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

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Message-ID: <91f1caa7-8005-6c8f-ce7c-84e5c8cee5f8@gmail.com>
Date:   Sun, 20 Sep 2020 18:55:28 +0200
From:   Jacek Anaszewski <jacek.anaszewski@...il.com>
To:     Marek Behun <marek.behun@....cz>
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 9/20/20 5:39 PM, Marek Behun wrote:
> 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?

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