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Message-ID: <20260126021401.1490163-1-wangyang.guo@intel.com>
Date: Mon, 26 Jan 2026 10:14:01 +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 v4] sched/clock: Avoid false sharing for sched_clock_irqtime
Read-mostly sched_clock_irqtime may share the same cacheline with
frequently updated nohz struct. Make it as static_key to avoid
false sharing issue.
Details:
In kernel 6.14, 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.
---
V4 -> V3:
- Avoid creating a new workqueue to disable static_key
- Specify kernel version for c2c result in changelog
V2 -> V3:
- Use static_key instead of a __read_mostly var.
V1 -> V2:
- Use __read_mostly instead of __cacheline_aligned to avoid wasting
spaces.
History:
v3: https://lore.kernel.org/all/20260116023945.1849329-1-wangyang.guo@intel.com/
v2: https://lore.kernel.org/all/20260113074807.3404180-1-wangyang.guo@intel.com/
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: K Prateek Nayak <kprateek.nayak@....com>
Suggested-by: Peter Zijlstra <peterz@...radead.org>
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>
---
arch/x86/kernel/tsc.c | 2 --
kernel/sched/clock.c | 3 +++
kernel/sched/cputime.c | 8 ++++----
kernel/sched/sched.h | 4 ++--
4 files changed, 9 insertions(+), 8 deletions(-)
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 87e749106dda..9a62e18d1bff 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1142,7 +1142,6 @@ static void tsc_cs_mark_unstable(struct clocksource *cs)
tsc_unstable = 1;
if (using_native_sched_clock())
clear_sched_clock_stable();
- disable_sched_clock_irqtime();
pr_info("Marking TSC unstable due to clocksource watchdog\n");
}
@@ -1212,7 +1211,6 @@ void mark_tsc_unstable(char *reason)
tsc_unstable = 1;
if (using_native_sched_clock())
clear_sched_clock_stable();
- disable_sched_clock_irqtime();
pr_info("Marking TSC unstable due to %s\n", reason);
clocksource_mark_unstable(&clocksource_tsc_early);
diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index f5e6dd6a6b3a..2ae4fbf13431 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -173,6 +173,7 @@ notrace static void __sched_clock_work(struct work_struct *work)
scd->tick_gtod, __gtod_offset,
scd->tick_raw, __sched_clock_offset);
+ disable_sched_clock_irqtime();
static_branch_disable(&__sched_clock_stable);
}
@@ -238,6 +239,8 @@ static int __init sched_clock_init_late(void)
if (__sched_clock_stable_early)
__set_sched_clock_stable();
+ else
+ disable_sched_clock_irqtime(); /* disable if clock unstable. */
return 0;
}
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 7097de2c8cda..959a86206c64 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -12,6 +12,8 @@
#ifdef CONFIG_IRQ_TIME_ACCOUNTING
+DEFINE_STATIC_KEY_FALSE(sched_clock_irqtime);
+
/*
* There are no locks covering percpu hardirq/softirq time.
* They are only modified in vtime_account, on corresponding CPU
@@ -25,16 +27,14 @@
*/
DEFINE_PER_CPU(struct irqtime, cpu_irqtime);
-int sched_clock_irqtime;
-
void enable_sched_clock_irqtime(void)
{
- sched_clock_irqtime = 1;
+ static_branch_enable(&sched_clock_irqtime);
}
void disable_sched_clock_irqtime(void)
{
- sched_clock_irqtime = 0;
+ static_branch_disable(&sched_clock_irqtime);
}
static void irqtime_account_delta(struct irqtime *irqtime, u64 delta,
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index adfb6e3409d7..ec963314287a 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -3172,11 +3172,11 @@ struct irqtime {
};
DECLARE_PER_CPU(struct irqtime, cpu_irqtime);
-extern int sched_clock_irqtime;
+DECLARE_STATIC_KEY_FALSE(sched_clock_irqtime);
static inline int irqtime_enabled(void)
{
- return sched_clock_irqtime;
+ return static_branch_likely(&sched_clock_irqtime);
}
/*
--
2.47.3
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