Re: Control of IRQ Affinities from Userspace

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2025-10-30 14:20:01 [+0000], Preclik, Tobias wrote:
> Dear Linux RT Experts,
> 
> the Linux kernel exposes two userspace interfaces to control IRQ
> affinities in the procfs [1]. First, /proc/irq/default_smp_affinity
> specifies the default affinity mask which gets applied as initial
> affinity mask for newly registering IRQs. Second,
> /proc/irq/${IRQ_NO}/smp_affinity which can be used to specify an
> affinity mask for a given IRQ.
> 
> For tuning the I/O path of RT applications we use the second interface
> to relocate IRQs to cores dedicated to run RT applications. However, we
> have observed that certain situations, such as interface bring-up or
> loading BPF/XDP programs, can cause the IRQ affinity mask to be lost.
> Specifically, some network drivers, particularly those based on stmmac,
> ignore the IRQ affinity mask set from userspace and overwrite it with
> decisions from IRQ rebalancing [2]. This driver behavior prevents
> consistent control of IRQ affinities from userspace, impacting the
> tuning of the I/O path for RT applications.
> 
> I would greatly appreciate any comments or guidance on this issue.

The usage of irq_set_affinity_hint() is not uncommon within the
networking drivers. It is probably a pity if the request happens on each
ifdown/ up but in this case it happens each time you add/ remove a XDP
program.

But the interrupt should be managed by the kernel. Looking at
irq_do_set_affinity() there is:
|         if (irqd_affinity_is_managed(data) &&
|             housekeeping_enabled(HK_TYPE_MANAGED_IRQ)) {
|                 const struct cpumask *hk_mask;
|
|                 hk_mask = housekeeping_cpumask(HK_TYPE_MANAGED_IRQ);
|
|                 cpumask_and(tmp_mask, mask, hk_mask);
|                 if (!cpumask_intersects(tmp_mask, cpu_online_mask))
|                         prog_mask = mask;
|                 else
|                         prog_mask = tmp_mask;
|         } else {

so if the IRQ is managed and you have IRQ isolation enabled then it
should exclude the non-isolated CPUs. Would that work?

> Best regards,
> Tobias Preclik

Sebastian