linux-kernel - Re: [RFC PATCH 0/5] irq: sysfs interface improvements for SMP affinity control

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Message-ID: <b3105ace22a8f5f3a56adceffd28dec6b522ecca.camel@redhat.com>
Date:   Wed, 31 May 2023 18:54:11 -0400
From:   Radu Rendec <rrendec@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org
Cc:     Marc Zyngier <maz@...nel.org>
Subject: Re: [RFC PATCH 0/5] irq: sysfs interface improvements for SMP
 affinity control

On Wed, 2023-05-31 at 15:09 +0200, Thomas Gleixner wrote:
> From a conceptual POV I understand why this is required, which makes me
> hate this chained mechanism even more.
> 
> [cut]
> 
> AFAICT, there is no real technical reason at least not for the vast
> majority of usage sites why the demultiplex handler cannot run inside a
> regular interrupt handler context.

Thanks for taking the time to explain everything in such detail.
Much appreciated!

> So I personally would prefer to get rid of this chained oddball and just
> have consistent mechanisms for dealing with interrupts, which would just
> avoid exposing the affinity files in two different places.

Does this mean that if I came across a chained driver that lacked
affinity support, then changing it to use regular interrupts via
request_irq() would be a viable solution to enable affinity support
and you would consider accepting such a patch?

Taking a step back, in the case of hierarchical domains where *no*
multiplexing is involved, do you consider setting .irq_set_affinity()
to irq_chip_set_affinity_parent() a good way to enable affinity support
(assuming, of course, the driver lacks such support originally)?

At the end of the day, what I'm trying to do is find a way to enable
affinity support for a few specific drivers where it's lacking. My
initial impression, after reading Marc's message[1], was that the fix
lay at the system level, at least for multiplexing drivers. Hence my
naive attempt at a system-level fix. It is now becoming clear that the
fix will have to be evaluated/addressed at the driver level, on a case
by case basis.

As a side note, one thing I particularly like about your approach is
that it doesn't require any changes to irqbalance.

> Providing information about child/parent relationship is an orthogonal
> issue.

Agreed. Do you see any value in doing that? And, if yes, is reusing
(struct irq_desc).parent_irq and irq_set_parent() a good way of doing
it? FWIW, a multiplexing driver could do that regardless of how it
registers a handler for the parent interrupt (chained/regular).

> If there is some good reason (aside of the chained muck) to have sysfs
> based affinity management, then I'm not objecting as long as the
> functionality is the same, i.e. effective affinity needs be exposed too.

I can't think of any other reason. AFAICT, chained interrupts are the
only interrupts that are active but not visible in procfs. For any
other purpose, the procfs interface is good as it is.

Thanks,
Radu

[1] https://lore.kernel.org/all/87fslr7ygl.wl-maz@kernel.org/