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

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Date:   Fri, 06 Dec 2019 15:22:53 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     John Garry <john.garry@...wei.com>
Cc:     <tglx@...utronix.de>, <chenxiang66@...ilicon.com>,
        <bigeasy@...utronix.de>, <linux-kernel@...r.kernel.org>,
        <hare@...e.com>, <ming.lei@...hat.com>, <hch@....de>,
        <axboe@...nel.dk>, <bvanassche@....org>, <peterz@...radead.org>,
        <mingo@...hat.com>
Subject: Re: [PATCH RFC 1/1] genirq: Make threaded handler use irq affinity  for managed interrupt

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@...wei.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...