答复: [PATCH] hrtimers: Update new CPU's next event in hrtimers_cpu_dying()

["Wangshaobo (bobo)" <bobo.shaobowang@huawei.com>]

-----邮件原件-----
发件人: Frederic Weisbecker [mailto:frederic@kernel.org] 
发送时间: 2025年7月8日 20:41
收件人: wangxiongfeng (C) <wangxiongfeng2@huawei.com>
抄送: anna-maria@linutronix.de; tglx@linutronix.de; linux-kernel@vger.kernel.org; Xiexiuqi <xiexiuqi@huawei.com>; Wangshaobo (bobo) <bobo.shaobowang@huawei.com>
主题: 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@huawei.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;

Could you explain in detail why this judgment is added? Is it due to security issues or efficiency impact?

- Wang ShaoBo


	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);