linux-kernel - Re: [PATCH] leds: trigger: ledtrig-cpu:: enforce NR

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Message-Id: <59044fff-6531-4425-8ccb-e21e17664f26@app.fastmail.com>
Date: Wed, 25 Jun 2025 12:24:51 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Lee Jones" <lee@...nel.org>, "Arnd Bergmann" <arnd@...nel.org>
Cc: "Pavel Machek" <pavel@...nel.org>, "Hans de Goede" <hansg@...nel.org>,
 linux-leds@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] leds: trigger: ledtrig-cpu:: enforce NR_CPUS limit in Kconfig

On Wed, Jun 25, 2025, at 11:24, Lee Jones wrote:
> On Fri, 20 Jun 2025, Arnd Bergmann wrote:
>> 
>> Enforce this limit in Kconfig instead to avoid the build failure.
>
> I have so many questions!
>
> - This number seems arbitrary - what is the limiting factor?
>   - Character size for printing?

That is my best guess, yes. The original commit 8f88731d052d
("led-triggers: create a trigger for CPU activity") already
had this, plus

#define MAX_NAME_LEN 8
snprintf(trig->name, MAX_NAME_LEN, "cpu%d", cpu);

which comes out to 10000 cpus (0 through 9999) in a NUL-terminated
string. Obviously we could have a higher limit here but would
technically still need to check that.

> - What platform did you test this on to blow through that number?

I was only compile-testing with a crazy number of CPUs, to see
what would break, in particular when there are per-cpu structures
or cpumask_t variables on the stack. I sent fixes for the ones
that broke the build, but don't expect anything to actually
need this at runtime. Some of the fixes I sent also address
possible stack overflows that happen with a more realistic
number of CPUs.

> - What if we really do want 10000+ CPUs?
>   - And what will that LED array look like to support them all?

abcc131292aa ("ledtrig-cpu: Limit to 8 CPUs") actually added a
runtime limit to eight CPUS, which is probably more reasonable.

> - If we're enforcing here, is the BUILD_BUG_ON() now superfluous?

Yes, but it documents the requirement better than just the
Kconfig dependency. Since there is a runtime limit, we could
probably just drop the compile-time check and not add the
Kconfig check.

      Arnd