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From:   Nitesh Lal <nilal@redhat.com>
Date:   Fri, 25 Jun 2021 13:52:04 -0400
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Subject: Re: [PATCH v1 01/14] genirq: Provide new interfaces for affinity hints
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On Thu, Jun 17, 2021 at 2:23 PM Nitesh Narayan Lal <nitesh@redhat.com> wrote:
>
> From: Thomas Gleixner <tglx@linutronix.de>
>
> The discussion about removing the side effect of irq_set_affinity_hint() of
> actually applying the cpumask (if not NULL) as affinity to the interrupt,
> unearthed a few unpleasantries:
>
>   1) The modular perf drivers rely on the current behaviour for the very
>      wrong reasons.
>
>   2) While none of the other drivers prevents user space from changing
>      the affinity, a cursorily inspection shows that there are at least
>      expectations in some drivers.
>
> #1 needs to be cleaned up anyway, so that's not a problem
>
> #2 might result in subtle regressions especially when irqbalanced (which
>    nowadays ignores the affinity hint) is disabled.
>
> Provide new interfaces:
>
>   irq_update_affinity_hint()  - Only sets the affinity hint pointer
>   irq_set_affinity_and_hint() - Set the pointer and apply the affinity to
>                                 the interrupt
>
> Make irq_set_affinity_hint() a wrapper around irq_apply_affinity_hint() and
> document it to be phased out.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Nitesh Narayan Lal <nitesh@redhat.com>
> Link: https://lore.kernel.org/r/20210501021832.743094-1-jesse.brandeburg@intel.com
> ---
>  include/linux/interrupt.h | 41 ++++++++++++++++++++++++++++++++++++++-
>  kernel/irq/manage.c       |  8 ++++----
>  2 files changed, 44 insertions(+), 5 deletions(-)
>
> diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
> index 2ed65b01c961..4ca491a76033 100644
> --- a/include/linux/interrupt.h
> +++ b/include/linux/interrupt.h
> @@ -328,7 +328,46 @@ extern int irq_force_affinity(unsigned int irq, const struct cpumask *cpumask);
>  extern int irq_can_set_affinity(unsigned int irq);
>  extern int irq_select_affinity(unsigned int irq);
>
> -extern int irq_set_affinity_hint(unsigned int irq, const struct cpumask *m);
> +extern int __irq_apply_affinity_hint(unsigned int irq, const struct cpumask *m,
> +                                    bool setaffinity);
> +
> +/**
> + * irq_update_affinity_hint - Update the affinity hint
> + * @irq:       Interrupt to update
> + * @cpumask:   cpumask pointer (NULL to clear the hint)
> + *
> + * Updates the affinity hint, but does not change the affinity of the interrupt.
> + */
> +static inline int
> +irq_update_affinity_hint(unsigned int irq, const struct cpumask *m)
> +{
> +       return __irq_apply_affinity_hint(irq, m, false);
> +}
> +
> +/**
> + * irq_set_affinity_and_hint - Update the affinity hint and apply the provided
> + *                          cpumask to the interrupt
> + * @irq:       Interrupt to update
> + * @cpumask:   cpumask pointer (NULL to clear the hint)
> + *
> + * Updates the affinity hint and if @cpumask is not NULL it applies it as
> + * the affinity of that interrupt.
> + */
> +static inline int
> +irq_set_affinity_and_hint(unsigned int irq, const struct cpumask *m)
> +{
> +       return __irq_apply_affinity_hint(irq, m, true);
> +}
> +
> +/*
> + * Deprecated. Use irq_update_affinity_hint() or irq_set_affinity_and_hint()
> + * instead.
> + */
> +static inline int irq_set_affinity_hint(unsigned int irq, const struct cpumask *m)
> +{
> +       return irq_set_affinity_and_hint(irq, m);
> +}
> +
>  extern int irq_update_affinity_desc(unsigned int irq,
>                                     struct irq_affinity_desc *affinity);
>
> diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
> index ef30b4762947..837b63e63111 100644
> --- a/kernel/irq/manage.c
> +++ b/kernel/irq/manage.c
> @@ -487,7 +487,8 @@ int irq_force_affinity(unsigned int irq, const struct cpumask *cpumask)
>  }
>  EXPORT_SYMBOL_GPL(irq_force_affinity);
>
> -int irq_set_affinity_hint(unsigned int irq, const struct cpumask *m)
> +int __irq_apply_affinity_hint(unsigned int irq, const struct cpumask *m,
> +                             bool setaffinity)
>  {
>         unsigned long flags;
>         struct irq_desc *desc = irq_get_desc_lock(irq, &flags, IRQ_GET_DESC_CHECK_GLOBAL);
> @@ -496,12 +497,11 @@ int irq_set_affinity_hint(unsigned int irq, const struct cpumask *m)
>                 return -EINVAL;
>         desc->affinity_hint = m;
>         irq_put_desc_unlock(desc, flags);
> -       /* set the initial affinity to prevent every interrupt being on CPU0 */
> -       if (m)
> +       if (m && setaffinity)
>                 __irq_set_affinity(irq, m, false);
>         return 0;
>  }
> -EXPORT_SYMBOL_GPL(irq_set_affinity_hint);
> +EXPORT_SYMBOL_GPL(__irq_apply_affinity_hint);
>
>  static void irq_affinity_notify(struct work_struct *work)
>  {
> --
> 2.27.0
>

It turns out that this patch has an issue. The new interfaces are not
added under the #ifdef (CONFIG_SMP)'s else section.
I will fix it and send a v2 with other changes.


--
Thanks

Nitesh