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Date:   Thu, 7 Nov 2019 03:10:32 +0900
From:   Keith Busch <kbusch@kernel.org>
To:     Jon Derrick <jonathan.derrick@intel.com>
Cc:     Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        Bjorn Helgaas <helgaas@kernel.org>, linux-pci@vger.kernel.org
Subject: Re: [PATCH 3/3] PCI: vmd: Use managed irq affinities
Message-ID: <20191106181032.GD29853@redsun51.ssa.fujisawa.hgst.com>
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On Wed, Nov 06, 2019 at 04:40:08AM -0700, Jon Derrick wrote:
> Using managed IRQ affinities sets up the VMD affinities identically to
> the child devices when those devices vector counts are limited by VMD.
> This promotes better affinity handling as interrupts won't necessarily
> need to pass context between non-local CPUs. One pre-vector is reserved
> for the slow interrupt and not considered in the affinity algorithm.

This only works if all devices have exactly the same number of interrupts
as the parent VMD host bridge. If a child device has less, the device
will stop working if you offline a cpu: the child device may have a
resource affined to other online cpus, but the VMD device affinity is to
that single offline cpu.