# Re: \[PATCH v10 00/12\] sched, steal\_governor: Introduce preferred CPUs and steal-driven vCPU backoff

- From: Shrikanth Hegde \<sshegde@linux.ibm.com\>
- Date: 2026-08-27 13:37:13 
- List: virtualization
- Page: https://ratatoskr.run/virtualization/2026/08/17469864
- Thread: https://ratatoskr.run/linux-doc/2026/08/17399965/t.md

```text


On 8/27/26 4:20 PM, Dietmar Eggemann wrote:
> Hi Shrikanth,
>

Hi Dietmar. Thanks for checking it.

> On 17.08.26 09:39, Shrikanth Hegde wrote:
>> Hi.
>>
>> In addition to what's currently planned for v11 which was posted here,
>> https://lore.kernel.org/all/895a058a-475e-42ca-
>> a7a3-2c854598eea4@linux.ibm.com/
>>
>> I was going through sashiko's comments at:
>> https://sashiko.dev/#/patchset/20260812054033.95658-1-
>> sshegde%40linux.ibm.com
>> This has revealed some gaps. Thanks to some really nice insights too.
>> Report quality improving day by day!
>>
>>
>> Vincent, Dietmar, please check the 32-bit task issue fix on ARM64.
>
> See below.
>
>> On 8/12/26 11:10 AM, Shrikanth Hegde wrote:
>
> [...]
>
>> Issue1: Possible crash on 32-bit tasks on ARM64.
>> =======
>>>> +static inline bool task_can_sched_on_preferred(int cpu, struct
>>>> task_struct *p)
>>>> +{
>>>> +    if (cpu_preferred(cpu))
>>>> +        return false;
>>>> +
>>>> +    /* Only FAIR tasks honor preferred CPU state */
>>>> +    if (unlikely(p->sched_class != &fair_sched_class))
>>>> +        return false;
>>>> +
>>>> +    return cpumask_intersects(p->cpus_ptr, cpu_preferred_mask);
>>>> +}
>>> Does this intersection check need to account for the architectural CPU
>>> mask?
>>> On asymmetric systems, 32-bit tasks are architecturally restricted by
>>> task_cpu_possible_mask(). If a 32-bit task's mask intersects with
>>> 64-bit-only preferred CPUs, this function might return true, causing
>>> is_cpu_allowed() to falsely return false for valid 32-bit non-
>>> preferred CPUs.
>>> Since 64-bit CPUs are rightfully rejected by task_allowed_on_cpu(),
>>> all CPUs
>>> end up rejected. Could this regression cause the select_fallback_rq()
>>> loop
>>> to exhaust all options and hit the BUG() case for 32-bit tasks?
>>
>> Fix:
>> ====
>> I wasn;t aware of this case, thanks to sashiko for bring it up.
>> Yes, it could potentially cause a BUG in select_fallback_rq.
>>
>> Do a simple check if mask differ from possible mask which indicates we
>> are on 32-bit task on 64 bit
>> kernel. Do the below. I think that should solve it.
>>
>>   static inline bool task_can_sched_on_preferred(int cpu, struct
>> task_struct *p)
>>   {
>> +    const struct cpumask *valid_mask;
>> +    int i;
>> [...]
>> +    valid_mask = task_cpu_possible_mask(p);
>> +    if (likely(valid_mask == cpu_possible_mask))
>> +        return cpumask_intersects(p->cpus_ptr, cpu_preferred_mask);
>> +
>> +    /* 32-bit task */
>> +    for_each_cpu_and(i, p->cpus_ptr, cpu_preferred_mask) {
>> +        if (cpumask_test_cpu(i, valid_mask))
>> +            return true;
>> +    }
>
> I assume the question is whether task_can_sched_on_preferred() would
> have to be changed:
>
> - return cpumask_intersects(p->cpus_ptr, cpu_preferred_mask);
> + return cpumask_first_and_and(p->cpus_ptr, cpu_preferred_mask,
> +                               task_cpu_possible_mask(p)) < nr_cpu_ids;
>> so that cpu_preferred_mask can play together nicely with the 'asymmetric
> AArch32 EL0 (executing 32-bit Arm userspace under an AArch64 kernel)
> support' feature on some mobile Arm64 Socs.
>
> IMHO, this is not necessary since for those tasks p->cpus_ptr is always
> a subset of task_cpu_possible_mask(p). 'p->cpus_ptr ∩
> cpu_preferred_mask' already cannot contain an architecturally impossible
> CPU for those 32-bit Arm userspace tasks.
>
> [...]
>

Based on the report, I thought there maybe cases where p->cpus_ptr may contain.
Specifically after looking at:

static inline bool task_allowed_on_cpu(struct task_struct *p, int cpu)
{
        /* When not in the task's cpumask, no point in looking further. */
        if (!cpumask_test_cpu(cpu, p->cpus_ptr))
                return false;

        /* Can @cpu run a user thread? */
        if (!(p->flags & PF_KTHREAD) && !task_cpu_possible(cpu, p))
                return false;

        return true;
}

If p->cpus_ptr is cannot contain an architecturally impossible, then check for
task_cpu_possible again is necessary? I thought there may be cases.
So i kept the defensive check not to fall into BUG later on.

If you think cpumask_intersects(p->cpus_ptr, cpu_preferred_mask) is sufficient and cover
all cases of 32 bit tasks, then i can drop that v11 change specific to 32-bit tasks.

What do you suggest?
```