Re: [PATCH v3 3/4] net: mana: Allow irq_setup() to skip cpus for affinity

Shradha Gupta
2025-05-15T04:51:19Z

On Wed, May 14, 2025 at 01:58:40PM -0400, Yury Norov wrote:
> On Wed, May 14, 2025 at 05:26:45PM +0000, Michael Kelley wrote:
> > > Hope that helps.
> >
> > Yes, that helps! So the key to understanding "weight" is that
> > NUMA locality is preferred over sibling dislocality.
> >
> > This is a great summary! All or most of it should go as a
> > comment describing the function and what it is trying to do.
>
> OK, please consider applying:
>
> >From abdf5cc6dabd7f433b1d1e66db86333a33a2cd15 Mon Sep 17 00:00:00 2001
> From: Yury Norov [NVIDIA] <yury.norov@gmail.com>
> Date: Wed, 14 May 2025 13:45:26 -0400
> Subject: [PATCH] net: mana: explain irq_setup() algorithm
>
> Commit 91bfe210e196 ("net: mana: add a function to spread IRQs per CPUs")
> added the irq_setup() function that distributes IRQs on CPUs according
> to a tricky heuristic. The corresponding commit message explains the
> heuristic.
>
> Duplicate it in the source code to make available for readers without
> digging git in history. Also, add more detailed explanation about how
> the heuristics is implemented.
>
> Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
> ---
> .../net/ethernet/microsoft/mana/gdma_main.c | 41 +++++++++++++++++++
> 1 file changed, 41 insertions(+)
>
> diff --git a/drivers/net/ethernet/microsoft/mana/gdma_main.c b/drivers/net/ethernet/microsoft/mana/gdma_main.c
> index 4ffaf7588885..f9e8d4d1ba3a 100644
> --- a/drivers/net/ethernet/microsoft/mana/gdma_main.c
> +++ b/drivers/net/ethernet/microsoft/mana/gdma_main.c
> @@ -1288,6 +1288,47 @@ void mana_gd_free_res_map(struct gdma_resource *r)
> r->size = 0;
> }
>
> +/*
> + * Spread on CPUs with the following heuristics:
> + *
> + * 1. No more than one IRQ per CPU, if possible;
> + * 2. NUMA locality is the second priority;
> + * 3. Sibling dislocality is the last priority.
> + *
> + * Let's consider this topology:
> + *
> + * Node 0 1
> + * Core 0 1 2 3
> + * CPU 0 1 2 3 4 5 6 7
> + *
> + * The most performant IRQ distribution based on the above topology
> + * and heuristics may look like this:
> + *
> + * IRQ Nodes Cores CPUs
> + * 0 1 0 0-1
> + * 1 1 1 2-3
> + * 2 1 0 0-1
> + * 3 1 1 2-3
> + * 4 2 2 4-5
> + * 5 2 3 6-7
> + * 6 2 2 4-5
> + * 7 2 3 6-7
> + *
> + * The heuristics is implemented as follows.
> + *
> + * The outer for_each() loop resets the 'weight' to the actual number
> + * of CPUs in the hop. Then inner for_each() loop decrements it by the
> + * number of sibling groups (cores) while assigning first set of IRQs
> + * to each group. IRQs 0 and 1 above are distributed this way.
> + *
> + * Now, because NUMA locality is more important, we should walk the
> + * same set of siblings and assign 2nd set of IRQs (2 and 3), and it's
> + * implemented by the medium while() loop. We do like this unless the
> + * number of IRQs assigned on this hop will not become equal to number
> + * of CPUs in the hop (weight == 0). Then we switch to the next hop and
> + * do the same thing.
> + */
> +
> static int irq_setup(unsigned int *irqs, unsigned int len, int node)
> {
> const struct cpumask *next, *prev = cpu_none_mask;

Thank you Yury,

I will include this patch in the patchset with the next version.

> --
> 2.43.0