Re: [PATCH v2 5/7] x86/irq: deal with old_cpu_mask for interrupts in movement in fixup_irqs()

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Jun 11, 2024 at 02:45:09PM +0200, Jan Beulich wrote:
> On 10.06.2024 16:20, Roger Pau Monne wrote:
> > Given the current logic it's possible for ->arch.old_cpu_mask to get out of
> > sync: if a CPU set in old_cpu_mask is offlined and then onlined
> > again without old_cpu_mask having been updated the data in the mask will no
> > longer be accurate, as when brought back online the CPU will no longer have
> > old_vector configured to handle the old interrupt source.
> > 
> > If there's an interrupt movement in progress, and the to be offlined CPU 
> > (which
> > is the call context) is in the old_cpu_mask clear it and update the mask, 
> > so it
> > doesn't contain stale data.
> 
> This imo is too __cpu_disable()-centric. In the code you cover the
> smp_send_stop() case afaict, where it's all _other_ CPUs which are being
> offlined. As we're not meaning to bring CPUs online again in that case,
> dealing with the situation likely isn't needed. Yet the description should
> imo at least make clear that the case was considered.

What about adding the following paragraph:

Note that when the system is going down fixup_irqs() will be called by
smp_send_stop() from CPU 0 with a mask with only CPU 0 on it,
effectively asking to move all interrupts to the current caller (CPU
0) which is the only CPU online.  In that case we don't care to
migrate interrupts that are in the process of being moved, as it's
likely we won't be able to move all interrupts to CPU 0 due to vector
shortage anyway.

> 
> > @@ -2589,6 +2589,28 @@ void fixup_irqs(const cpumask_t *mask, bool verbose)
> >                                 affinity);
> >          }
> >  
> > +        if ( desc->arch.move_in_progress &&
> > +             !cpumask_test_cpu(cpu, &cpu_online_map) &&
> 
> This part of the condition is, afaict, what covers (excludes) the
> smp_send_stop() case. Might be nice to have a brief comment here, thus
> also clarifying ...

Would you be fine with:

        if ( desc->arch.move_in_progress &&
             /*
              * Only attempt to migrate if the current CPU is going
              * offline, otherwise the whole system is going down and
              * leaving stale interrupts is fine.
              */
             !cpumask_test_cpu(cpu, &cpu_online_map) &&
             cpumask_test_cpu(cpu, desc->arch.old_cpu_mask) )


> > +             cpumask_test_cpu(cpu, desc->arch.old_cpu_mask) )
> > +        {
> > +            /*
> > +             * This CPU is going offline, remove it from 
> > ->arch.old_cpu_mask
> > +             * and possibly release the old vector if the old mask becomes
> > +             * empty.
> > +             *
> > +             * Note cleaning ->arch.old_cpu_mask is required if the CPU is
> > +             * brought offline and then online again, as when re-onlined 
> > the
> > +             * per-cpu vector table will no longer have ->arch.old_vector
> > +             * setup, and hence ->arch.old_cpu_mask would be stale.
> > +             */
> > +            cpumask_clear_cpu(cpu, desc->arch.old_cpu_mask);
> > +            if ( cpumask_empty(desc->arch.old_cpu_mask) )
> > +            {
> > +                desc->arch.move_in_progress = 0;
> > +                release_old_vec(desc);
> > +            }
> 
> ... that none of this is really wanted or necessary in that other case.
> Assuming my understanding above is correct, the code change itself is
> once again

It is.  For the smp_send_stop() case we don't care much about leaving
stale data around, as the system is going down.  It's also likely
impossible to move all interrupts to CPU0 due to vector shortage, so
some interrupts will be left assigned to different CPUs.

> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> yet here I'm uncertain whether to offer on-commit editing, as it's not
> really clear to me whether there's a dependencies on patch 4.

No, in principle it should be fine to skip patch 4, but I would like
to do another round of testing before confirming.

Thanks, Roger.