linux-kernel - [PATCH v9 2/2] sched: Annotate sched_clock_irqtime with __read

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Message-Id: <20250511030800.1900-3-laoar.shao@gmail.com>
Date: Sun, 11 May 2025 11:08:00 +0800
From: Yafang Shao <laoar.shao@...il.com>
To: mingo@...hat.com,
	peterz@...radead.org,
	mkoutny@...e.com,
	hannes@...xchg.org
Cc: juri.lelli@...hat.com,
	vincent.guittot@...aro.org,
	dietmar.eggemann@....com,
	rostedt@...dmis.org,
	bsegall@...gle.com,
	mgorman@...e.de,
	vschneid@...hat.com,
	surenb@...gle.com,
	linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org,
	lkp@...el.com,
	Yafang Shao <laoar.shao@...il.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: [PATCH v9 2/2] sched: Annotate sched_clock_irqtime with __read_mostly

Eric reported an issue [0] as follows,
: rebalance_domains() can attempt to change sched_balance_running
: more than 350,000 times per second on our servers.

: If sched_clock_irqtime and sched_balance_running share the
: same cache line, we see a very high cost on hosts with 480 threads
: dealing with many interrupts.

While the rebalance_domains() issue has been resolved [1], we should
proactively annotate sched_clock_irqtime with __read_mostly to prevent
potential cacheline false sharing. This optimization is particularly
justified since sched_clock_irqtime is only modified during TSC instability
events.

Link: https://lore.kernel.org/all/20250423174634.3009657-1-edumazet@google.com/ [0]
Link: https://lore.kernel.org/all/20250416035823.1846307-1-tim.c.chen@linux.intel.com/ [1]

Reported-by: Eric Dumazet <edumazet@...gle.com>
Debugged-by: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: Yafang Shao <laoar.shao@...il.com>
Cc: Eric Dumazet <edumazet@...gle.com>
---
 kernel/sched/cputime.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 6dab4854c6c0..c499a42ceda4 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -22,7 +22,7 @@
  */
 DEFINE_PER_CPU(struct irqtime, cpu_irqtime);
 
-int sched_clock_irqtime;
+int __read_mostly sched_clock_irqtime;
 
 void enable_sched_clock_irqtime(void)
 {
-- 
2.43.5