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

[Xiongfeng Wang <wangxiongfeng2@huawei.com>]

Hi Frederic,

On 2025/7/8 20:40, Frederic Weisbecker wrote:
> 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:

This modification can solve the problem. Thanks a lot.

> 
> 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;

Once we remove this check. The problem disappears.
Also I noticed that we check whether highres is active before we really program
the hardware in __hrtimer_reprogram(). So if the above check in
retrigger_next_event() is just to decrease extra cost, I think we can remove it.

Thanks,
Xiongfeng

> -
>  	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);
>  
> .
>