hrtimers: Update new CPU's next event in hrtimers_cpu_dying()

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

Posted by Xiongfeng Wang 3 months ago

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);
 
-- 
2.20.1

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

Posted by Frederic Weisbecker 3 months ago

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;
-
 	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);

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

Posted by Xiongfeng Wang 3 months ago

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);
>  
> .
>

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

Posted by Frederic Weisbecker 3 months ago

Le Wed, Jul 09, 2025 at 11:18:14AM +0800, Xiongfeng Wang a écrit :
> 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.

Thanks! Feel free to send an updated version of the patch.

> > 
> > 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

I think it was also to remove extra base locking.

But then again, this is a slow path so this should be harmless.

Thanks.

-- 
Frederic Weisbecker
SUSE Labs

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

Posted by Wangshaobo (bobo) 3 months ago

-----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);

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

Posted by Wangshaobo (bobo) 3 months ago

在 2025/7/9 0:23, Wangshaobo (bobo) 写道:
> -----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);
>   
>