Re: ledtrig-cpu: Limit to 4 CPUs

[Jacek Anaszewski]

On 9/20/20 7:33 PM, Marek Behun wrote:
> On Sun, 20 Sep 2020 18:55:28 +0200
> Jacek Anaszewski <jacek.anaszewski@xxxxxxxxx> wrote:
>
> > 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.
>
> Hi Jacek,
>
> I understand (and Pavel does for sure too) that many people
> currently have that possibility, that they have access to machines with
> many CPUs and many LEDs. We also understand that currently it is
> possible for users to set 1847th LED to trigger on activity on CPU ID
> 1337. What we are suggesting is that practically no one uses this, and
> for those 10 people who do, well it would be better for them to migrate
> to new ABI than for kernel developers having forever maintain this
> legacy ABI.
>
> Legacy drivers get removed from kernel from time to time, if no one
> uses them. So I think Pavel's proposal (although I may not agree with
> the limit 4) has some merit. If we try this, and someone complains, we
> can then discuss. If we don't try, we may never know.

Just go ahead without my ack. I just wanted not to let it go without
any discussion. At least we leave a trace...

> > > 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