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

[Atish Patra <atish.patra@wdc.com>]

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

> 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.

# echo 0 > /proc[  280.810000] plic: plic_set_affinity: set affinity [0-1]
[  280.810000] plic: plic_set_affinity: cpu = [0] irq = 4
# echo 1 > /proc[  286.290000] plic: plic_set_affinity: set affinity [0]
[  286.290000] plic: plic_set_affinity: cpu = [0] irq = 4
# echo 2 > /proc[  292.130000] plic: plic_set_affinity: set affinity [1]
[  292.130000] plic: plic_set_affinity: cpu = [1] irq = 4
# echo 3 > /proc[  297.750000] plic: plic_set_affinity: set affinity [0-1]
[  297.750000] plic: plic_set_affinity: cpu = [0] irq = 4

# 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

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.

Regards,
Atish
> +	}
> +

> +	irq_data_update_effective_affinity(d, cpumask_of(cpu));
> +
> +	return IRQ_SET_MASK_OK_DONE;
> +}
> +#endif
> +
>   static struct irq_chip plic_chip = {
>   	.name		= "SiFive PLIC",
>   	/*
> @@ -114,6 +142,9 @@ static struct irq_chip plic_chip = {
>   	 */
>   	.irq_enable	= plic_irq_enable,
>   	.irq_disable	= plic_irq_disable,
> +#ifdef CONFIG_SMP
> +	.irq_set_affinity = plic_set_affinity,
> +#endif
>   };
>   
>   static int plic_irqdomain_map(struct irq_domain *d, unsigned int irq,
>