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

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Date:   Sun, 20 Sep 2020 19:49:52 +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 7:33 PM, Marek Behun wrote:
> On Sun, 20 Sep 2020 18:55:28 +0200
> Jacek Anaszewski <jacek.anaszewski@...il.com> wrote:
> 
>> 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.
> 
> 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