linux-kernel - Re: [RFC] genirq: Add effective CPU index retrieving interface

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Date:   Tue, 10 Aug 2021 16:12:53 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Shung-Hsi Yu <shung-hsi.yu@...e.com>, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, netdev@...r.kernel.org,
        linux-pci@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>,
        Nitesh Lal <nilal@...hat.com>
Subject: Re: [RFC] genirq: Add effective CPU index retrieving interface

On Mon, Jun 07 2021 at 15:33, Shung-Hsi Yu wrote:
> Most driver's IRQ spreading scheme is naive compared to the IRQ spreading
> scheme introduced since IRQ subsystem rework, so it better to rely
> request_irq() to spread IRQ out.
>
> However, drivers that care about performance enough also tends to try
> allocating memory on the same NUMA node on which the IRQ handler will run.
> For such driver to rely on request_irq() for IRQ spreading, we also need to
> provide an interface to retrieve the CPU index after calling
> request_irq().

So if you are interested in the resulting NUMA node, then why exposing a
random CPU out of the affinity mask instead of exposing a function to
retrieve the NUMA node?
  
> +/**
> + * irq_get_effective_cpu - Retrieve the effective CPU index
> + * @irq:	Target interrupt to retrieve effective CPU index
> + *
> + * When the effective affinity cpumask has multiple CPU toggled, it just
> + * returns the first CPU in the cpumask.
> + */
> +int irq_get_effective_cpu(unsigned int irq)
> +{
> +	struct irq_data *data = irq_get_irq_data(irq);

This can be NULL.

> +	struct cpumask *m;
> +
> +	m = irq_data_get_effective_affinity_mask(data);
> +	return cpumask_first(m);
> +}

Thanks,

        tglx