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Date:   Fri, 16 Oct 2020 14:20:46 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Nitesh Narayan Lal <nitesh@redhat.com>
Cc:     linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        linux-pci@vger.kernel.org, intel-wired-lan@lists.osuosl.org,
        frederic@kernel.org, mtosatti@redhat.com, sassmann@redhat.com,
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        helgaas@kernel.org, jeffrey.t.kirsher@intel.com,
        jacob.e.keller@intel.com, jlelli@redhat.com, hch@infradead.org,
        bhelgaas@google.com, mike.marciniszyn@intel.com,
        dennis.dalessandro@intel.com, thomas.lendacky@amd.com,
        jiri@nvidia.com, mingo@redhat.com, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, lgoncalv@redhat.com
Subject: Re: [PATCH v4 4/4] PCI: Limit pci_alloc_irq_vectors() to
 housekeeping CPUs
Message-ID: <20201016122046.GP2611@hirez.programming.kicks-ass.net>
References: <20200928183529.471328-1-nitesh@redhat.com>
 <20200928183529.471328-5-nitesh@redhat.com>
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On Mon, Sep 28, 2020 at 02:35:29PM -0400, Nitesh Narayan Lal wrote:
> If we have isolated CPUs dedicated for use by real-time tasks, we try to
> move IRQs to housekeeping CPUs from the userspace to reduce latency
> overhead on the isolated CPUs.
> 
> If we allocate too many IRQ vectors, moving them all to housekeeping CPUs
> may exceed per-CPU vector limits.
> 
> When we have isolated CPUs, limit the number of vectors allocated by
> pci_alloc_irq_vectors() to the minimum number required by the driver, or
> to one per housekeeping CPU if that is larger.
> 
> Signed-off-by: Nitesh Narayan Lal <nitesh@redhat.com>
> ---
>  drivers/pci/msi.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
> index 30ae4ffda5c1..8c156867803c 100644
> --- a/drivers/pci/msi.c
> +++ b/drivers/pci/msi.c
> @@ -23,6 +23,7 @@
>  #include <linux/slab.h>
>  #include <linux/irqdomain.h>
>  #include <linux/of_irq.h>
> +#include <linux/sched/isolation.h>
>  
>  #include "pci.h"
>  
> @@ -1191,8 +1192,25 @@ int pci_alloc_irq_vectors_affinity(struct pci_dev *dev, unsigned int min_vecs,
>  				   struct irq_affinity *affd)
>  {
>  	struct irq_affinity msi_default_affd = {0};
> +	unsigned int hk_cpus;
>  	int nvecs = -ENOSPC;
>  
> +	hk_cpus = housekeeping_num_online_cpus(HK_FLAG_MANAGED_IRQ);
> +
> +	/*
> +	 * If we have isolated CPUs for use by real-time tasks, to keep the
> +	 * latency overhead to a minimum, device-specific IRQ vectors are moved
> +	 * to the housekeeping CPUs from the userspace by changing their
> +	 * affinity mask. Limit the vector usage to keep housekeeping CPUs from
> +	 * running out of IRQ vectors.
> +	 */
> +	if (hk_cpus < num_online_cpus()) {
> +		if (hk_cpus < min_vecs)
> +			max_vecs = min_vecs;
> +		else if (hk_cpus < max_vecs)
> +			max_vecs = hk_cpus;

is that:

		max_vecs = clamp(hk_cpus, min_vecs, max_vecs);

Also, do we really need to have that conditional on hk_cpus <
num_online_cpus()? That is, why can't we do this unconditionally?

And what are the (desired) semantics vs hotplug? Using a cpumask without
excluding hotplug is racy.

> +	}
> +
>  	if (flags & PCI_IRQ_AFFINITY) {
>  		if (!affd)
>  			affd = &msi_default_affd;
> -- 
> 2.18.2
>