net: mana: explain irq_setup() algorithm

JIRA: https://issues.redhat.com/browse/RHEL-109580

commit 4607617
Author: Yury Norov <yury.norov@gmail.com>
Date:   Wed Jun 11 07:10:29 2025 -0700

    net: mana: explain irq_setup() algorithm

    Commit 91bfe21 ("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 <yury.norov@gmail.com>
    Signed-off-by: Shradha Gupta <shradhagupta@linux.microsoft.com>

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>

Author: Maxim Levitsky

drivers/net/ethernet/microsoft/mana/gdma_main.c

@@ -1367,6 +1367,47 @@ 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;