linux-kernel - Re: ledtrig-cpu: Limit to 4 CPUs

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Date:   Sun, 20 Sep 2020 17:39:05 +0200
From:   Marek Behun <marek.behun@....cz>
To:     Jacek Anaszewski <jacek.anaszewski@...il.com>
Cc:     Pavel Machek <pavel@....cz>, linux-leds@...r.kernel.org,
        dmurphy@...com,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: ledtrig-cpu: Limit to 4 CPUs

On Sun, 20 Sep 2020 16:15:09 +0200
Jacek Anaszewski <jacek.anaszewski@...il.com> wrote:

> Hi Pavel,
> 
> On 9/19/20 11:38 AM, Pavel Machek wrote:
> > commit 318681d3e019e39354cc6c2155a7fd1bb8e8084d
> > Author: Pavel Machek <pavel@....cz>
> > Date:   Sat Sep 19 11:34:58 2020 +0200
> > 
> >      ledtrig-cpu: Limit to 4 CPUs
> >      
> >      Some machines have thousands of CPUs... and trigger mechanisms was not
> >      really meant for thousands of triggers. I doubt anyone uses this
> >      trigger on many-CPU machine; but if they do, they'll need to do it
> >      properly.
> >      
> >      Signed-off-by: Pavel Machek <pavel@....cz>
> > 
> > diff --git a/drivers/leds/trigger/ledtrig-cpu.c b/drivers/leds/trigger/ledtrig-cpu.c
> > index 869976d1b734..b7e00b09b137 100644
> > --- a/drivers/leds/trigger/ledtrig-cpu.c
> > +++ b/drivers/leds/trigger/ledtrig-cpu.c
> > @@ -2,14 +2,18 @@
> >   /*
> >    * ledtrig-cpu.c - LED trigger based on CPU activity
> >    *
> > - * This LED trigger will be registered for each possible CPU and named as
> > - * cpu0, cpu1, cpu2, cpu3, etc.
> > + * This LED trigger will be registered for first four CPUs and named
> > + * as cpu0, cpu1, cpu2, cpu3. There's additional trigger called cpu that
> > + * is on when any CPU is active.
> > + *
> > + * If you want support for arbitrary number of CPUs, make it one trigger,
> > + * with additional sysfs file selecting which CPU to watch.
> >    *
> >    * It can be bound to any LED just like other triggers using either a
> >    * board file or via sysfs interface.
> >    *
> >    * An API named ledtrig_cpu is exported for any user, who want to add CPU
> > - * activity indication in their code
> > + * activity indication in their code.
> >    *
> >    * Copyright 2011 Linus Walleij <linus.walleij@...aro.org>
> >    * Copyright 2011 - 2012 Bryan Wu <bryan.wu@...onical.com>
> > @@ -145,6 +149,9 @@ static int __init ledtrig_cpu_init(void)
> >   	for_each_possible_cpu(cpu) {
> >   		struct led_trigger_cpu *trig = &per_cpu(cpu_trig, cpu);
> >   
> > +		if (cpu > 4)  
> 
> NACK. The workaround for this trigger was implemented for a reason -
> to make it working on platforms with arbitrary number of logical cpus.
> I've got 8, so I am discriminated now. Not saying, that it precludes
> trigger registration with no single line of warning.
> Regardless of that - you have no guarantee that you're not breaking
> anyone - "I doubt" is not a sufficient argument.
> 

If that is the case Jacek, I would try 16 and then see if people
complain. Do you really think that someone sets a specific LED to
trigger on activity on CPU id > 16?

If you do not agree, then I think we should implement a "cpu" trigger
where the cpu ID (or maybe mask of multiple CPUs) is configurable via
another sysfs file. And then declare current cpu trigger (with names
"cpu%d") as legacy.

Marek