)
@@ -12955,7 +12955,7 @@ static void rq_online_fair(struct rq *rq)
 
 	update_runtime_enabled(rq);
 
-	/* Fixup nr_busy_cpus and nohz stats. */
+	/* Fixup nr_idle_cpus and nohz stats. */
 	cpu_sd_reenter_nohz_balance(rq);
 }
 
@@ -12969,7 +12969,7 @@ static void rq_offline_fair(struct rq *rq)
 	/* Ensure that we remove rq contribution to group share: */
 	clear_tg_offline_cfs_rqs(rq);
 
-	/* Fixup nr_busy_cpus and nohz stats. */
+	/* Fixup nr_idle_cpus and nohz stats. */
 	cpu_sd_exit_nohz_balance(rq);
 }
 
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index a059641e12e5..0b0257937a97 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -2581,7 +2581,6 @@ build_sched_domains(const struct cpumask *cpu_map, struct sched_domain_attr *att
 				int llc_id = cpumask_first(sched_domain_span(sd));
 
 				sd->shared = *per_cpu_ptr(d.sds, llc_id);
-				atomic_set(&sd->shared->nr_busy_cpus, sd->span_weight);
 				atomic_inc(&sd->shared->ref);
 			}
 
-- 
2.34.1