Commit f9a25f776d78 ("cpusets: Rebuild root domain deadline accounting information")
enabled rebuilding sched domain on cpuset and hotplug operations to
correct deadline accounting.

Rebuilding sched domain is a slow operation and we see 10+ ms delay on
suspend-resume because of that.

Since nothing is expected to change on suspend-resume operation; skip
rebuilding the sched domains to regain the time lost.

Debugged-by: Rick Yiu <rickyiu@google.com>
Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
---

    Changes in v2:
    
    	* Remove redundant check in update_tasks_root_domain() (Thanks Waiman)
    
    v1 link:
    
    	https://lore.kernel.org/lkml/20221216233501.gh6m75e7s66dmjgo@airbuntu/

 kernel/cgroup/cpuset.c  | 3 +++
 kernel/sched/deadline.c | 3 +++
 2 files changed, 6 insertions(+)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index a29c0b13706b..9a45f083459c 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1088,6 +1088,9 @@ static void rebuild_root_domains(void)
 	lockdep_assert_cpus_held();
 	lockdep_assert_held(&sched_domains_mutex);
 
+	if (cpuhp_tasks_frozen)
+		return;
+
 	rcu_read_lock();
 
 	/*
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 0d97d54276cc..42c1143a3956 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2575,6 +2575,9 @@ void dl_clear_root_domain(struct root_domain *rd)
 {
 	unsigned long flags;
 
+	if (cpuhp_tasks_frozen)
+		return;
+
 	raw_spin_lock_irqsave(&rd->dl_bw.lock, flags);
 	rd->dl_bw.total_bw = 0;
 	raw_spin_unlock_irqrestore(&rd->dl_bw.lock, flags);
-- 
2.25.1

On 1/20/23 14:48, Qais Yousef wrote:
> Commit f9a25f776d78 ("cpusets: Rebuild root domain deadline accounting information")
> enabled rebuilding sched domain on cpuset and hotplug operations to
> correct deadline accounting.
>
> Rebuilding sched domain is a slow operation and we see 10+ ms delay on
> suspend-resume because of that.
>
> Since nothing is expected to change on suspend-resume operation; skip
> rebuilding the sched domains to regain the time lost.
>
> Debugged-by: Rick Yiu <rickyiu@google.com>
> Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> ---
>
>      Changes in v2:
>      
>      	* Remove redundant check in update_tasks_root_domain() (Thanks Waiman)
>      
>      v1 link:
>      
>      	https://lore.kernel.org/lkml/20221216233501.gh6m75e7s66dmjgo@airbuntu/
>
>   kernel/cgroup/cpuset.c  | 3 +++
>   kernel/sched/deadline.c | 3 +++
>   2 files changed, 6 insertions(+)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index a29c0b13706b..9a45f083459c 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1088,6 +1088,9 @@ static void rebuild_root_domains(void)
>   	lockdep_assert_cpus_held();
>   	lockdep_assert_held(&sched_domains_mutex);
>   
> +	if (cpuhp_tasks_frozen)
> +		return;
> +
>   	rcu_read_lock();
>   
>   	/*
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 0d97d54276cc..42c1143a3956 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -2575,6 +2575,9 @@ void dl_clear_root_domain(struct root_domain *rd)
>   {
>   	unsigned long flags;
>   
> +	if (cpuhp_tasks_frozen)
> +		return;
> +
>   	raw_spin_lock_irqsave(&rd->dl_bw.lock, flags);
>   	rd->dl_bw.total_bw = 0;
>   	raw_spin_unlock_irqrestore(&rd->dl_bw.lock, flags);

cpuhp_tasks_frozen is set when thaw_secondary_cpus() or 
freeze_secondary_cpus() is called. I don't know the exact suspend/resume 
calling sequences, will cpuhp_tasks_frozen be cleared at the end of 
resume sequence? Maybe we should make sure that rebuild_root_domain() is 
called at least once at the end of resume operation.

Cheers,
Longman

On 01/20/23 17:16, Waiman Long wrote:
> 
> On 1/20/23 14:48, Qais Yousef wrote:
> > Commit f9a25f776d78 ("cpusets: Rebuild root domain deadline accounting information")
> > enabled rebuilding sched domain on cpuset and hotplug operations to
> > correct deadline accounting.
> > 
> > Rebuilding sched domain is a slow operation and we see 10+ ms delay on
> > suspend-resume because of that.
> > 
> > Since nothing is expected to change on suspend-resume operation; skip
> > rebuilding the sched domains to regain the time lost.
> > 
> > Debugged-by: Rick Yiu <rickyiu@google.com>
> > Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> > ---
> > 
> >      Changes in v2:
> >      	* Remove redundant check in update_tasks_root_domain() (Thanks Waiman)
> >      v1 link:
> >      	https://lore.kernel.org/lkml/20221216233501.gh6m75e7s66dmjgo@airbuntu/
> > 
> >   kernel/cgroup/cpuset.c  | 3 +++
> >   kernel/sched/deadline.c | 3 +++
> >   2 files changed, 6 insertions(+)
> > 
> > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> > index a29c0b13706b..9a45f083459c 100644
> > --- a/kernel/cgroup/cpuset.c
> > +++ b/kernel/cgroup/cpuset.c
> > @@ -1088,6 +1088,9 @@ static void rebuild_root_domains(void)
> >   	lockdep_assert_cpus_held();
> >   	lockdep_assert_held(&sched_domains_mutex);
> > +	if (cpuhp_tasks_frozen)
> > +		return;
> > +
> >   	rcu_read_lock();
> >   	/*
> > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > index 0d97d54276cc..42c1143a3956 100644
> > --- a/kernel/sched/deadline.c
> > +++ b/kernel/sched/deadline.c
> > @@ -2575,6 +2575,9 @@ void dl_clear_root_domain(struct root_domain *rd)
> >   {
> >   	unsigned long flags;
> > +	if (cpuhp_tasks_frozen)
> > +		return;
> > +
> >   	raw_spin_lock_irqsave(&rd->dl_bw.lock, flags);
> >   	rd->dl_bw.total_bw = 0;
> >   	raw_spin_unlock_irqrestore(&rd->dl_bw.lock, flags);
> 
> cpuhp_tasks_frozen is set when thaw_secondary_cpus() or
> freeze_secondary_cpus() is called. I don't know the exact suspend/resume
> calling sequences, will cpuhp_tasks_frozen be cleared at the end of resume
> sequence? Maybe we should make sure that rebuild_root_domain() is called at
> least once at the end of resume operation.

Very good questions. It made me look at the logic again and I realize now that
the way force_build behaves is causing this issue.

I *think* we should just make the call rebuild_root_domains() only if
cpus_updated in cpuset_hotplug_workfn().

cpuset_cpu_active() seems to be the source of force_rebuild in my case; which
seems to be called only after the last cpu is back online (what you suggest).
In this case we can end up with cpus_updated = false, but force_rebuild = true.

Now you added a couple of new users to force_rebuild in 4b842da276a8a; I'm
trying to figure out what the conditions would be there. It seems we can have
corner cases for cpus_update might not trigger correctly?

Could the below be a good cure?

AFAICT we must rebuild the root domains if something has changed in cpuset.
Which should be captured by either having:

	* cpus_updated = true
	* force_rebuild && !cpuhp_tasks_frozen

/me goes to test the patch

--->8---

	diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
	index a29c0b13706b..363e4459559f 100644
	--- a/kernel/cgroup/cpuset.c
	+++ b/kernel/cgroup/cpuset.c
	@@ -1079,6 +1079,8 @@ static void update_tasks_root_domain(struct cpuset *cs)
		css_task_iter_end(&it);
	 }

	+static bool need_rebuild_rd = true;
	+
	 static void rebuild_root_domains(void)
	 {
		struct cpuset *cs = NULL;
	@@ -1088,6 +1090,9 @@ static void rebuild_root_domains(void)
		lockdep_assert_cpus_held();
		lockdep_assert_held(&sched_domains_mutex);

	+       if (!need_rebuild_rd)
	+               return;
	+
		rcu_read_lock();

		/*
	@@ -3627,7 +3632,9 @@ static void cpuset_hotplug_workfn(struct work_struct *work)
		/* rebuild sched domains if cpus_allowed has changed */
		if (cpus_updated || force_rebuild) {
			force_rebuild = false;
	+               need_rebuild_rd = cpus_updated || (force_rebuild && !cpuhp_tasks_frozen);
			rebuild_sched_domains();
	+               need_rebuild_rd = true;
		}

		free_cpumasks(NULL, ptmp);


--->8---

Thanks!

--
Qais Yousef

On 1/25/23 11:35, Qais Yousef wrote:
> On 01/20/23 17:16, Waiman Long wrote:
>> On 1/20/23 14:48, Qais Yousef wrote:
>>> Commit f9a25f776d78 ("cpusets: Rebuild root domain deadline accounting information")
>>> enabled rebuilding sched domain on cpuset and hotplug operations to
>>> correct deadline accounting.
>>>
>>> Rebuilding sched domain is a slow operation and we see 10+ ms delay on
>>> suspend-resume because of that.
>>>
>>> Since nothing is expected to change on suspend-resume operation; skip
>>> rebuilding the sched domains to regain the time lost.
>>>
>>> Debugged-by: Rick Yiu <rickyiu@google.com>
>>> Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
>>> ---
>>>
>>>       Changes in v2:
>>>       	* Remove redundant check in update_tasks_root_domain() (Thanks Waiman)
>>>       v1 link:
>>>       	https://lore.kernel.org/lkml/20221216233501.gh6m75e7s66dmjgo@airbuntu/
>>>
>>>    kernel/cgroup/cpuset.c  | 3 +++
>>>    kernel/sched/deadline.c | 3 +++
>>>    2 files changed, 6 insertions(+)
>>>
>>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>>> index a29c0b13706b..9a45f083459c 100644
>>> --- a/kernel/cgroup/cpuset.c
>>> +++ b/kernel/cgroup/cpuset.c
>>> @@ -1088,6 +1088,9 @@ static void rebuild_root_domains(void)
>>>    	lockdep_assert_cpus_held();
>>>    	lockdep_assert_held(&sched_domains_mutex);
>>> +	if (cpuhp_tasks_frozen)
>>> +		return;
>>> +
>>>    	rcu_read_lock();
>>>    	/*
>>> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
>>> index 0d97d54276cc..42c1143a3956 100644
>>> --- a/kernel/sched/deadline.c
>>> +++ b/kernel/sched/deadline.c
>>> @@ -2575,6 +2575,9 @@ void dl_clear_root_domain(struct root_domain *rd)
>>>    {
>>>    	unsigned long flags;
>>> +	if (cpuhp_tasks_frozen)
>>> +		return;
>>> +
>>>    	raw_spin_lock_irqsave(&rd->dl_bw.lock, flags);
>>>    	rd->dl_bw.total_bw = 0;
>>>    	raw_spin_unlock_irqrestore(&rd->dl_bw.lock, flags);
>> cpuhp_tasks_frozen is set when thaw_secondary_cpus() or
>> freeze_secondary_cpus() is called. I don't know the exact suspend/resume
>> calling sequences, will cpuhp_tasks_frozen be cleared at the end of resume
>> sequence? Maybe we should make sure that rebuild_root_domain() is called at
>> least once at the end of resume operation.
> Very good questions. It made me look at the logic again and I realize now that
> the way force_build behaves is causing this issue.
>
> I *think* we should just make the call rebuild_root_domains() only if
> cpus_updated in cpuset_hotplug_workfn().
>
> cpuset_cpu_active() seems to be the source of force_rebuild in my case; which
> seems to be called only after the last cpu is back online (what you suggest).
> In this case we can end up with cpus_updated = false, but force_rebuild = true.
>
> Now you added a couple of new users to force_rebuild in 4b842da276a8a; I'm
> trying to figure out what the conditions would be there. It seems we can have
> corner cases for cpus_update might not trigger correctly?
>
> Could the below be a good cure?
>
> AFAICT we must rebuild the root domains if something has changed in cpuset.
> Which should be captured by either having:
>
> 	* cpus_updated = true
> 	* force_rebuild && !cpuhp_tasks_frozen
>
> /me goes to test the patch
>
> --->8---
>
> 	diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> 	index a29c0b13706b..363e4459559f 100644
> 	--- a/kernel/cgroup/cpuset.c
> 	+++ b/kernel/cgroup/cpuset.c
> 	@@ -1079,6 +1079,8 @@ static void update_tasks_root_domain(struct cpuset *cs)
> 		css_task_iter_end(&it);
> 	 }
>
> 	+static bool need_rebuild_rd = true;
> 	+
> 	 static void rebuild_root_domains(void)
> 	 {
> 		struct cpuset *cs = NULL;
> 	@@ -1088,6 +1090,9 @@ static void rebuild_root_domains(void)
> 		lockdep_assert_cpus_held();
> 		lockdep_assert_held(&sched_domains_mutex);
>
> 	+       if (!need_rebuild_rd)
> 	+               return;
> 	+
> 		rcu_read_lock();
>
> 		/*
> 	@@ -3627,7 +3632,9 @@ static void cpuset_hotplug_workfn(struct work_struct *work)
> 		/* rebuild sched domains if cpus_allowed has changed */
> 		if (cpus_updated || force_rebuild) {
> 			force_rebuild = false;
> 	+               need_rebuild_rd = cpus_updated || (force_rebuild && !cpuhp_tasks_frozen);
> 			rebuild_sched_domains();
> 	+               need_rebuild_rd = true;

You do the force_check check after it is set to false in the previous 
statement which is definitely not correct. So it will be false whenever 
cpus_updated is false.

If you just want to skip rebuild_sched_domains() call for hotplug, why 
don't just skip the call here if the condition is right? Like

         /* rebuild sched domains if cpus_allowed has changed */
         if (cpus_updated || (force_rebuild && !cpuhp_tasks_frozen)) {
                 force_rebuild = false;
                 rebuild_sched_domains();
         }

Still, we will need to confirm that cpuhp_tasks_frozen will be cleared 
outside of the suspend/resume cycle.

Cheers,
Longman

On 01/29/23 21:49, Waiman Long wrote:
> On 1/25/23 11:35, Qais Yousef wrote:
> > On 01/20/23 17:16, Waiman Long wrote:
> > > On 1/20/23 14:48, Qais Yousef wrote:
> > > > Commit f9a25f776d78 ("cpusets: Rebuild root domain deadline accounting information")
> > > > enabled rebuilding sched domain on cpuset and hotplug operations to
> > > > correct deadline accounting.
> > > > 
> > > > Rebuilding sched domain is a slow operation and we see 10+ ms delay on
> > > > suspend-resume because of that.
> > > > 
> > > > Since nothing is expected to change on suspend-resume operation; skip
> > > > rebuilding the sched domains to regain the time lost.
> > > > 
> > > > Debugged-by: Rick Yiu <rickyiu@google.com>
> > > > Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> > > > ---
> > > > 
> > > >       Changes in v2:
> > > >       	* Remove redundant check in update_tasks_root_domain() (Thanks Waiman)
> > > >       v1 link:
> > > >       	https://lore.kernel.org/lkml/20221216233501.gh6m75e7s66dmjgo@airbuntu/
> > > > 
> > > >    kernel/cgroup/cpuset.c  | 3 +++
> > > >    kernel/sched/deadline.c | 3 +++
> > > >    2 files changed, 6 insertions(+)
> > > > 
> > > > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> > > > index a29c0b13706b..9a45f083459c 100644
> > > > --- a/kernel/cgroup/cpuset.c
> > > > +++ b/kernel/cgroup/cpuset.c
> > > > @@ -1088,6 +1088,9 @@ static void rebuild_root_domains(void)
> > > >    	lockdep_assert_cpus_held();
> > > >    	lockdep_assert_held(&sched_domains_mutex);
> > > > +	if (cpuhp_tasks_frozen)
> > > > +		return;
> > > > +
> > > >    	rcu_read_lock();
> > > >    	/*
> > > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > > > index 0d97d54276cc..42c1143a3956 100644
> > > > --- a/kernel/sched/deadline.c
> > > > +++ b/kernel/sched/deadline.c
> > > > @@ -2575,6 +2575,9 @@ void dl_clear_root_domain(struct root_domain *rd)
> > > >    {
> > > >    	unsigned long flags;
> > > > +	if (cpuhp_tasks_frozen)
> > > > +		return;
> > > > +
> > > >    	raw_spin_lock_irqsave(&rd->dl_bw.lock, flags);
> > > >    	rd->dl_bw.total_bw = 0;
> > > >    	raw_spin_unlock_irqrestore(&rd->dl_bw.lock, flags);
> > > cpuhp_tasks_frozen is set when thaw_secondary_cpus() or
> > > freeze_secondary_cpus() is called. I don't know the exact suspend/resume
> > > calling sequences, will cpuhp_tasks_frozen be cleared at the end of resume
> > > sequence? Maybe we should make sure that rebuild_root_domain() is called at
> > > least once at the end of resume operation.
> > Very good questions. It made me look at the logic again and I realize now that
> > the way force_build behaves is causing this issue.
> > 
> > I *think* we should just make the call rebuild_root_domains() only if
> > cpus_updated in cpuset_hotplug_workfn().
> > 
> > cpuset_cpu_active() seems to be the source of force_rebuild in my case; which
> > seems to be called only after the last cpu is back online (what you suggest).
> > In this case we can end up with cpus_updated = false, but force_rebuild = true.
> > 
> > Now you added a couple of new users to force_rebuild in 4b842da276a8a; I'm
> > trying to figure out what the conditions would be there. It seems we can have
> > corner cases for cpus_update might not trigger correctly?
> > 
> > Could the below be a good cure?
> > 
> > AFAICT we must rebuild the root domains if something has changed in cpuset.
> > Which should be captured by either having:
> > 
> > 	* cpus_updated = true
> > 	* force_rebuild && !cpuhp_tasks_frozen
> > 
> > /me goes to test the patch
> > 
> > --->8---
> > 
> > 	diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> > 	index a29c0b13706b..363e4459559f 100644
> > 	--- a/kernel/cgroup/cpuset.c
> > 	+++ b/kernel/cgroup/cpuset.c
> > 	@@ -1079,6 +1079,8 @@ static void update_tasks_root_domain(struct cpuset *cs)
> > 		css_task_iter_end(&it);
> > 	 }
> > 
> > 	+static bool need_rebuild_rd = true;
> > 	+
> > 	 static void rebuild_root_domains(void)
> > 	 {
> > 		struct cpuset *cs = NULL;
> > 	@@ -1088,6 +1090,9 @@ static void rebuild_root_domains(void)
> > 		lockdep_assert_cpus_held();
> > 		lockdep_assert_held(&sched_domains_mutex);
> > 
> > 	+       if (!need_rebuild_rd)
> > 	+               return;
> > 	+
> > 		rcu_read_lock();
> > 
> > 		/*
> > 	@@ -3627,7 +3632,9 @@ static void cpuset_hotplug_workfn(struct work_struct *work)
> > 		/* rebuild sched domains if cpus_allowed has changed */
> > 		if (cpus_updated || force_rebuild) {
> > 			force_rebuild = false;
> > 	+               need_rebuild_rd = cpus_updated || (force_rebuild && !cpuhp_tasks_frozen);
> > 			rebuild_sched_domains();
> > 	+               need_rebuild_rd = true;
> 
> You do the force_check check after it is set to false in the previous
> statement which is definitely not correct. So it will be false whenever
> cpus_updated is false.
> 
> If you just want to skip rebuild_sched_domains() call for hotplug, why don't

We just need to skip rebuild_root_domains(). I think rebuild_sched_domains()
should still happen.

The issue, AFAIU, is that we assume this hotplug operation results in changes
in cpuset and the DEADLINE accounting is now wrong and must be re-calculated.
But s2ram will cause hotplug operation without actually changing the cpuset
configuration - the re-calculation is not required. But it'd be good to get
a confirmation from Juri.

> just skip the call here if the condition is right? Like
> 
>         /* rebuild sched domains if cpus_allowed has changed */
>         if (cpus_updated || (force_rebuild && !cpuhp_tasks_frozen)) {
>                 force_rebuild = false;
>                 rebuild_sched_domains();
>         }
> 
> Still, we will need to confirm that cpuhp_tasks_frozen will be cleared
> outside of the suspend/resume cycle.

I think it's fine to use this variable from the cpuhp callback context only.
Which I think this cpuset workfn is considered an extension of.

But you're right, I can't use cpuhp_tasks_frozen directly in
rebuild_root_domains() as I did in v1 because it doesn't get cleared after
calling the last _cpu_up(). force_rebuild will only be set after the last cpu
is brought online though - so this should happen once at the end.

(will update the comment too)

It seems I still need more time to study the code. What appeared simple, looks
is actually not..


Cheers

--
Qais Yousef

On 1/29/23 21:49, Waiman Long wrote:
> On 1/25/23 11:35, Qais Yousef wrote:
>> On 01/20/23 17:16, Waiman Long wrote:
>>> On 1/20/23 14:48, Qais Yousef wrote:
>>>> Commit f9a25f776d78 ("cpusets: Rebuild root domain deadline 
>>>> accounting information")
>>>> enabled rebuilding sched domain on cpuset and hotplug operations to
>>>> correct deadline accounting.
>>>>
>>>> Rebuilding sched domain is a slow operation and we see 10+ ms delay on
>>>> suspend-resume because of that.
>>>>
>>>> Since nothing is expected to change on suspend-resume operation; skip
>>>> rebuilding the sched domains to regain the time lost.
>>>>
>>>> Debugged-by: Rick Yiu <rickyiu@google.com>
>>>> Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
>>>> ---
>>>>
>>>>       Changes in v2:
>>>>           * Remove redundant check in update_tasks_root_domain() 
>>>> (Thanks Waiman)
>>>>       v1 link:
>>>> https://lore.kernel.org/lkml/20221216233501.gh6m75e7s66dmjgo@airbuntu/
>>>>
>>>>    kernel/cgroup/cpuset.c  | 3 +++
>>>>    kernel/sched/deadline.c | 3 +++
>>>>    2 files changed, 6 insertions(+)
>>>>
>>>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>>>> index a29c0b13706b..9a45f083459c 100644
>>>> --- a/kernel/cgroup/cpuset.c
>>>> +++ b/kernel/cgroup/cpuset.c
>>>> @@ -1088,6 +1088,9 @@ static void rebuild_root_domains(void)
>>>>        lockdep_assert_cpus_held();
>>>>        lockdep_assert_held(&sched_domains_mutex);
>>>> +    if (cpuhp_tasks_frozen)
>>>> +        return;
>>>> +
>>>>        rcu_read_lock();
>>>>        /*
>>>> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
>>>> index 0d97d54276cc..42c1143a3956 100644
>>>> --- a/kernel/sched/deadline.c
>>>> +++ b/kernel/sched/deadline.c
>>>> @@ -2575,6 +2575,9 @@ void dl_clear_root_domain(struct root_domain 
>>>> *rd)
>>>>    {
>>>>        unsigned long flags;
>>>> +    if (cpuhp_tasks_frozen)
>>>> +        return;
>>>> +
>>>>        raw_spin_lock_irqsave(&rd->dl_bw.lock, flags);
>>>>        rd->dl_bw.total_bw = 0;
>>>>        raw_spin_unlock_irqrestore(&rd->dl_bw.lock, flags);
>>> cpuhp_tasks_frozen is set when thaw_secondary_cpus() or
>>> freeze_secondary_cpus() is called. I don't know the exact 
>>> suspend/resume
>>> calling sequences, will cpuhp_tasks_frozen be cleared at the end of 
>>> resume
>>> sequence? Maybe we should make sure that rebuild_root_domain() is 
>>> called at
>>> least once at the end of resume operation.
>> Very good questions. It made me look at the logic again and I realize 
>> now that
>> the way force_build behaves is causing this issue.
>>
>> I *think* we should just make the call rebuild_root_domains() only if
>> cpus_updated in cpuset_hotplug_workfn().
>>
>> cpuset_cpu_active() seems to be the source of force_rebuild in my 
>> case; which
>> seems to be called only after the last cpu is back online (what you 
>> suggest).
>> In this case we can end up with cpus_updated = false, but 
>> force_rebuild = true.
>>
>> Now you added a couple of new users to force_rebuild in 
>> 4b842da276a8a; I'm
>> trying to figure out what the conditions would be there. It seems we 
>> can have
>> corner cases for cpus_update might not trigger correctly?
>>
>> Could the below be a good cure?
>>
>> AFAICT we must rebuild the root domains if something has changed in 
>> cpuset.
>> Which should be captured by either having:
>>
>>     * cpus_updated = true
>>     * force_rebuild && !cpuhp_tasks_frozen
>>
>> /me goes to test the patch
>>
>> --->8---
>>
>>     diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>>     index a29c0b13706b..363e4459559f 100644
>>     --- a/kernel/cgroup/cpuset.c
>>     +++ b/kernel/cgroup/cpuset.c
>>     @@ -1079,6 +1079,8 @@ static void update_tasks_root_domain(struct 
>> cpuset *cs)
>>         css_task_iter_end(&it);
>>      }
>>
>>     +static bool need_rebuild_rd = true;
>>     +
>>      static void rebuild_root_domains(void)
>>      {
>>         struct cpuset *cs = NULL;
>>     @@ -1088,6 +1090,9 @@ static void rebuild_root_domains(void)
>>         lockdep_assert_cpus_held();
>>         lockdep_assert_held(&sched_domains_mutex);
>>
>>     +       if (!need_rebuild_rd)
>>     +               return;
>>     +
>>         rcu_read_lock();
>>
>>         /*
>>     @@ -3627,7 +3632,9 @@ static void cpuset_hotplug_workfn(struct 
>> work_struct *work)
>>         /* rebuild sched domains if cpus_allowed has changed */
>>         if (cpus_updated || force_rebuild) {
>>             force_rebuild = false;
>>     +               need_rebuild_rd = cpus_updated || (force_rebuild 
>> && !cpuhp_tasks_frozen);
>>             rebuild_sched_domains();
>>     +               need_rebuild_rd = true;
>
> You do the force_check check after it is set to false in the previous 
> statement which is definitely not correct. So it will be false 
> whenever cpus_updated is false.
>
> If you just want to skip rebuild_sched_domains() call for hotplug, why 
> don't just skip the call here if the condition is right? Like
>
>         /* rebuild sched domains if cpus_allowed has changed */
>         if (cpus_updated || (force_rebuild && !cpuhp_tasks_frozen)) {
>                 force_rebuild = false;
>                 rebuild_sched_domains();
>         }
>
> Still, we will need to confirm that cpuhp_tasks_frozen will be cleared 
> outside of the suspend/resume cycle.

BTW, you also need to expand the comment to explain why we need to check 
for cpuhp_tasks_frozen.

Cheers,
Longman

On 01/25/23 16:35, Qais Yousef wrote:
> On 01/20/23 17:16, Waiman Long wrote:
> > 
> > On 1/20/23 14:48, Qais Yousef wrote:
> > > Commit f9a25f776d78 ("cpusets: Rebuild root domain deadline accounting information")
> > > enabled rebuilding sched domain on cpuset and hotplug operations to
> > > correct deadline accounting.
> > > 
> > > Rebuilding sched domain is a slow operation and we see 10+ ms delay on
> > > suspend-resume because of that.
> > > 
> > > Since nothing is expected to change on suspend-resume operation; skip
> > > rebuilding the sched domains to regain the time lost.
> > > 
> > > Debugged-by: Rick Yiu <rickyiu@google.com>
> > > Signed-off-by: Qais Yousef (Google) <qyousef@layalina.io>
> > > ---
> > > 
> > >      Changes in v2:
> > >      	* Remove redundant check in update_tasks_root_domain() (Thanks Waiman)
> > >      v1 link:
> > >      	https://lore.kernel.org/lkml/20221216233501.gh6m75e7s66dmjgo@airbuntu/
> > > 
> > >   kernel/cgroup/cpuset.c  | 3 +++
> > >   kernel/sched/deadline.c | 3 +++
> > >   2 files changed, 6 insertions(+)
> > > 
> > > diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> > > index a29c0b13706b..9a45f083459c 100644
> > > --- a/kernel/cgroup/cpuset.c
> > > +++ b/kernel/cgroup/cpuset.c
> > > @@ -1088,6 +1088,9 @@ static void rebuild_root_domains(void)
> > >   	lockdep_assert_cpus_held();
> > >   	lockdep_assert_held(&sched_domains_mutex);
> > > +	if (cpuhp_tasks_frozen)
> > > +		return;
> > > +
> > >   	rcu_read_lock();
> > >   	/*
> > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > > index 0d97d54276cc..42c1143a3956 100644
> > > --- a/kernel/sched/deadline.c
> > > +++ b/kernel/sched/deadline.c
> > > @@ -2575,6 +2575,9 @@ void dl_clear_root_domain(struct root_domain *rd)
> > >   {
> > >   	unsigned long flags;
> > > +	if (cpuhp_tasks_frozen)
> > > +		return;
> > > +
> > >   	raw_spin_lock_irqsave(&rd->dl_bw.lock, flags);
> > >   	rd->dl_bw.total_bw = 0;
> > >   	raw_spin_unlock_irqrestore(&rd->dl_bw.lock, flags);
> > 
> > cpuhp_tasks_frozen is set when thaw_secondary_cpus() or
> > freeze_secondary_cpus() is called. I don't know the exact suspend/resume
> > calling sequences, will cpuhp_tasks_frozen be cleared at the end of resume
> > sequence? Maybe we should make sure that rebuild_root_domain() is called at
> > least once at the end of resume operation.
> 
> Very good questions. It made me look at the logic again and I realize now that
> the way force_build behaves is causing this issue.
> 
> I *think* we should just make the call rebuild_root_domains() only if
> cpus_updated in cpuset_hotplug_workfn().
> 
> cpuset_cpu_active() seems to be the source of force_rebuild in my case; which
> seems to be called only after the last cpu is back online (what you suggest).
> In this case we can end up with cpus_updated = false, but force_rebuild = true.
> 
> Now you added a couple of new users to force_rebuild in 4b842da276a8a; I'm
> trying to figure out what the conditions would be there. It seems we can have
> corner cases for cpus_update might not trigger correctly?
> 
> Could the below be a good cure?
> 
> AFAICT we must rebuild the root domains if something has changed in cpuset.
> Which should be captured by either having:
> 
> 	* cpus_updated = true
> 	* force_rebuild && !cpuhp_tasks_frozen
> 
> /me goes to test the patch

It works fine.

Can we assume cpus_udpated will always be false in suspend/resume cycle? I can
then check for (force_rebuild && !cpuhp_tasks_frozen) directly in
rebuild_root_domains().


Thanks!

--
Qais Yousef

> 
> --->8---
> 
> 	diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> 	index a29c0b13706b..363e4459559f 100644
> 	--- a/kernel/cgroup/cpuset.c
> 	+++ b/kernel/cgroup/cpuset.c
> 	@@ -1079,6 +1079,8 @@ static void update_tasks_root_domain(struct cpuset *cs)
> 		css_task_iter_end(&it);
> 	 }
> 
> 	+static bool need_rebuild_rd = true;
> 	+
> 	 static void rebuild_root_domains(void)
> 	 {
> 		struct cpuset *cs = NULL;
> 	@@ -1088,6 +1090,9 @@ static void rebuild_root_domains(void)
> 		lockdep_assert_cpus_held();
> 		lockdep_assert_held(&sched_domains_mutex);
> 
> 	+       if (!need_rebuild_rd)
> 	+               return;
> 	+
> 		rcu_read_lock();
> 
> 		/*
> 	@@ -3627,7 +3632,9 @@ static void cpuset_hotplug_workfn(struct work_struct *work)
> 		/* rebuild sched domains if cpus_allowed has changed */
> 		if (cpus_updated || force_rebuild) {
> 			force_rebuild = false;
> 	+               need_rebuild_rd = cpus_updated || (force_rebuild && !cpuhp_tasks_frozen);
> 			rebuild_sched_domains();
> 	+               need_rebuild_rd = true;
> 		}
> 
> 		free_cpumasks(NULL, ptmp);
> 
> 
> --->8---
> 
> Thanks!
> 
> --
> Qais Yousef