Re: [PATCH RFC 1/1] genirq: Make threaded handler use irq affinity for managed interrupt

Marc Zyngier
2019-12-06T15:23:05Z

Hi John,

On 2019-12-06 14:35, John Garry wrote:
> Currently the cpu allowed mask for the threaded part of a threaded
> irq
> handler will be set to the effective affinity of the hard irq.
>
> Typically the effective affinity of the hard irq will be for a single
> cpu. As such,
> the threaded handler would always run on the same cpu as the hard
> irq.
>
> We have seen scenarios in high data-rate throughput testing that the
> cpu
> handling the interrupt can be totally saturated handling both the
> hard
> interrupt and threaded handler parts, limiting throughput.
>
> For when the interrupt is managed, allow the threaded part to run on
> all
> cpus in the irq affinity mask.
>
> Signed-off-by: John Garry <john.garry@huawei.com>
> ---
> kernel/irq/manage.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
> index 1753486b440c..8e7f8e758a88 100644
> --- a/kernel/irq/manage.c
> +++ b/kernel/irq/manage.c
> @@ -968,7 +968,11 @@ irq_thread_check_affinity(struct irq_desc *desc,
> struct irqaction *action)
> if (cpumask_available(desc->irq_common_data.affinity)) {
> const struct cpumask *m;
>
> - m = irq_data_get_effective_affinity_mask(&desc->irq_data);
> + if (irqd_affinity_is_managed(&desc->irq_data))
> + m = desc->irq_common_data.affinity;
> + else
> + m = irq_data_get_effective_affinity_mask(
> + &desc->irq_data);
> cpumask_copy(mask, m);
> } else {
> valid = false;

Although I completely understand that there are cases where you
really want to let your thread roam all CPUs, I feel like changing
this based on a seemingly unrelated property is likely to trigger
yet another whack-a-mole episode. I'd feel much more comfortable
if there was a way to let the IRQ subsystem know about what is best.

Shouldn't the endpoint driver know better about it? Note that
I have no data supporting an approach or the other, hence playing
the role of the village idiot here.

Thanks,

M.
--
Jazz is not dead. It just smells funny...