Re: [PATCH v2 4/4] irqchip: sifive-plic: Implement irq_set_affinity() for SMP host

[Anup Patel <anup@brainfault.org>]

On Fri, Nov 30, 2018 at 11:29 AM Atish Patra <atish.patra@wdc.com> wrote:
>
> On 11/27/18 2:04 AM, Anup Patel wrote:
> > Currently on SMP host, all CPUs take external interrupts routed via
> > PLIC. All CPUs will try to claim a given external interrupt but only
> > one of them will succeed while other CPUs would simply resume whatever
> > they were doing before. This means if we have N CPUs then for every
> > external interrupt N-1 CPUs will always fail to claim it and waste
> > their CPU time.
> >
> > Instead of above, external interrupts should be taken by only one CPU
> > and we should have provision to explicity specify IRQ affinity from
> s/explicity/explicitly

Sure, I will update it.

>
> > kernel-space or user-space.
> >
> > This patch provides irq_set_affinity() implementation for PLIC driver.
> > It also updates irq_enable() such that PLIC interrupts are only enabled
> > for one of CPUs specified in IRQ affinity mask.
> >
> > With this patch in-place, we can change IRQ affinity at any-time from
> > user-space using procfs.
> >
> > Example:
> >
> > / # cat /proc/interrupts
> >             CPU0       CPU1       CPU2       CPU3
> >    8:         44          0          0          0  SiFive PLIC   8  virtio0
> >   10:         48          0          0          0  SiFive PLIC  10  ttyS0
> > IPI0:        55        663         58        363  Rescheduling interrupts
> > IPI1:         0          1          3         16  Function call interrupts
> > / #
> > / #
> > / # echo 4 > /proc/irq/10/smp_affinity
> > / #
> > / # cat /proc/interrupts
> >             CPU0       CPU1       CPU2       CPU3
> >    8:         45          0          0          0  SiFive PLIC   8  virtio0
> >   10:        160          0         17          0  SiFive PLIC  10  ttyS0
> > IPI0:        68        693         77        410  Rescheduling interrupts
> > IPI1:         0          2          3         16  Function call interrupts
> >
> > Signed-off-by: Anup Patel <anup@brainfault.org>
> > ---
> >   drivers/irqchip/irq-sifive-plic.c | 35 +++++++++++++++++++++++++++++--
> >   1 file changed, 33 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/irqchip/irq-sifive-plic.c b/drivers/irqchip/irq-sifive-plic.c
> > index ffd4deaca057..fec7da3797fa 100644
> > --- a/drivers/irqchip/irq-sifive-plic.c
> > +++ b/drivers/irqchip/irq-sifive-plic.c
> > @@ -98,14 +98,42 @@ static void plic_irq_toggle(const struct cpumask *mask, int hwirq, int enable)
> >
> >   static void plic_irq_enable(struct irq_data *d)
> >   {
> > -     plic_irq_toggle(irq_data_get_affinity_mask(d), d->hwirq, 1);
> > +     unsigned int cpu = cpumask_any_and(irq_data_get_affinity_mask(d),
> > +                                        cpu_online_mask);
> > +     WARN_ON(cpu >= nr_cpu_ids);
> > +     plic_irq_toggle(cpumask_of(cpu), d->hwirq, 1);
> >   }
> >
> >   static void plic_irq_disable(struct irq_data *d)
> >   {
> > -     plic_irq_toggle(irq_data_get_affinity_mask(d), d->hwirq, 0);
> > +     plic_irq_toggle(cpu_possible_mask, d->hwirq, 0);
> >   }
> >
> > +#ifdef CONFIG_SMP
> > +static int plic_set_affinity(struct irq_data *d, const struct cpumask *mask_val,
> > +                         bool force)
> > +{
> > +     unsigned int cpu;
> > +
> > +     if (!force)
> > +             cpu = cpumask_any_and(mask_val, cpu_online_mask);
> > +     else
> > +             cpu = cpumask_first(mask_val);
> > +
> > +     if (cpu >= nr_cpu_ids)
> > +             return -EINVAL;
> > +
> > +     if (!irqd_irq_disabled(d)) {
> > +             plic_irq_toggle(cpu_possible_mask, d->hwirq, 0);
> > +             plic_irq_toggle(cpumask_of(cpu), d->hwirq, 1);
>
> irq is disabled for a fraction of time for cpu as well.
> You can use cpumask_andnot to avoid that.
>
>
> Moreover, something is weird here. I tested the patch in Unleashed with
> a debug statement.
>
> Here are the cpumask plic_set_affinity receives.

The smp_affinity in procfs takes hex values as input.

1 = CPU0
2 = CPU1
3 = CPU0-1
4 = CPU2
... and so on ...

>
> # echo 0 > /proc[  280.810000] plic: plic_set_affinity: set affinity [0-1]
> [  280.810000] plic: plic_set_affinity: cpu = [0] irq = 4

OK, this is strange.

> # echo 1 > /proc[  286.290000] plic: plic_set_affinity: set affinity [0]
> [  286.290000] plic: plic_set_affinity: cpu = [0] irq = 4

This is correct.

> # echo 2 > /proc[  292.130000] plic: plic_set_affinity: set affinity [1]
> [  292.130000] plic: plic_set_affinity: cpu = [1] irq = 4

This is correct.

> # echo 3 > /proc[  297.750000] plic: plic_set_affinity: set affinity [0-1]
> [  297.750000] plic: plic_set_affinity: cpu = [0] irq = 4

This is correct.

>
> # echo 2 > /proc/irq/4/smp_affinity
> [  322.850000] plic: plic_set_affinity: set affinity [1]
> [  322.850000] plic: plic_set_affinity: cpu = [1] irq = 4

This is correct.

>
> I have not figured out why it receive cpu mask for 0 & 3.
> Not sure if logical cpu id to hart id mapping is responsible for other
> two case. I will continue to test tomorrow.

Except value '0', all cases are correct.

Regards,
Anup