Re: [PATCH v1 1/1] irqchip/gic-v3-its: fixup IRQ affinities to account for online CPUs

[David Decotigny]

--
David Decotigny -- Platforms, US SVL MAT3

On Sat, Mar 5, 2022 at 3:24 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> On Fri, 04 Mar 2022 19:52:38 +0000,
> David Decotigny <decot+git@xxxxxxxxxx> wrote:
> >
> > From: David Decotigny <ddecotig@xxxxxxxxxx>
> >
> > In some cases (eg. when booting with maxcpus=X), it is possible that
> > the preset IRQ affinity masks don't intersect with the set of online
> > CPUs. This patch extends the fallback strategy implemented when
> > IRQD_AFFINITY_MANAGED is not set to all cases. This is logged the
> > first time that happens.
> >
> > Fixes: c5d6082d35e0 ("irqchip/gic-v3-its: Balance initial LPI affinity across CPUs")
> >
>
> Missing SoB?
>
> > ---
> >  drivers/irqchip/irq-gic-v3-its.c | 13 ++++++-------
> >  1 file changed, 6 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
> > index cd772973114a..de862fd9ad73 100644
> > --- a/drivers/irqchip/irq-gic-v3-its.c
> > +++ b/drivers/irqchip/irq-gic-v3-its.c
> > @@ -1618,11 +1618,6 @@ static int its_select_cpu(struct irq_data *d,
> >               /* Try the intersection of the affinity and online masks */
> >               cpumask_and(tmpmask, aff_mask, cpu_online_mask);
> >
> > -             /* If that doesn't fly, the online mask is the last resort */
> > -             if (cpumask_empty(tmpmask))
> > -                     cpumask_copy(tmpmask, cpu_online_mask);
> > -
> > -             cpu = cpumask_pick_least_loaded(d, tmpmask);
> >       } else {
> >               cpumask_and(tmpmask, irq_data_get_affinity_mask(d), cpu_online_mask);
> >
> > @@ -1630,9 +1625,13 @@ static int its_select_cpu(struct irq_data *d,
> >               if ((its_dev->its->flags & ITS_FLAGS_WORKAROUND_CAVIUM_23144) &&
> >                   node != NUMA_NO_NODE)
> >                       cpumask_and(tmpmask, tmpmask, cpumask_of_node(node));
> > -
> > -             cpu = cpumask_pick_least_loaded(d, tmpmask);
> >       }
> > +
> > +     /* If that doesn't fly, the online mask is the last resort */
> > +     if (WARN_ON_ONCE(cpumask_empty(tmpmask)))
> > +             cpumask_copy(tmpmask, cpu_online_mask);
> > +
> > +     cpu = cpumask_pick_least_loaded(d, tmpmask);
> >  out:
> >       free_cpumask_var(tmpmask);
> >
>
> Known issue, see [1] and [2].
>
> I don't think the above is the right approach. For managed interrupts,
> we shouldn't try and enable these interrupts until the CPUs are up,
> and activating them on *another* CPU breaks the abstraction entirely.
>
> The right way to do it would be to not activate them (marking them as
> shutdown) until the CPUs are up and running, similarly to what x86
> does (but we obviously can't reuse the matrix allocator for that).
> Looking into this is on my TODO list, just didn't get the time to work
> on it.

Indeed: thanks for
https://lore.kernel.org/lkml/20220307190625.254426-1-maz@xxxxxxxxxx/ ,
worked for us !

... And then please ignore my patch.

>
>         M.
>
> [1] https://lore.kernel.org/r/78615d08-1764-c895-f3b7-bfddfbcbdfb9@xxxxxxxxxx
> [2] https://lore.kernel.org/r/20220124073440.88598-1-wangxiongfeng2@xxxxxxxxxx
>
> --
> Without deviation from the norm, progress is not possible.