linux-kernel - Re: [PATCH] hrtimers: Update new CPU's next event in hrtimers_cpu

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Message-ID: <aG0RxASJqknbaUkM@localhost.localdomain>
Date: Tue, 8 Jul 2025 14:40:36 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Xiongfeng Wang <wangxiongfeng2@...wei.com>
Cc: anna-maria@...utronix.de, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, xiexiuqi@...wei.com,
	bobo.shaobowang@...wei.com
Subject: Re: [PATCH] hrtimers: Update new CPU's next event in
 hrtimers_cpu_dying()

Le Tue, Jul 08, 2025 at 06:17:27PM +0800, Xiongfeng Wang a écrit :
> When testing softirq based hrtimers on an ARM32 board, with high
> resolution mode and nohz are both inactive, softirq based hrtimers
> failed to trigger when moved away from an offline CPU. The flowpath
> is as follows.
> 
> CPU0				CPU1
> 				softirq based hrtimers are queued
> 				offline CPU1
> 				move hrtimers to CPU0 in hrtimers_cpu_dying()
> 				send IPI to CPU0 to retrigger next event
> 'softirq_expires_next' is KTIME_MAX
> call retrigger_next_event()
> highres and nohz is inactive,just return
> 'softirq_expires_next' is not updated
> hrtimer softirq is never triggered
> 
> Some softirq based hrtimers are queued on CPU1. Then we offline CPU1.
> hrtimers_cpu_dying() moves hrtimers from CPU1 to CPU0, and then it send
> a IPI to CPU0 to let CPU0 call retrigger_next_event(). But high
> resolution mode and nohz are both inactive. So retrigger_next_event()
> just returned. 'softirq_expires_next' is never updated and remains
> KTIME_MAX. So hrtimer softirq is never raised.
> 
> To fix this issue, we call hrtimer_update_next_event() in
> hrtimers_cpu_dying() to update 'softirq_expires_next' for the new CPU.
> It also update hardirq hrtimer's next event, but it should have no bad
> effect.
> 
> Fixes: 5c0930ccaad5 ("hrtimers: Push pending hrtimers away from outgoing CPU earlier")
> Signed-off-by: Xiongfeng Wang <wangxiongfeng2@...wei.com>
> ---
>  kernel/time/hrtimer.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
> index 30899a8cc52c..ff97eb36c116 100644
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -2298,8 +2298,11 @@ int hrtimers_cpu_dying(unsigned int dying_cpu)
>  	/*
>  	 * The migration might have changed the first expiring softirq
>  	 * timer on this CPU. Update it.
> +	 * We also need to update 'softirq_expires_next' here, because it will
> +	 * not be updated in retrigger_next_event() if high resolution mode
> +	 * and nohz are both inactive.
>  	 */
> -	__hrtimer_get_next_event(new_base, HRTIMER_ACTIVE_SOFT);
> +	hrtimer_update_next_event(new_base);
>  	/* Tell the other CPU to retrigger the next event */
>  	smp_call_function_single(ncpu, retrigger_next_event, NULL, 0);

It seems that a similar problem can happen while enqueueing a timer
from an offline CPU (see the call to smp_call_function_single_async()).

How about this (untested) instead? retrigger_next_event, is not a fast
path so we don't care about rare extra cost:

diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 30899a8cc52c..e8c479329282 100644
--- a/kernel/time/hrtimer.c
+++ b/kernel/time/hrtimer.c
@@ -787,10 +787,10 @@ static void retrigger_next_event(void *arg)
 	 * of the next expiring timer is enough. The return from the SMP
 	 * function call will take care of the reprogramming in case the
 	 * CPU was in a NOHZ idle sleep.
+	 *
+	 * In periodic low resolution mode, the next softirq expiration
+	 * must also be updated.
 	 */
-	if (!hrtimer_hres_active(base) && !tick_nohz_active)
-		return;
-
 	raw_spin_lock(&base->lock);
 	hrtimer_update_base(base);
 	if (hrtimer_hres_active(base))
@@ -2295,11 +2295,6 @@ int hrtimers_cpu_dying(unsigned int dying_cpu)
 				     &new_base->clock_base[i]);
 	}
 
-	/*
-	 * The migration might have changed the first expiring softirq
-	 * timer on this CPU. Update it.
-	 */
-	__hrtimer_get_next_event(new_base, HRTIMER_ACTIVE_SOFT);
 	/* Tell the other CPU to retrigger the next event */
 	smp_call_function_single(ncpu, retrigger_next_event, NULL, 0);