Re: ledtrig-cpu: Limit to 4 CPUs

[Jacek Anaszewski]

On 9/20/20 5:39 PM, Marek Behun wrote:
> On Sun, 20 Sep 2020 16:15:09 +0200
> Jacek Anaszewski <jacek.anaszewski@xxxxxxxxx> wrote:
>
> > Hi Pavel,
> >
> > On 9/19/20 11:38 AM, Pavel Machek wrote:
> > > commit 318681d3e019e39354cc6c2155a7fd1bb8e8084d
> > > Author: Pavel Machek <pavel@xxxxxx>
> > > 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@xxxxxx>
> > >
> > > 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@xxxxxxxxxx>
> > >     * Copyright 2011 - 2012 Bryan Wu <bryan.wu@xxxxxxxxxxxxx>
> > > @@ -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?

I have an access to the machine with 80 cpus, so I could once
get surprised not being able to find cpuN triggers not being
listed among available triggers.

And say that I have a solution where I install 80 userspace LEDs
(drivers/leds/uleds.c) and register them on each cpuN triggers to get
notifications on how cpus work.

> 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.

Yes, we can do that, and even mark the cpu trigger as legacy but we
cannot prevent people from using it if that was present in kernel
for many years.

--
Best regards,
Jacek Anaszewski