linux-kernel - Re: [PATCH 0/3] Enable MSI affinity support for dwc PCI

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Message-ID: <wzegusdukhm4e3bogyx7lfgjvjqd342mfielw2gzynquohnugf@wzqdbycndn6i>
Date: Fri, 3 Oct 2025 22:02:37 +0530
From: Manivannan Sadhasivam <mani@...nel.org>
To: Radu Rendec <rrendec@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, 
	Bjorn Helgaas <bhelgaas@...gle.com>, Rob Herring <robh@...nel.org>, 
	Krzysztof Wilczyński <kwilczynski@...nel.org>, Lorenzo Pieralisi <lpieralisi@...nel.org>, 
	Jingoo Han <jingoohan1@...il.com>, Brian Masney <bmasney@...hat.com>, 
	Eric Chanudet <echanude@...hat.com>, Alessandro Carminati <acarmina@...hat.com>, 
	Jared Kangas <jkangas@...hat.com>, linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Daniel Tsai <danielsftsai@...gle.com>, Marek Behún <kabel@...nel.org>, 
	Krishna Chaitanya Chundru <quic_krichai@...cinc.com>
Subject: Re: [PATCH 0/3] Enable MSI affinity support for dwc PCI

+ folks who were part of previous attempts

On Fri, Oct 03, 2025 at 12:04:18PM -0400, Radu Rendec wrote:
> Various attempts have been made so far to support CPU affinity control
> for (de)multiplexed interrupts. Some examples are [1] and [2]. That work
> was centered around the idea to control the parent interrupt's CPU
> affinity, since the child interrupt handler runs in the context of the
> parent interrupt handler, on whatever CPU it was triggered.
> 
> This is a new attempt based on a different approach. Instead of touching
> the parent interrupt's CPU affinity, the child interrupt is allowed to
> freely change its affinity setting, independently of the parent. If the
> interrupt handler happens to be triggered on an "incompatible" CPU (a
> CPU that's not part of the child interrupt's affinity mask), the handler
> is redirected and runs in IRQ work context on a "compatible" CPU. This
> is a direct follow up to the (unsubmitted) patches that Thomas Gleixner
> proposed in [3].
> 
> The first patch adds support for interrupt redirection to the IRQ core,
> without making any functional change to irqchip drivers. The other two
> patches modify the dwc PCI core driver to enable interrupt redirection
> using the new infrastructure added in the first patch.
> 
> Thomas, however, I made a small design change to your original patches.
> Instead of keeping track of the parent interrupt's affinity setting (or
> rather the first CPU in its affinity mask) and attempting to pick the
> same CPU for the child (as the target CPU) if possible, I just check if
> the child handler fires on a CPU that's part of its affinity mask (which
> is already stored anyway). As an optimization for the case when the
> current CPU is *not* part of the mask and the handler needs to be
> redirected, I pre-calculate and store the first CPU in the mask, at the
> time when the child affinity is set. In my opinion, this is simpler and
> cleaner, at the expense of a cpumask_test_cpu() call on the fast path,
> because:
> - It no longer needs to keep track of the parent interrupt's affinity
>   setting.
> - If the parent interrupt can run on more than one CPU, the child can
>   also run on any of those CPUs without being redirected (in case the
>   child's affinity mask is the same as the parent's or a superset).
> 
> Last but not least, since most of the code in these patches is your
> code, I took the liberty to add your From and Signed-off-by tags to
> properly attribute authorship. I hope that's all right, and if for any
> reason you don't want that, then please accept my apologies and I will
> remove them in a future version. Of course, you can always remove them
> yourself if you want (assuming the patches are merged at some point),
> since you are the maintainer :)
> 
> [1] https://lore.kernel.org/all/20220502102137.764606ee@thinkpad/
> [2] https://lore.kernel.org/all/20230530214550.864894-1-rrendec@redhat.com/
> [3] https://lore.kernel.org/linux-pci/878qpg4o4t.ffs@tglx/
> 
> Radu Rendec (3):
>   genirq: Add interrupt redirection infrastructure
>   PCI: dwc: Code cleanup
>   PCI: dwc: Enable MSI affinity support
> 
>  .../pci/controller/dwc/pcie-designware-host.c | 123 ++++++++----------
>  drivers/pci/controller/dwc/pcie-designware.h  |   7 +-
>  include/linux/irq.h                           |   6 +
>  include/linux/irqdesc.h                       |  11 +-
>  kernel/irq/chip.c                             |  20 +++
>  kernel/irq/irqdesc.c                          |  51 +++++++-
>  kernel/irq/manage.c                           |  16 ++-
>  7 files changed, 154 insertions(+), 80 deletions(-)
> 
> -- 
> 2.51.0
> 

-- 
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