Re: [PATCH v8 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()

[Juri Lelli]

Hi,

On 22/08/17 14:53, Byungchul Park wrote:
> On Mon, Aug 21, 2017 at 02:44:58PM +0100, Juri Lelli wrote:
> > Hi,
> > On 18/08/17 17:21, Byungchul Park wrote:
> > > It would be better to try to check other siblings first if
> > > SD_PREFER_SIBLING is flaged when pushing tasks - migration.
> > > 
> > > Signed-off-by: Byungchul Park 
> > 
> > Mmm, this looks like Peter's proposed patch, maybe add (at least) a
> > Suggested-by: him ?
> 
> Hi Juri,
> 
> Why not. I will add it from the next spin.
> 
> BTW, is it enough? I don't know the way I should do, whenever I got
> thankful suggestions. I really want to add them as a separate patch
> which can be stacked on my patches _if possible_. But in case that
> it's better to merge them into one like this, I don't know how.
> 

It usually depends on the entity of the suggestion (is it expressed with
a sentence, actual code or a proper patch?) and what the person
suggesting it is fine with. I usually simply ask. :)

> I mean I will add 'Suggested-by' from now on - I learned what I should
> do (at least) in this case thanks to Juri, but I'm still not sure if
> it's enough.
> 
> Speaking of which, I have something to ask Peterz and Ingo for. I really
> want to interact with maintainers actively e.g. asking ways they prefer.
> But it takes too much long to get responses from them e.g. at most 2
> monthes in case rushing them. I should have decided and done what the
> best I think is, than asking.
> 
> It would be very appriciated if you pay more attention.
> 

I try to be as responsive as I can (I think this applies to everyone)
and I apologize if from time to time it takes too much to reply. Balance
between upstream and product work means that there are times when one of
the two gets a bit delayed, I'm afraid.

Please keep asking questions, propose solutions and chase people! :)

> > > @@ -1376,8 +1399,7 @@ static int find_later_rq(struct task_struct *task)
> > >   return this_cpu;
> > >   }
> > >  
> > > - best_cpu = cpumask_first_and(later_mask,
> > > - sched_domain_span(sd));
> > > + best_cpu = find_cpu(later_mask, sd, prefer);
> > >   /*
> > >* Last chance: if a cpu being in both later_mask
> > >* and current sd span is valid, that becomes our
> > > @@ -1385,6 +1407,26 @@ static int find_later_rq(struct task_struct *task)
> > >* already under consideration through later_mask.
> > >*/
> > 
> > It seems that the comment above should be updated as well.
> 
> How? Could you explain it more?
> 

Simply removing it might just work.

Thanks,

- Juri

Re: [PATCH v8 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()

[Juri Lelli]

Hi,

On 22/08/17 14:53, Byungchul Park wrote:
> On Mon, Aug 21, 2017 at 02:44:58PM +0100, Juri Lelli wrote:
> > Hi,
> > On 18/08/17 17:21, Byungchul Park wrote:
> > > It would be better to try to check other siblings first if
> > > SD_PREFER_SIBLING is flaged when pushing tasks - migration.
> > > 
> > > Signed-off-by: Byungchul Park 
> > 
> > Mmm, this looks like Peter's proposed patch, maybe add (at least) a
> > Suggested-by: him ?
> 
> Hi Juri,
> 
> Why not. I will add it from the next spin.
> 
> BTW, is it enough? I don't know the way I should do, whenever I got
> thankful suggestions. I really want to add them as a separate patch
> which can be stacked on my patches _if possible_. But in case that
> it's better to merge them into one like this, I don't know how.
> 

It usually depends on the entity of the suggestion (is it expressed with
a sentence, actual code or a proper patch?) and what the person
suggesting it is fine with. I usually simply ask. :)

> I mean I will add 'Suggested-by' from now on - I learned what I should
> do (at least) in this case thanks to Juri, but I'm still not sure if
> it's enough.
> 
> Speaking of which, I have something to ask Peterz and Ingo for. I really
> want to interact with maintainers actively e.g. asking ways they prefer.
> But it takes too much long to get responses from them e.g. at most 2
> monthes in case rushing them. I should have decided and done what the
> best I think is, than asking.
> 
> It would be very appriciated if you pay more attention.
> 

I try to be as responsive as I can (I think this applies to everyone)
and I apologize if from time to time it takes too much to reply. Balance
between upstream and product work means that there are times when one of
the two gets a bit delayed, I'm afraid.

Please keep asking questions, propose solutions and chase people! :)

> > > @@ -1376,8 +1399,7 @@ static int find_later_rq(struct task_struct *task)
> > >   return this_cpu;
> > >   }
> > >  
> > > - best_cpu = cpumask_first_and(later_mask,
> > > - sched_domain_span(sd));
> > > + best_cpu = find_cpu(later_mask, sd, prefer);
> > >   /*
> > >* Last chance: if a cpu being in both later_mask
> > >* and current sd span is valid, that becomes our
> > > @@ -1385,6 +1407,26 @@ static int find_later_rq(struct task_struct *task)
> > >* already under consideration through later_mask.
> > >*/
> > 
> > It seems that the comment above should be updated as well.
> 
> How? Could you explain it more?
> 

Simply removing it might just work.

Thanks,

- Juri

Re: [PATCH v8 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()

[Byungchul Park]

On Tue, Aug 22, 2017 at 02:53:25PM +0900, Byungchul Park wrote:
> On Mon, Aug 21, 2017 at 02:44:58PM +0100, Juri Lelli wrote:
> > Hi,
> > On 18/08/17 17:21, Byungchul Park wrote:
> > > It would be better to try to check other siblings first if
> > > SD_PREFER_SIBLING is flaged when pushing tasks - migration.
> > > 
> > > Signed-off-by: Byungchul Park 
> > 
> > Mmm, this looks like Peter's proposed patch, maybe add (at least) a
> > Suggested-by: him ?
> 
> Hi Juri,
> 
> Why not. I will add it from the next spin.
> 
> BTW, is it enough? I don't know the way I should do, whenever I got
> thankful suggestions. I really want to add them as a separate patch
> which can be stacked on my patches _if possible_. But in case that
> it's better to merge them into one like this, I don't know how.
> 
> I mean I will add 'Suggested-by' from now on - I learned what I should
> do (at least) in this case thanks to Juri, but I'm still not sure if
> it's enough.
> 
> Speaking of which, I have something to ask Peterz and Ingo for. I really
> want to interact with maintainers actively e.g. asking ways they prefer.
> But it takes too much long to get responses from them e.g. at most 2
> monthes in case rushing them. I should have decided and done what the
> best I think is, than asking.
> 
> It would be very appriciated if you pay more attention.
> 
> > > @@ -1376,8 +1399,7 @@ static int find_later_rq(struct task_struct *task)
> > >   return this_cpu;
> > >   }
> > >  
> > > - best_cpu = cpumask_first_and(later_mask,
> > > - sched_domain_span(sd));
> > > + best_cpu = find_cpu(later_mask, sd, prefer);
> > >   /*
> > >* Last chance: if a cpu being in both later_mask
> > >* and current sd span is valid, that becomes our
> > > @@ -1385,6 +1407,26 @@ static int find_later_rq(struct task_struct *task)
> > >* already under consideration through later_mask.
> > >*/
> > 
> > It seems that the comment above should be updated as well.
> 
> How? Could you explain it more?

Let me try it by myself.. Please fix me at the next spin if needed.

Thank you,
Byungchul

Re: [PATCH v8 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()

[Byungchul Park]

On Tue, Aug 22, 2017 at 02:53:25PM +0900, Byungchul Park wrote:
> On Mon, Aug 21, 2017 at 02:44:58PM +0100, Juri Lelli wrote:
> > Hi,
> > On 18/08/17 17:21, Byungchul Park wrote:
> > > It would be better to try to check other siblings first if
> > > SD_PREFER_SIBLING is flaged when pushing tasks - migration.
> > > 
> > > Signed-off-by: Byungchul Park 
> > 
> > Mmm, this looks like Peter's proposed patch, maybe add (at least) a
> > Suggested-by: him ?
> 
> Hi Juri,
> 
> Why not. I will add it from the next spin.
> 
> BTW, is it enough? I don't know the way I should do, whenever I got
> thankful suggestions. I really want to add them as a separate patch
> which can be stacked on my patches _if possible_. But in case that
> it's better to merge them into one like this, I don't know how.
> 
> I mean I will add 'Suggested-by' from now on - I learned what I should
> do (at least) in this case thanks to Juri, but I'm still not sure if
> it's enough.
> 
> Speaking of which, I have something to ask Peterz and Ingo for. I really
> want to interact with maintainers actively e.g. asking ways they prefer.
> But it takes too much long to get responses from them e.g. at most 2
> monthes in case rushing them. I should have decided and done what the
> best I think is, than asking.
> 
> It would be very appriciated if you pay more attention.
> 
> > > @@ -1376,8 +1399,7 @@ static int find_later_rq(struct task_struct *task)
> > >   return this_cpu;
> > >   }
> > >  
> > > - best_cpu = cpumask_first_and(later_mask,
> > > - sched_domain_span(sd));
> > > + best_cpu = find_cpu(later_mask, sd, prefer);
> > >   /*
> > >* Last chance: if a cpu being in both later_mask
> > >* and current sd span is valid, that becomes our
> > > @@ -1385,6 +1407,26 @@ static int find_later_rq(struct task_struct *task)
> > >* already under consideration through later_mask.
> > >*/
> > 
> > It seems that the comment above should be updated as well.
> 
> How? Could you explain it more?

Let me try it by myself.. Please fix me at the next spin if needed.

Thank you,
Byungchul

Re: [PATCH v8 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()

[Byungchul Park]

On Mon, Aug 21, 2017 at 03:07:57PM +0100, Juri Lelli wrote:
> > Consider a 4 core, SMT2 system:
> > 
> > LLC [0 - 7]
> > 
> > SMT [0,1] [2,3] [4,5] [6,7]
> > 
> > If we do a wake-up on CPU0, we'll find CPU1, mark that as fallback,
> > continue up the domain tree, exclude 0,1 from 0-7 and find CPU2.
> > 
> > A next wakeup on CPU0 does the same and will find CPU3, fully loading
> > that core, instead of considering CPU4 first.
> > 
> 
> Ah, right, I see. Thanks for explaining.
> 
> Byungchul, maybe you could add this explanation as a comment?

Yes. Good idea. I will add it.

Thank you,
Byungchul

Re: [PATCH v8 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()

[Byungchul Park]

On Mon, Aug 21, 2017 at 03:07:57PM +0100, Juri Lelli wrote:
> > Consider a 4 core, SMT2 system:
> > 
> > LLC [0 - 7]
> > 
> > SMT [0,1] [2,3] [4,5] [6,7]
> > 
> > If we do a wake-up on CPU0, we'll find CPU1, mark that as fallback,
> > continue up the domain tree, exclude 0,1 from 0-7 and find CPU2.
> > 
> > A next wakeup on CPU0 does the same and will find CPU3, fully loading
> > that core, instead of considering CPU4 first.
> > 
> 
> Ah, right, I see. Thanks for explaining.
> 
> Byungchul, maybe you could add this explanation as a comment?

Yes. Good idea. I will add it.

Thank you,
Byungchul

Re: [PATCH v8 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()

[Byungchul Park]

On Mon, Aug 21, 2017 at 02:44:58PM +0100, Juri Lelli wrote:
> Hi,
> On 18/08/17 17:21, Byungchul Park wrote:
> > It would be better to try to check other siblings first if
> > SD_PREFER_SIBLING is flaged when pushing tasks - migration.
> > 
> > Signed-off-by: Byungchul Park 
> 
> Mmm, this looks like Peter's proposed patch, maybe add (at least) a
> Suggested-by: him ?

Hi Juri,

Why not. I will add it from the next spin.

BTW, is it enough? I don't know the way I should do, whenever I got
thankful suggestions. I really want to add them as a separate patch
which can be stacked on my patches _if possible_. But in case that
it's better to merge them into one like this, I don't know how.

I mean I will add 'Suggested-by' from now on - I learned what I should
do (at least) in this case thanks to Juri, but I'm still not sure if
it's enough.

Speaking of which, I have something to ask Peterz and Ingo for. I really
want to interact with maintainers actively e.g. asking ways they prefer.
But it takes too much long to get responses from them e.g. at most 2
monthes in case rushing them. I should have decided and done what the
best I think is, than asking.

It would be very appriciated if you pay more attention.

> > @@ -1376,8 +1399,7 @@ static int find_later_rq(struct task_struct *task)
> > return this_cpu;
> > }
> >  
> > -   best_cpu = cpumask_first_and(later_mask,
> > -   sched_domain_span(sd));
> > +   best_cpu = find_cpu(later_mask, sd, prefer);
> > /*
> >  * Last chance: if a cpu being in both later_mask
> >  * and current sd span is valid, that becomes our
> > @@ -1385,6 +1407,26 @@ static int find_later_rq(struct task_struct *task)
> >  * already under consideration through later_mask.
> >  */
> 
> It seems that the comment above should be updated as well.

How? Could you explain it more?

Thanks,
Byungchul

Re: [PATCH v8 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()

[Byungchul Park]

On Mon, Aug 21, 2017 at 02:44:58PM +0100, Juri Lelli wrote:
> Hi,
> On 18/08/17 17:21, Byungchul Park wrote:
> > It would be better to try to check other siblings first if
> > SD_PREFER_SIBLING is flaged when pushing tasks - migration.
> > 
> > Signed-off-by: Byungchul Park 
> 
> Mmm, this looks like Peter's proposed patch, maybe add (at least) a
> Suggested-by: him ?

Hi Juri,

Why not. I will add it from the next spin.

BTW, is it enough? I don't know the way I should do, whenever I got
thankful suggestions. I really want to add them as a separate patch
which can be stacked on my patches _if possible_. But in case that
it's better to merge them into one like this, I don't know how.

I mean I will add 'Suggested-by' from now on - I learned what I should
do (at least) in this case thanks to Juri, but I'm still not sure if
it's enough.

Speaking of which, I have something to ask Peterz and Ingo for. I really
want to interact with maintainers actively e.g. asking ways they prefer.
But it takes too much long to get responses from them e.g. at most 2
monthes in case rushing them. I should have decided and done what the
best I think is, than asking.

It would be very appriciated if you pay more attention.

> > @@ -1376,8 +1399,7 @@ static int find_later_rq(struct task_struct *task)
> > return this_cpu;
> > }
> >  
> > -   best_cpu = cpumask_first_and(later_mask,
> > -   sched_domain_span(sd));
> > +   best_cpu = find_cpu(later_mask, sd, prefer);
> > /*
> >  * Last chance: if a cpu being in both later_mask
> >  * and current sd span is valid, that becomes our
> > @@ -1385,6 +1407,26 @@ static int find_later_rq(struct task_struct *task)
> >  * already under consideration through later_mask.
> >  */
> 
> It seems that the comment above should be updated as well.

How? Could you explain it more?

Thanks,
Byungchul

Re: [PATCH v8 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()

[Juri Lelli]

On 21/08/17 15:56, Peter Zijlstra wrote:
> On Mon, Aug 21, 2017 at 02:44:58PM +0100, Juri Lelli wrote:
> 
> > Also, I'm not sure what Peter meant with
> > 
> > "But still this isn't quite right, because when we consider this for SMT
> > (as was the intent here) we'll happily occupy a full sibling core over
> > finding an empty one."
> 
> Consider a 4 core, SMT2 system:
> 
> LLC   [0 - 7]
> 
> SMT   [0,1] [2,3] [4,5] [6,7]
> 
> If we do a wake-up on CPU0, we'll find CPU1, mark that as fallback,
> continue up the domain tree, exclude 0,1 from 0-7 and find CPU2.
> 
> A next wakeup on CPU0 does the same and will find CPU3, fully loading
> that core, instead of considering CPU4 first.
> 

Ah, right, I see. Thanks for explaining.

Byungchul, maybe you could add this explanation as a comment?

> Doing this 'right' is difficult and expensive :-/
> 

Re: [PATCH v8 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()

[Juri Lelli]

On 21/08/17 15:56, Peter Zijlstra wrote:
> On Mon, Aug 21, 2017 at 02:44:58PM +0100, Juri Lelli wrote:
> 
> > Also, I'm not sure what Peter meant with
> > 
> > "But still this isn't quite right, because when we consider this for SMT
> > (as was the intent here) we'll happily occupy a full sibling core over
> > finding an empty one."
> 
> Consider a 4 core, SMT2 system:
> 
> LLC   [0 - 7]
> 
> SMT   [0,1] [2,3] [4,5] [6,7]
> 
> If we do a wake-up on CPU0, we'll find CPU1, mark that as fallback,
> continue up the domain tree, exclude 0,1 from 0-7 and find CPU2.
> 
> A next wakeup on CPU0 does the same and will find CPU3, fully loading
> that core, instead of considering CPU4 first.
> 

Ah, right, I see. Thanks for explaining.

Byungchul, maybe you could add this explanation as a comment?

> Doing this 'right' is difficult and expensive :-/
> 

Re: [PATCH v8 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()

[Peter Zijlstra]

On Mon, Aug 21, 2017 at 02:44:58PM +0100, Juri Lelli wrote:

> Also, I'm not sure what Peter meant with
> 
> "But still this isn't quite right, because when we consider this for SMT
> (as was the intent here) we'll happily occupy a full sibling core over
> finding an empty one."

Consider a 4 core, SMT2 system:

LLC [0 - 7]

SMT [0,1] [2,3] [4,5] [6,7]

If we do a wake-up on CPU0, we'll find CPU1, mark that as fallback,
continue up the domain tree, exclude 0,1 from 0-7 and find CPU2.

A next wakeup on CPU0 does the same and will find CPU3, fully loading
that core, instead of considering CPU4 first.

Doing this 'right' is difficult and expensive :-/

Re: [PATCH v8 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()

[Peter Zijlstra]

On Mon, Aug 21, 2017 at 02:44:58PM +0100, Juri Lelli wrote:

> Also, I'm not sure what Peter meant with
> 
> "But still this isn't quite right, because when we consider this for SMT
> (as was the intent here) we'll happily occupy a full sibling core over
> finding an empty one."

Consider a 4 core, SMT2 system:

LLC [0 - 7]

SMT [0,1] [2,3] [4,5] [6,7]

If we do a wake-up on CPU0, we'll find CPU1, mark that as fallback,
continue up the domain tree, exclude 0,1 from 0-7 and find CPU2.

A next wakeup on CPU0 does the same and will find CPU3, fully loading
that core, instead of considering CPU4 first.

Doing this 'right' is difficult and expensive :-/

Re: [PATCH v8 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()

[Juri Lelli]

Hi,

On 18/08/17 17:21, Byungchul Park wrote:
> It would be better to try to check other siblings first if
> SD_PREFER_SIBLING is flaged when pushing tasks - migration.
> 
> Signed-off-by: Byungchul Park 

Mmm, this looks like Peter's proposed patch, maybe add (at least) a
Suggested-by: him ?

https://marc.info/?l=linux-kernel=150176183807073

Also, I'm not sure what Peter meant with

"But still this isn't quite right, because when we consider this for SMT
(as was the intent here) we'll happily occupy a full sibling core over
finding an empty one."

since we are still using the later_mask, which should not include full
cores (unless it is the one with the lates deadline)?

> ---
>  kernel/sched/deadline.c | 55 
> ++---
>  1 file changed, 52 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 0223694..115250b 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1319,12 +1319,35 @@ static struct task_struct 
> *pick_earliest_pushable_dl_task(struct rq *rq, int cpu
>  
>  static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask_dl);
>  
> +/*
> + * Find the first cpu in: mask & sd & ~prefer
> + */
> +static int find_cpu(const struct cpumask *mask,
> + const struct sched_domain *sd,
> + const struct sched_domain *prefer)
> +{
> + const struct cpumask *sds = sched_domain_span(sd);
> + const struct cpumask *ps  = prefer ? sched_domain_span(prefer) : NULL;
> + int cpu;
> +
> + for_each_cpu(cpu, mask) {
> + if (!cpumask_test_cpu(cpu, sds))
> + continue;
> + if (ps && cpumask_test_cpu(cpu, ps))
> + continue;
> + break;
> + }
> +
> + return cpu;
> +}
> +
>  static int find_later_rq(struct task_struct *task)
>  {
> - struct sched_domain *sd;
> + struct sched_domain *sd, *prefer = NULL;
>   struct cpumask *later_mask = 
> this_cpu_cpumask_var_ptr(local_cpu_mask_dl);
>   int this_cpu = smp_processor_id();
>   int cpu = task_cpu(task);
> + int fallback_cpu = -1;
>  
>   /* Make sure the mask is initialized first */
>   if (unlikely(!later_mask))
> @@ -1376,8 +1399,7 @@ static int find_later_rq(struct task_struct *task)
>   return this_cpu;
>   }
>  
> - best_cpu = cpumask_first_and(later_mask,
> - sched_domain_span(sd));
> + best_cpu = find_cpu(later_mask, sd, prefer);
>   /*
>* Last chance: if a cpu being in both later_mask
>* and current sd span is valid, that becomes our
> @@ -1385,6 +1407,26 @@ static int find_later_rq(struct task_struct *task)
>* already under consideration through later_mask.
>*/

It seems that the comment above should be updated as well.

Thanks,

- Juri

Re: [PATCH v8 1/2] sched/deadline: Add support for SD_PREFER_SIBLING on find_later_rq()

[Juri Lelli]

Hi,

On 18/08/17 17:21, Byungchul Park wrote:
> It would be better to try to check other siblings first if
> SD_PREFER_SIBLING is flaged when pushing tasks - migration.
> 
> Signed-off-by: Byungchul Park 

Mmm, this looks like Peter's proposed patch, maybe add (at least) a
Suggested-by: him ?

https://marc.info/?l=linux-kernel=150176183807073

Also, I'm not sure what Peter meant with

"But still this isn't quite right, because when we consider this for SMT
(as was the intent here) we'll happily occupy a full sibling core over
finding an empty one."

since we are still using the later_mask, which should not include full
cores (unless it is the one with the lates deadline)?

> ---
>  kernel/sched/deadline.c | 55 
> ++---
>  1 file changed, 52 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 0223694..115250b 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1319,12 +1319,35 @@ static struct task_struct 
> *pick_earliest_pushable_dl_task(struct rq *rq, int cpu
>  
>  static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask_dl);
>  
> +/*
> + * Find the first cpu in: mask & sd & ~prefer
> + */
> +static int find_cpu(const struct cpumask *mask,
> + const struct sched_domain *sd,
> + const struct sched_domain *prefer)
> +{
> + const struct cpumask *sds = sched_domain_span(sd);
> + const struct cpumask *ps  = prefer ? sched_domain_span(prefer) : NULL;
> + int cpu;
> +
> + for_each_cpu(cpu, mask) {
> + if (!cpumask_test_cpu(cpu, sds))
> + continue;
> + if (ps && cpumask_test_cpu(cpu, ps))
> + continue;
> + break;
> + }
> +
> + return cpu;
> +}
> +
>  static int find_later_rq(struct task_struct *task)
>  {
> - struct sched_domain *sd;
> + struct sched_domain *sd, *prefer = NULL;
>   struct cpumask *later_mask = 
> this_cpu_cpumask_var_ptr(local_cpu_mask_dl);
>   int this_cpu = smp_processor_id();
>   int cpu = task_cpu(task);
> + int fallback_cpu = -1;
>  
>   /* Make sure the mask is initialized first */
>   if (unlikely(!later_mask))
> @@ -1376,8 +1399,7 @@ static int find_later_rq(struct task_struct *task)
>   return this_cpu;
>   }
>  
> - best_cpu = cpumask_first_and(later_mask,
> - sched_domain_span(sd));
> + best_cpu = find_cpu(later_mask, sd, prefer);
>   /*
>* Last chance: if a cpu being in both later_mask
>* and current sd span is valid, that becomes our
> @@ -1385,6 +1407,26 @@ static int find_later_rq(struct task_struct *task)
>* already under consideration through later_mask.
>*/

It seems that the comment above should be updated as well.

Thanks,

- Juri