Re: [PATCH v2] tick/nohz: Avoid unused timekeeping_max_deferment() calls

[Usama Arif <usama.arif@linux.dev>]

On Fri, 18 Sep 2026 06:34:08 -0700 Usama Arif <usama.arif@linux.dev> wrote:

> tick_nohz_next_event() limits a CPU's sleep interval to the maximum
> deferment supported by the current clocksource when that CPU owns the
> do_timer() duty. If the duty is unassigned, the limit also applies when
> the CPU's TS_FLAG_DO_TIMER_LAST flag is set.
> 
> After the early timer checks, the function currently reads the maximum
> deferment unconditionally. It then replaces the result with KTIME_MAX
> unless one of the two conditions above applies.
> 
> timekeeping_max_deferment() performs a seqcount-protected read of the
> shared timekeeper and follows its clocksource pointer. Check the do_timer
> state first and avoid this work when the result would be discarded. This
> leaves the resulting expiry unchanged and reduces accesses to timekeeper
> data that is modified regularly.
> 
> On x86-64 this removes 18-20 dynamically executed instructions, including
> the call, from the common non-owner path when the seqcount does not retry.
> 
> Signed-off-by: Usama Arif <usama.arif@linux.dev>
> ---
> v1 -> v2:
> - Remove the unnecessary comment and delta variable (Frederic Weisbecker).
> ---
>  kernel/time/tick-sched.c | 23 ++++++++++++-----------
>  1 file changed, 12 insertions(+), 11 deletions(-)

Hi,

Just wanted to check if there are any comments or reviews for this patch?

Thanks!
Usama

> 
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index c8f2c4a503b08..a7893a079a83f 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -816,7 +816,7 @@ u64 get_jiffies_update(unsigned long *basej)
>   */
>  static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
>  {
> -	u64 basemono, next_tick, delta, expires;
> +	u64 basemono, next_tick, expires;
>  	unsigned long basejiff;
>  	int tick_cpu;
>  
> @@ -856,8 +856,7 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
>  	 * If the tick is due in the next period, keep it ticking or
>  	 * force prod the timer.
>  	 */
> -	delta = next_tick - basemono;
> -	if (delta <= (u64)TICK_NSEC) {
> +	if (next_tick - basemono <= (u64)TICK_NSEC) {
>  		/*
>  		 * We've not stopped the tick yet, and there's a timer in the
>  		 * next period, so no point in stopping it either, bail.
> @@ -873,17 +872,19 @@ static ktime_t tick_nohz_next_event(struct tick_sched *ts, int cpu)
>  	 * the sleep time to the timekeeping 'max_deferment' value.
>  	 * Otherwise we can sleep as long as we want.
>  	 */
> -	delta = timekeeping_max_deferment();
>  	tick_cpu = READ_ONCE(tick_do_timer_cpu);
>  	if (tick_cpu != cpu &&
> -	    (tick_cpu != TICK_DO_TIMER_NONE || !tick_sched_flag_test(ts, TS_FLAG_DO_TIMER_LAST)))
> -		delta = KTIME_MAX;
> -
> -	/* Calculate the next expiry time */
> -	if (delta < (KTIME_MAX - basemono))
> -		expires = basemono + delta;
> -	else
> +	    (tick_cpu != TICK_DO_TIMER_NONE || !tick_sched_flag_test(ts, TS_FLAG_DO_TIMER_LAST))) {
>  		expires = KTIME_MAX;
> +	} else {
> +		expires = timekeeping_max_deferment();
> +
> +		/* Calculate the next expiry time */
> +		if (expires < (KTIME_MAX - basemono))
> +			expires += basemono;
> +		else
> +			expires = KTIME_MAX;
> +	}
>  
>  	ts->timer_expires = min_t(u64, expires, next_tick);
>  
> -- 
> 2.53.0-Meta
> 
>