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

[Shrikanth Hegde]

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@xxxxxxxxxxxxx/
> >
> > 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?