linux-kernel - [PATCH v2] sched/clock: Avoid false sharing for sched_clock

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Message-ID: <20260113074807.3404180-1-wangyang.guo@intel.com>
Date: Tue, 13 Jan 2026 15:48:07 +0800
From: Wangyang Guo <wangyang.guo@...el.com>
To: K Prateek Nayak <kprateek.nayak@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	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>
Cc: linux-kernel@...r.kernel.org,
	Wangyang Guo <wangyang.guo@...el.com>,
	Shrikanth Hegde <sshegde@...ux.ibm.com>,
	Benjamin Lei <benjamin.lei@...el.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Tianyou Li <tianyou.li@...el.com>
Subject: [PATCH v2] sched/clock: Avoid false sharing for sched_clock_irqtime

Read-mostly sched_clock_irqtime may share the same cacheline with
frequently updated nohz struct. Mark it as __read_mostly to avoid
false sharing issue.

Details:
We observed ~3% cycles hotspots in irqtime_account_irq when running
SPECjbb2015 in a 2-sockets system. Most of cycles spent in reading
sched_clock_irqtime, which is a read-mostly var.

perf c2c (cachelien view) shows it has false sharing with nohz struct:
     Num RmtHitm LclHitm  Offset records             Symbol
   6.25%   0.00%   0.00%   0x0       4   [k] _nohz_idle_balance.isra.0
  18.75% 100.00%   0.00%   0x8      14   [k] nohz_balance_exit_idle
   6.25%   0.00%   0.00%   0x8       8   [k] nohz_balance_enter_idle
   6.25%   0.00%   0.00%   0xc       8   [k] sched_balance_newidle
   6.25%   0.00%   0.00%  0x10      31   [k] nohz_balancer_kick
   6.25%   0.00%   0.00%  0x20      16   [k] sched_balance_newidle
  37.50%   0.00%   0.00%  0x38      50   [k] irqtime_account_irq
   6.25%   0.00%   0.00%  0x38      47   [k] account_process_tick
   6.25%   0.00%   0.00%  0x38      12   [k] account_idle_ticks

Offsets:
*  0x0 -- nohz.idle_cpu_mask (r)
*  0x8 -- nohz.nr_cpus (w)
* 0x38 -- sched_clock_irqtime (r), not in nohz, but share cacheline

The layout in /proc/kallsyms can also confirm that:
ffffffff88600d40 b nohz
ffffffff88600d68 B arch_needs_tick_broadcast
ffffffff88600d6c b __key.264
ffffffff88600d6c b __key.265
ffffffff88600d70 b dl_generation
ffffffff88600d78 b sched_clock_irqtime

With the patch applied, irqtime_account_irq hotspot disappear.

Changes since V1:
- Use __read_mostly instead of __cacheline_aligned to avoid wasting
  spaces.

History:
  v1: https://lore.kernel.org/all/20260113022958.3379650-1-wangyang.guo@intel.com/
  prev discussions: https://lore.kernel.org/all/20251211055612.4071266-1-wangyang.guo@intel.com/T/#u

Suggested-by: Shrikanth Hegde <sshegde@...ux.ibm.com>
Reported-by: Benjamin Lei <benjamin.lei@...el.com>
Reviewed-by: Tim Chen <tim.c.chen@...ux.intel.com>
Reviewed-by: Tianyou Li <tianyou.li@...el.com>
Signed-off-by: Wangyang Guo <wangyang.guo@...el.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 7097de2c8cda..fc8f8081c48f 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -25,7 +25,7 @@
  */
 DEFINE_PER_CPU(struct irqtime, cpu_irqtime);
 
-int sched_clock_irqtime;
+int sched_clock_irqtime __read_mostly;
 
 void enable_sched_clock_irqtime(void)
 {
-- 
2.47.3