In select_idle_core(), the idle core returned may have no cpu
allowed. I think the idle core returned for the task is the one
that can be allowed to run. I insist on this semantics.
In select_idle_cpu(), if select_idle_core() can not find the
idle core, one reason is that the core is not allowed for the
task, but the core itself is idle from the point of
sds->has_idle_cores. I insist on this semantics.
No others, just two additional check.
---
kernel/sched/fair.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f6a05d9b5443..a44aca5095d3 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6213,7 +6213,7 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu
*idle_cpu = cpu;
}
- if (idle)
+ if (idle && *idle_cpu != -1)
return core;
cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
@@ -6324,7 +6324,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
}
}
- if (has_idle_core)
+ if (has_idle_core && *idle_cpu != -1)
set_idle_cores(target, false);
if (sched_feat(SIS_PROP) && !has_idle_core) {
--
2.32.0
On Sun, Oct 10, 2021 at 02:09:41AM +0800, Tao Zhou wrote:
> In select_idle_core(), the idle core returned may have no cpu
> allowed. I think the idle core returned for the task is the one
> that can be allowed to run. I insist on this semantics.
>
> In select_idle_cpu(), if select_idle_core() can not find the
> idle core, one reason is that the core is not allowed for the
> task, but the core itself is idle from the point of
> sds->has_idle_cores. I insist on this semantics.
>
> No others, just two additional check.
> ---
> kernel/sched/fair.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f6a05d9b5443..a44aca5095d3 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6213,7 +6213,7 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu
> *idle_cpu = cpu;
> }
>
> - if (idle)
> + if (idle && *idle_cpu != -1)
> return core;
In that case, core would be nr_cpu_ids (==nr_cpumask_bits), and then the caller checks:
(unsigned)i < nr_cpumask_bits
> cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
> @@ -6324,7 +6324,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> }
> }
>
> - if (has_idle_core)
> + if (has_idle_core && *idle_cpu != -1)
> set_idle_cores(target, false);
And this one I'm completely failing, why shouldn't we mark the core as
non-idle when there is a single idle CPU found? That's just worng.
Hi Peter,
On Sun, Oct 10, 2021 at 12:50:57AM +0200, Peter Zijlstra wrote:
> On Sun, Oct 10, 2021 at 02:09:41AM +0800, Tao Zhou wrote:
> > In select_idle_core(), the idle core returned may have no cpu
> > allowed. I think the idle core returned for the task is the one
> > that can be allowed to run. I insist on this semantics.
> >
> > In select_idle_cpu(), if select_idle_core() can not find the
> > idle core, one reason is that the core is not allowed for the
> > task, but the core itself is idle from the point of
> > sds->has_idle_cores. I insist on this semantics.
> >
> > No others, just two additional check.
> > ---
> > kernel/sched/fair.c | 4 ++--
> > 1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index f6a05d9b5443..a44aca5095d3 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -6213,7 +6213,7 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu
> > *idle_cpu = cpu;
> > }
> >
> > - if (idle)
> > + if (idle && *idle_cpu != -1)
> > return core;
>
> In that case, core would be nr_cpu_ids (==nr_cpumask_bits), and then the caller checks:
>
> (unsigned)i < nr_cpumask_bits
Thank you for reply.
If (1)there is no idle core or (2)the idle core has no allowed cpu, we return -1.
Originally, just (1) has happened, we return -1. The (2) is what I want to add.
If we find idle core and has allowed cpu in the core, is it better to return
@*idle_cpu.
if (idle && *idle_cpu != -1)
return *idle_cpu;
This @*idle_cpu is the allowed cpu in the idle core. We do not promise anything
about the @core(target) is the allowed cpu until we hit in select_task_rq() -->
select_fallback_rq(). And the select_fallback_rq() will return a different cpu
than the @core or @*idle_cpu.
> > cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
> > @@ -6324,7 +6324,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> > }
> > }
> >
> > - if (has_idle_core)
> > + if (has_idle_core && *idle_cpu != -1)
> > set_idle_cores(target, false);
>
> And this one I'm completely failing, why shouldn't we mark the core as
> non-idle when there is a single idle CPU found? That's just worng.
When @has_idle_core is true, it implies for all cpu in the core the case
(1) or case (2) has happened. The (1) can be mark as non-idle. I conclude
to contradiction myself last time. The (2) is also seemed to be non-idle.
But, I think I am totally wrong because the sds->has_idle_cores is related
to the cpu not task. So, the affinity should not affect the decision of
sds->has_idle_cores.
Thanks,
Tao
On Sun, Oct 10, 2021 at 10:45 PM Tao Zhou <[email protected]> wrote:
>
> Hi Peter,
>
> On Sun, Oct 10, 2021 at 12:50:57AM +0200, Peter Zijlstra wrote:
> > On Sun, Oct 10, 2021 at 02:09:41AM +0800, Tao Zhou wrote:
> > > In select_idle_core(), the idle core returned may have no cpu
> > > allowed. I think the idle core returned for the task is the one
> > > that can be allowed to run. I insist on this semantics.
> > >
> > > In select_idle_cpu(), if select_idle_core() can not find the
> > > idle core, one reason is that the core is not allowed for the
> > > task, but the core itself is idle from the point of
> > > sds->has_idle_cores. I insist on this semantics.
> > >
> > > No others, just two additional check.
> > > ---
> > > kernel/sched/fair.c | 4 ++--
> > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index f6a05d9b5443..a44aca5095d3 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -6213,7 +6213,7 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu
> > > *idle_cpu = cpu;
> > > }
> > >
> > > - if (idle)
> > > + if (idle && *idle_cpu != -1)
> > > return core;
> >
> > In that case, core would be nr_cpu_ids (==nr_cpumask_bits), and then the caller checks:
> >
> > (unsigned)i < nr_cpumask_bits
>
> Thank you for reply.
>
>
> If (1)there is no idle core or (2)the idle core has no allowed cpu, we return -1.
> Originally, just (1) has happened, we return -1. The (2) is what I want to add.
I don't understand (2). before doing
for_each_cpu_wrap(cpu, cpus, target + 1) {
if (has_idle_core) {
i = select_idle_core(p, cpu, cpus, &idle_cpu);
if ((unsigned int)i < nr_cpumask_bits)
return i;
} else {
if (!--nr)
return -1;
idle_cpu = __select_idle_cpu(cpu, p);
if ((unsigned int)idle_cpu < nr_cpumask_bits)
break;
}
}
to select idle core, we have already done:
cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
so we are only scanning allowed cpus.
>
> If we find idle core and has allowed cpu in the core, is it better to return
> @*idle_cpu.
>
> if (idle && *idle_cpu != -1)
> return *idle_cpu;
>
> This @*idle_cpu is the allowed cpu in the idle core. We do not promise anything
> about the @core(target) is the allowed cpu until we hit in select_task_rq() -->
> select_fallback_rq(). And the select_fallback_rq() will return a different cpu
> than the @core or @*idle_cpu.
>
> > > cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
> > > @@ -6324,7 +6324,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> > > }
> > > }
> > >
> > > - if (has_idle_core)
> > > + if (has_idle_core && *idle_cpu != -1)
> > > set_idle_cores(target, false);
> >
> > And this one I'm completely failing, why shouldn't we mark the core as
> > non-idle when there is a single idle CPU found? That's just worng.
>
> When @has_idle_core is true, it implies for all cpu in the core the case
> (1) or case (2) has happened. The (1) can be mark as non-idle. I conclude
> to contradiction myself last time. The (2) is also seemed to be non-idle.
>
>
> But, I think I am totally wrong because the sds->has_idle_cores is related
> to the cpu not task. So, the affinity should not affect the decision of
> sds->has_idle_cores.
>
>
>
> Thanks,
> Tao
Thanks
barry
Hi Barry,
On Mon, Oct 11, 2021 at 01:19:57AM +1300, Barry Song wrote:
> On Sun, Oct 10, 2021 at 10:45 PM Tao Zhou <[email protected]> wrote:
> >
> > Hi Peter,
> >
> > On Sun, Oct 10, 2021 at 12:50:57AM +0200, Peter Zijlstra wrote:
> > > On Sun, Oct 10, 2021 at 02:09:41AM +0800, Tao Zhou wrote:
> > > > In select_idle_core(), the idle core returned may have no cpu
> > > > allowed. I think the idle core returned for the task is the one
> > > > that can be allowed to run. I insist on this semantics.
> > > >
> > > > In select_idle_cpu(), if select_idle_core() can not find the
> > > > idle core, one reason is that the core is not allowed for the
> > > > task, but the core itself is idle from the point of
> > > > sds->has_idle_cores. I insist on this semantics.
> > > >
> > > > No others, just two additional check.
> > > > ---
> > > > kernel/sched/fair.c | 4 ++--
> > > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > > index f6a05d9b5443..a44aca5095d3 100644
> > > > --- a/kernel/sched/fair.c
> > > > +++ b/kernel/sched/fair.c
> > > > @@ -6213,7 +6213,7 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu
> > > > *idle_cpu = cpu;
> > > > }
> > > >
> > > > - if (idle)
> > > > + if (idle && *idle_cpu != -1)
> > > > return core;
> > >
> > > In that case, core would be nr_cpu_ids (==nr_cpumask_bits), and then the caller checks:
> > >
> > > (unsigned)i < nr_cpumask_bits
> >
> > Thank you for reply.
> >
> >
> > If (1)there is no idle core or (2)the idle core has no allowed cpu, we return -1.
> > Originally, just (1) has happened, we return -1. The (2) is what I want to add.
>
> I don't understand (2). before doing
> for_each_cpu_wrap(cpu, cpus, target + 1) {
> if (has_idle_core) {
> i = select_idle_core(p, cpu, cpus, &idle_cpu);
> if ((unsigned int)i < nr_cpumask_bits)
> return i;
>
> } else {
> if (!--nr)
> return -1;
> idle_cpu = __select_idle_cpu(cpu, p);
> if ((unsigned int)idle_cpu < nr_cpumask_bits)
> break;
> }
> }
>
> to select idle core, we have already done:
> cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
>
> so we are only scanning allowed cpus.
Um.. You read top down.. and you are right.
The function itself semantics is important to me.
After a secondary recall and not thorough now, I realize that
cpus_ptr may be changed.
See code of this:
static void migrate_disable_switch(struct rq *rq, struct task_struct *p)
{
if (likely(!p->migration_disabled))
return;
if (p->cpus_ptr != &p->cpus_mask)
return;
/*
* Violates locking rules! see comment in __do_set_cpus_allowed().
*/
__do_set_cpus_allowed(p, cpumask_of(rq->cpu), SCA_MIGRATE_DISABLE);
}
This change is under the light of ->pi_lock.
That thing is quick to forget to me..
Not sure I am right. Thank you for remind.
If the cpu_ptr can be changed, you can not depend on the first AND
operation there.
> >
> > If we find idle core and has allowed cpu in the core, is it better to return
> > @*idle_cpu.
> >
> > if (idle && *idle_cpu != -1)
> > return *idle_cpu;
> >
> > This @*idle_cpu is the allowed cpu in the idle core. We do not promise anything
> > about the @core(target) is the allowed cpu until we hit in select_task_rq() -->
> > select_fallback_rq(). And the select_fallback_rq() will return a different cpu
> > than the @core or @*idle_cpu.
> >
> > > > cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
> > > > @@ -6324,7 +6324,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> > > > }
> > > > }
> > > >
> > > > - if (has_idle_core)
> > > > + if (has_idle_core && *idle_cpu != -1)
> > > > set_idle_cores(target, false);
> > >
> > > And this one I'm completely failing, why shouldn't we mark the core as
> > > non-idle when there is a single idle CPU found? That's just worng.
> >
> > When @has_idle_core is true, it implies for all cpu in the core the case
> > (1) or case (2) has happened. The (1) can be mark as non-idle. I conclude
> > to contradiction myself last time. The (2) is also seemed to be non-idle.
> >
> >
> > But, I think I am totally wrong because the sds->has_idle_cores is related
> > to the cpu not task. So, the affinity should not affect the decision of
> > sds->has_idle_cores.
> >
> >
> >
> > Thanks,
> > Tao
>
> Thanks
> barry
Thanks,
Tao
On Mon, Oct 11, 2021 at 3:26 AM Tao Zhou <[email protected]> wrote:
>
> Hi Barry,
>
> On Mon, Oct 11, 2021 at 01:19:57AM +1300, Barry Song wrote:
> > On Sun, Oct 10, 2021 at 10:45 PM Tao Zhou <[email protected]> wrote:
> > >
> > > Hi Peter,
> > >
> > > On Sun, Oct 10, 2021 at 12:50:57AM +0200, Peter Zijlstra wrote:
> > > > On Sun, Oct 10, 2021 at 02:09:41AM +0800, Tao Zhou wrote:
> > > > > In select_idle_core(), the idle core returned may have no cpu
> > > > > allowed. I think the idle core returned for the task is the one
> > > > > that can be allowed to run. I insist on this semantics.
> > > > >
> > > > > In select_idle_cpu(), if select_idle_core() can not find the
> > > > > idle core, one reason is that the core is not allowed for the
> > > > > task, but the core itself is idle from the point of
> > > > > sds->has_idle_cores. I insist on this semantics.
> > > > >
> > > > > No others, just two additional check.
> > > > > ---
> > > > > kernel/sched/fair.c | 4 ++--
> > > > > 1 file changed, 2 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > > > index f6a05d9b5443..a44aca5095d3 100644
> > > > > --- a/kernel/sched/fair.c
> > > > > +++ b/kernel/sched/fair.c
> > > > > @@ -6213,7 +6213,7 @@ static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpu
> > > > > *idle_cpu = cpu;
> > > > > }
> > > > >
> > > > > - if (idle)
> > > > > + if (idle && *idle_cpu != -1)
> > > > > return core;
> > > >
> > > > In that case, core would be nr_cpu_ids (==nr_cpumask_bits), and then the caller checks:
> > > >
> > > > (unsigned)i < nr_cpumask_bits
> > >
> > > Thank you for reply.
> > >
> > >
> > > If (1)there is no idle core or (2)the idle core has no allowed cpu, we return -1.
> > > Originally, just (1) has happened, we return -1. The (2) is what I want to add.
> >
> > I don't understand (2). before doing
> > for_each_cpu_wrap(cpu, cpus, target + 1) {
> > if (has_idle_core) {
> > i = select_idle_core(p, cpu, cpus, &idle_cpu);
> > if ((unsigned int)i < nr_cpumask_bits)
> > return i;
> >
> > } else {
> > if (!--nr)
> > return -1;
> > idle_cpu = __select_idle_cpu(cpu, p);
> > if ((unsigned int)idle_cpu < nr_cpumask_bits)
> > break;
> > }
> > }
> >
> > to select idle core, we have already done:
> > cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> >
> > so we are only scanning allowed cpus.
>
> Um.. You read top down.. and you are right.
> The function itself semantics is important to me.
>
> After a secondary recall and not thorough now, I realize that
> cpus_ptr may be changed.
>
>
> See code of this:
>
> static void migrate_disable_switch(struct rq *rq, struct task_struct *p)
> {
> if (likely(!p->migration_disabled))
> return;
>
> if (p->cpus_ptr != &p->cpus_mask)
> return;
>
> /*
> * Violates locking rules! see comment in __do_set_cpus_allowed().
> */
> __do_set_cpus_allowed(p, cpumask_of(rq->cpu), SCA_MIGRATE_DISABLE);
> }
>
>
> This change is under the light of ->pi_lock.
> That thing is quick to forget to me..
> Not sure I am right. Thank you for remind.
>
> If the cpu_ptr can be changed, you can not depend on the first AND
> operation there.
The explanation doesn't make any sense to me. We are scanning
based on the first AND operation. select_idle_core() is returning
*idle_cpu based on the cpumask after AND operation.
Even though cpumask can change after select_idle_core() is done
or before select_idle_core() is called, the return value is not wrong.
>
> > >
> > > If we find idle core and has allowed cpu in the core, is it better to return
> > > @*idle_cpu.
> > >
> > > if (idle && *idle_cpu != -1)
> > > return *idle_cpu;
> > >
> > > This @*idle_cpu is the allowed cpu in the idle core. We do not promise anything
> > > about the @core(target) is the allowed cpu until we hit in select_task_rq() -->
> > > select_fallback_rq(). And the select_fallback_rq() will return a different cpu
> > > than the @core or @*idle_cpu.
> > >
> > > > > cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
> > > > > @@ -6324,7 +6324,7 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
> > > > > }
> > > > > }
> > > > >
> > > > > - if (has_idle_core)
> > > > > + if (has_idle_core && *idle_cpu != -1)
> > > > > set_idle_cores(target, false);
> > > >
> > > > And this one I'm completely failing, why shouldn't we mark the core as
> > > > non-idle when there is a single idle CPU found? That's just worng.
> > >
> > > When @has_idle_core is true, it implies for all cpu in the core the case
> > > (1) or case (2) has happened. The (1) can be mark as non-idle. I conclude
> > > to contradiction myself last time. The (2) is also seemed to be non-idle.
> > >
> > >
> > > But, I think I am totally wrong because the sds->has_idle_cores is related
> > > to the cpu not task. So, the affinity should not affect the decision of
> > > sds->has_idle_cores.
> > >
> > >
> > >
> > > Thanks,
> > > Tao
> >
> > Thanks
> > barry
>
>
>
> Thanks,
> Tao