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

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Date:   Tue, 10 Aug 2021 10:35:13 -0400
From:   Nitesh Lal <nilal@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     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, Peter Zijlstra <peterz@...radead.org>,
        Jesse Brandeburg <jesse.brandeburg@...el.com>
Subject: Re: [RFC] genirq: Add effective CPU index retrieving interface

On Tue, Aug 10, 2021 at 10:13 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> 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?

Agreed, probably it will make more sense for the drivers to pass either the
local NUMA node index or NULL (in case they don't care about it) as a
parameter then at the time of allocation, we only find the best-fit CPUs
from that NUMA?

or, maybe we should do this by default, and if the local NUMA CPUs run out
of available vectors then we go to the other NUMA node CPUs.

>
> > +/**
> > + * 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
>


-- 
Thanks
Nitesh