Re: [PATCH v2 05/17] sched/core: allow only preferred CPUs in is_cpu_allowed

Shrikanth Hegde
2026-04-08T12:56:52Z

Hi Yury.

On 4/8/26 6:35 AM, Yury Norov wrote:
> On Wed, Apr 08, 2026 at 12:49:38AM +0530, Shrikanth Hegde wrote:
>>
>> When possible, choose a preferred CPUs to pick.
>>
>> Push task mechanism uses stopper thread which going to call
>> select_fallback_rq and use this mechanism to pick only a preferred CPU.
>>
>> When task is affined only to non-preferred CPUs it should continue to
>> run there. Detect that by checking if cpus_ptr and cpu_preferred_mask
>> interesect or not.
>>
>> Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
>> ---
>> kernel/sched/core.c | 17 ++++++++++++++---
>> kernel/sched/sched.h | 12 ++++++++++++
>> 2 files changed, 26 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 7ea05a7a717b..336e7c694eb7 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2463,9 +2463,16 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
>> if (is_migration_disabled(p))
>> return cpu_online(cpu);
>>
>> - /* Non kernel threads are not allowed during either online or offline. */
>> - if (!(p->flags & PF_KTHREAD))
>> - return cpu_active(cpu);
>> + /*
>> + * Non kernel threads are not allowed during either online or offline.
>> + * Ensure it is a preferred CPU to avoid further contention
>> + */
>> + if (!(p->flags & PF_KTHREAD)) {
>> + if (!cpu_active(cpu))
>> + return false;
>> + if (!cpu_preferred(cpu) && task_can_run_on_preferred_cpu(p))
>> + return false;
>> + }
>>
>> /* KTHREAD_IS_PER_CPU is always allowed. */
>> if (kthread_is_per_cpu(p))
>> @@ -2475,6 +2482,10 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
>> if (cpu_dying(cpu))
>> return false;
>>
>> + /* Try on preferred CPU first */
>> + if (!cpu_preferred(cpu) && task_can_run_on_preferred_cpu(p))
>> + return false;
>

First one was regular tasks, this is for unbound kernel threads.
Both will need. No?

> You repeat this for the 2nd time. The cpu_preferred() call should go
> inside task_can_run_on_preferred_cpu().

I want to keep this check for cpu_preferred() first. the reason being
it is inexpensive since it is bit check. Only if it fails, then one should
bother about task_can_run_on_preferred_cpu which is O(N) as you said.

I am using the task_can_run_on_preferred_cpu in push task mechanism too.
PATCH 10/17. I get there only on non-preferred CPU.
So can I keep as is?

>
> And can you please pick some shorter name?
>

task_has_preferred_cpus?

>> +
>> /* But are allowed during online. */
>> return cpu_online(cpu);
>> }
>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>> index 88e0c93b9e21..7271af2ca64f 100644
>> --- a/kernel/sched/sched.h
>> +++ b/kernel/sched/sched.h
>> @@ -4130,4 +4130,16 @@ DEFINE_CLASS_IS_UNCONDITIONAL(sched_change)
>>
>> #include "ext.h"
>>
>> +#ifdef CONFIG_PARAVIRT
>> +static inline bool task_can_run_on_preferred_cpu(struct task_struct *p)
>> +{
>> + return cpumask_intersects(p->cpus_ptr, cpu_preferred_mask);
>
> This makes is_cpu_allowed() O(N). Even if CONFIG_PARAVIRT is enabled,
> I think some people would prefer to avoid this. Also, select_fallback_rq()
> calls it in a loop, and this makes it O(N^2).
>
>
> /* Any allowed, online CPU? */
> for_each_cpu(dest_cpu, p->cpus_ptr) {
> if (!is_cpu_allowed(p, dest_cpu))
> continue;
>
> goto out;
> }
>
> You can keep it O(N):
> for_each_cpu_and(dest_cpu, p->cpus_ptr, cpu_preferred_mask) {
> ...
> }

This would leave tasks which has affinity only on non-preferred CPUs without a CPU.

That breaks below case,
600 CPUs, high steal time and hence preferred is 0-399.
In that state, user does "taskset -c 500 <stress-ng>", that task ends up
going to preferred CPUs since its affinity gets reset in the switch
block later in select_fallback_rq.

>
> Not sure how critical that path is, but this looks suspicious.
>

Fair point. But we come here only if cpu_preferred(cpu) == false.

When that happens, we expect the task running on will get preempted
to a preferred CPUs. If it migrated to a preferred cpu, then on-wards
it wont suffer the additional overhead of task_can_run_on_preferred_cpu

case where it would be O(N^2) is for tasks which have explicit affinity
on non-preferred CPUs. I see that as rare case.

For the majority of the cases, it should still be O(N) since cpu_preferred(cpu)
will be true.

Here is the benchmark data on system where there is 0 steal time.
It is dedicated LPAR(VM).

| Test | Baseline | NO STEAL | %diff to base| STEAL | %diff to base |
| | | MONITOR | | MONITOR | |
|---------------------------------|----------|----------|--------------|-----------|---------------|
| HackBench Process 20 groups | 2.70 | 2.67 | +1.11% | 2.66 | +1.48% |
| HackBench Process 40 groups | 5.31 | 5.26 | +0.94% | 5.30 | +0.19% |
| HackBench Process 60 groups | 7.95 | 7.82 | +1.64% | 7.90 | +0.63% |
| HackBench thread 10 Time | 1.61 | 1.59 | +1.24% | 1.56 | +3.11% |
| HackBench thread 20 Time | 2.82 | 2.83 | -0.35% | 2.80 | +0.71% |
| HackBench Process(Pipe) 20 Time | 1.51 | 1.50 | +0.66% | 1.47 | +2.65% |
| HackBench Process(Pipe) 40 Time | 2.78 | 2.69 | +3.24% | 2.69 | +3.24% |
| HackBench Process(Pipe) 60 Time | 3.73 | 3.76 | -0.80% | 3.64 | +2.41% |
| HackBench thread(Pipe) 10 Time | 0.94 | 0.91 | +3.19% | 0.91 | +3.19% |
| HackBench thread(Pipe) 20 Time | 1.58 | 1.58 | -0.00% | 1.59 | -0.63% |

>> +}
>> +#else
>> +static inline bool task_can_run_on_preferred_cpu(struct task_struct *p)
>> +{
>> + return true;
>> +}
>> +#endif
>
> Same comment as in patch 3. I believe, it's worth to declare cpu_preferred_mask
> unrelated to CONFIG_PARAVIRT, so that you'll not have to spread this
> ifdefery around.
>

Yes.

>> +
>> #endif /* _KERNEL_SCHED_SCHED_H */
>> --
>> 2.47.3