linux-kernel - Re: [PATCH v2] tick/sched: Limit non-timekeeper CPUs calling jiffies update

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Message-ID: <bfa0a61e-7cb8-4bdd-b913-1bf241f316c7@linux.ibm.com>
Date: Tue, 28 Oct 2025 11:39:30 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Steve Wahl <steve.wahl@....com>
Cc: Russ Anderson <rja@....com>, Dimitri Sivanich <sivanich@....com>,
        Kyle Meyer <kyle.meyer@....com>,
        Anna-Maria Behnsen <anna-maria@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>,
        Ingo Molnar <mingo@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] tick/sched: Limit non-timekeeper CPUs calling jiffies
 update



On 10/28/25 12:04 AM, Steve Wahl wrote:
> On large NUMA systems, while running a test program that saturates the
> inter-processor and inter-NUMA links, acquiring the jiffies_lock can
> be very expensive.  If the cpu designated to do jiffies updates
> (tick_do_timer_cpu) gets delayed and other cpus decide to do the
> jiffies update themselves, a large number of them decide to do so at
> the same time.  The inexpensive check against tick_next_period is far
> quicker than actually acquiring the lock, so most of these get in line
> to obtain the lock.  If obtaining the lock is slow enough, this
> spirals into the vast majority of CPUs continuously being stuck
> waiting for this lock, just to obtain it and find out that time has
> already been updated by another cpu. For example, on one random entry
> to kdb by manually-injected NMI, I saw 2912 of 3840 cpus stuck here.
> 
> To avoid this, allow only one non-timekeeper CPU to call
> tick_do_update_jiffies64() at any given time, resetting ts->stalled
> jiffies only if the jiffies update function is actually called.
> 
> With this change, manually interrupting the test I find at most two
> CPUs in the tick_do_update_jiffies64 function (the timekeeper and one
> other).
> 
> Signed-off-by: Steve Wahl <steve.wahl@....com>
> ---
> 
> v2: Rewritten to use an atomic to gate non-timekeeping cpus calling the
>      jiffies update, as suggested by tglx. Title of patch has changed
>      since trylock is no longer used.
> 
> v1 discussion: https://lore.kernel.org/all/20251013150959.298288-1-steve.wahl@hpe.com/
> 
>   kernel/time/tick-sched.c | 30 ++++++++++++++++++++++++++----
>   1 file changed, 26 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index c527b421c865..3ff3eb1f90d0 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -201,6 +201,27 @@ static inline void tick_sched_flag_clear(struct tick_sched *ts,
>   	ts->flags &= ~flag;
>   }
>   
> +/*
> + * Allow only one non-timekeeper CPU at a time update jiffies from
> + * the timer tick.
> + *
> + * Returns true if update was run.
> + */
> +static bool tick_limited_update_jiffies64(struct tick_sched *ts, ktime_t now)
> +{
> +	static atomic_t in_progress;
> +	int inp;
> +
> +	inp = atomic_read(&in_progress);
> +	if (inp || !atomic_try_cmpxchg(&in_progress, &inp, 1))
> +		return false;
> +

You come here if (ts->last_tick_jiffies == jiffies). So it may be not necessary to check again.

> +	if (ts->last_tick_jiffies == jiffies)
> +		tick_do_update_jiffies64(now);
> +	atomic_set(&in_progress, 0);
> +	return true;
> +}
> +
>   #define MAX_STALLED_JIFFIES 5
>   
>   static void tick_sched_do_timer(struct tick_sched *ts, ktime_t now)
> @@ -239,10 +260,11 @@ static void tick_sched_do_timer(struct tick_sched *ts, ktime_t now)
>   		ts->stalled_jiffies = 0;
>   		ts->last_tick_jiffies = READ_ONCE(jiffies);
>   	} else {
> -		if (++ts->stalled_jiffies == MAX_STALLED_JIFFIES) {
> -			tick_do_update_jiffies64(now);
> -			ts->stalled_jiffies = 0;
> -			ts->last_tick_jiffies = READ_ONCE(jiffies);
> +		if (++ts->stalled_jiffies >= MAX_STALLED_JIFFIES) {
> +			if (tick_limited_update_jiffies64(ts, now)) {
> +				ts->stalled_jiffies = 0;
> +				ts->last_tick_jiffies = READ_ONCE(jiffies);
> +			}
>   		}
>   	}
>   


Yes. This could help large systems.

Acked-by: Shrikanth Hegde <sshegde@...ux.ibm.com>