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Message-ID: <CAKfTPtDdfZkhE50iFqL=RmhjAWTFg7YfB+-oKB-SBM93O+aaGQ@mail.gmail.com>
Date: Tue, 3 Feb 2026 10:55:05 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Wangyang Guo <wangyang.guo@...el.com>
Cc: K Prateek Nayak <kprateek.nayak@....com>, Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
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>, linux-kernel@...r.kernel.org,
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: Re: [PATCH v7] sched/clock: Avoid false sharing for sched_clock_irqtime
On Tue, 27 Jan 2026 at 08:28, Wangyang Guo <wangyang.guo@...el.com> wrote:
>
> 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.
>
> The only user of disable_sched_clock_irqtime()
> is tsc_.*mark_unstable() which may be invoked under atomic context
> and require a workqueue to disable static_key. But both of them
> calls clear_sched_clock_stable() just before doing
> disable_sched_clock_irqtime(). We can reuse
> "sched_clock_work" to also disable sched_clock_irqtime().
>
> One additional case need to handle is if the tsc is marked unstable
> before late_initcall() phase, sched_clock_work will not be invoked
> and sched_clock_irqtime will stay enabled although clock is unstable:
> tsc_init()
> enable_sched_clock_irqtime() # irqtime accounting is enabled here
> ...
> if (unsynchronized_tsc()) # true
> mark_tsc_unstable()
> clear_sched_clock_stable()
> __sched_clock_stable_early = 0;
> ...
> if (static_key_count(&sched_clock_running.key) == 2)
> # Only happens at sched_clock_init_late()
> __clear_sched_clock_stable(); # Never executed
> ...
>
> # late_initcall() phase
> sched_clock_init_late()
> if (__sched_clock_stable_early) # Already false
> __set_sched_clock_stable(); # sched_clock is never marked stable
> # TSC unstable, but sched_clock_work won't run to disable irqtime
>
> So we need to disable_sched_clock_irqtime() in sched_clock_init_late()
> if clock is unstable.
>
> Reviewed-by: K Prateek Nayak <kprateek.nayak@....com>
> Tested-by: K Prateek Nayak <kprateek.nayak@....com>
> 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>
Acked-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
> v7 -> v6:
> - move irqtime_enabled() check to disable_sched_clock_irqtime()
>
> v6 -> v5:
> - Only disable_sched_clock_irqtime() if irqtime_enabled() in
> sched_lock_init_late() to avoid unnessary overhead.
>
> V5 -> v4:
> - Changelog update to reflect static_key changes
>
> 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:
> v6: https://lore.kernel.org/all/20260127044159.2254247-1-wangyang.guo@intel.com/
> v5: https://lore.kernel.org/all/20260127031602.1907377-1-wangyang.guo@intel.com/
> v4: https://lore.kernel.org/all/20260126021401.1490163-1-wangyang.guo@intel.com/
> 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
> ---
> arch/x86/kernel/tsc.c | 2 --
> kernel/sched/clock.c | 3 +++
> kernel/sched/cputime.c | 9 +++++----
> kernel/sched/sched.h | 4 ++--
> 4 files changed, 10 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..556a70f344d0 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 +27,15 @@
> */
> 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;
> + if (irqtime_enabled())
> + 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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