Re: [PATCH RFC v1 1/2] genirq/affinity: add support for limiting managed interrupts

[Jason Wang]

On Fri, Nov 1, 2024 at 11:12 AM mapicccy <guanjun@xxxxxxxxxxxxxxxxx> wrote:
>
>
>
> 2024年10月31日 18:50，Ming Lei <ming.lei@xxxxxxxxxx> 写道：
>
> On Thu, Oct 31, 2024 at 6:35 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
>
> On Thu, Oct 31 2024 at 15:46, guanjun@xxxxxxxxxxxxxxxxx wrote:
>
> #ifdef CONFIG_SMP
>
> +static unsigned int __read_mostly managed_irqs_per_node;
> +static struct cpumask managed_irqs_cpumsk[MAX_NUMNODES] __cacheline_aligned_in_smp = {
> +     [0 ... MAX_NUMNODES-1] = {CPU_BITS_ALL}
> +};
>
> +static void __group_prepare_affinity(struct cpumask *premask,
> +                                  cpumask_var_t *node_to_cpumask)
> +{
> +     nodemask_t nodemsk = NODE_MASK_NONE;
> +     unsigned int ncpus, n;
> +
> +     get_nodes_in_cpumask(node_to_cpumask, premask, &nodemsk);
> +
> +     for_each_node_mask(n, nodemsk) {
> +             cpumask_and(&managed_irqs_cpumsk[n], &managed_irqs_cpumsk[n], premask);
> +             cpumask_and(&managed_irqs_cpumsk[n], &managed_irqs_cpumsk[n], node_to_cpumask[n]);
>
>
> How is this managed_irqs_cpumsk array protected against concurrency?
>
> +             ncpus = cpumask_weight(&managed_irqs_cpumsk[n]);
> +             if (ncpus < managed_irqs_per_node) {
> +                     /* Reset node n to current node cpumask */
> +                     cpumask_copy(&managed_irqs_cpumsk[n], node_to_cpumask[n]);
>
>
> This whole logic is incomprehensible and aside of the concurrency
> problem it's broken when CPUs are made present at run-time because these
> cpu masks are static and represent the stale state of the last
> invocation.
>
> Given the limitations of the x86 vector space, which is not going away
> anytime soon, there are only two options IMO to handle such a scenario.
>
>   1) Tell the nvme/block layer to disable queue affinity management
>
>
> +1
>
> There are other use cases, such as cpu isolation, which can benefit from
> this way too.
>
> https://lore.kernel.org/linux-nvme/20240702104112.4123810-1-ming.lei@xxxxxxxxxx/
>

I wonder if we need to do the same for virtio-blk.

>
> Thanks for your reminder. However, in this link only modified the NVMe driver,
> but there is the same issue in the virtio net driver as well.

I guess you meant virtio-blk actually?

>
> Guanjun
>
>
> Thanks,
>

Thanks