Re: [PATCH] hrtimer: Refresh softirq deadline after clock changes in low-res mode

[Frederic Weisbecker]

Le Sun, Nov 01, 2026 at 07:23:44PM +0200, Avi Weiss a écrit :
> In periodic low-resolution mode, hrtimer_run_queues() updates the clock
> base offsets at each tick but does not refresh softirq_expires_next.
> When CLOCK_REALTIME is moved forward, an expired soft hrtimer can
> therefore remain pending until the old monotonic deadline is reached.
> 
> Reevaluate the next expiry when hrtimer_update_base() observes a new
> clock-was-set sequence, before checking whether to raise HRTIMER_SOFTIRQ.
> 
> Fixes: 5da70160462e ("hrtimer: Implement support for softirq based hrtimers")
> Signed-off-by: Avi Weiss <thnkslprpt@xxxxxxxxx>
> ---
>  kernel/time/hrtimer.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
> index 17dd38a6cee7..5d1df371e931 100644
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -2289,6 +2289,7 @@ void hrtimer_run_queues(void)
>  	struct hrtimer_cpu_base *cpu_base = this_cpu_ptr(&hrtimer_bases);
>  	unsigned long flags;
>  	ktime_t now;
> +	unsigned int seq;
>  
>  	if (hrtimer_hres_active(cpu_base))
>  		return;
> @@ -2306,7 +2307,10 @@ void hrtimer_run_queues(void)
>  	}
>  
>  	raw_spin_lock_irqsave(&cpu_base->lock, flags);
> +	seq = cpu_base->clock_was_set_seq;
>  	now = hrtimer_update_base(cpu_base);
> +	if (seq != cpu_base->clock_was_set_seq)
> +		hrtimer_update_next_event(cpu_base);

It took some time for me to understand the issue because retrigger_next_event()
takes care of that already.

But if the clock is set and hrtimer_run_queues() happens to run before the base
is locked and update_needs_ipi() is called, the IPI won't happen.

It could be worth adding that to the changelog.

Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>

-- 
Frederic Weisbecker
SUSE Labs