Subject: Re: [PATCH v2] powerpc: Warn about use of smt_snooze_delay

From: Michael Ellerman <[email-masked]>
Date: Tue, 04 Aug 2020 21:59:44 +1000

Joel Stanley <[email-masked]> writes:
> It's not done anything for a long time. Save the percpu variable, and
> emit a warning to remind users to not expect it to do anything.
>
> Fixes: 3fa8cad82b94 ("powerpc/pseries/cpuidle: smt-snooze-delay cleanup.")
> Cc: [email-masked] # v3.14
> Signed-off-by: Joel Stanley <[email-masked]>
> --
> v2:
>  Use pr_warn instead of WARN
>  Reword and print proccess name with pid in message
>  Leave CPU_FTR_SMT test in
>  Add Fixes line
>
> mpe, if you don't agree then feel free to drop the cc stable.
>
> Testing 'ppc64_cpu --smt=off' on a 24 core / 4 SMT system it's quite noisy
> as the online/offline loop that ppc64_cpu runs is slow.

Hmm, that is pretty spammy.

I know I suggested the ratelimit, but I was thinking it would print once
for each ppc64_cpu invocation, not ~30 times.

How about pr_warn_once(), that should still be sufficient for people to
notice if they're looking for it.

...
> diff --git a/arch/powerpc/kernel/sysfs.c b/arch/powerpc/kernel/sysfs.c
> index 571b3259697e..ba6d4cee19ef 100644
> --- a/arch/powerpc/kernel/sysfs.c
> +++ b/arch/powerpc/kernel/sysfs.c
> @@ -32,29 +32,26 @@
>  
>  static DEFINE_PER_CPU(struct cpu, cpu_devices);
>  
> -/*
> - * SMT snooze delay stuff, 64-bit only for now
> - */
> -
>  #ifdef CONFIG_PPC64
>  
> -/* Time in microseconds we delay before sleeping in the idle loop */
> -static DEFINE_PER_CPU(long, smt_snooze_delay) = { 100 };
> +/*
> + * Snooze delay has not been hooked up since 3fa8cad82b94 ("powerpc/pseries/cpuidle:
> + * smt-snooze-delay cleanup.") and has been broken even longer. As was foretold in
> + * 2014:
> + *
> + *  "ppc64_util currently utilises it. Once we fix ppc64_util, propose to clean
> + *  up the kernel code."
> + *
> + * At some point in the future this code should be removed.
> + */
>  
>  static ssize_t store_smt_snooze_delay(struct device *dev,
>  				      struct device_attribute *attr,
>  				      const char *buf,
>  				      size_t count)
>  {
> -	struct cpu *cpu = container_of(dev, struct cpu, dev);
> -	ssize_t ret;
> -	long snooze;
> -
> -	ret = sscanf(buf, "%ld", &snooze);
> -	if (ret != 1)
> -		return -EINVAL;
> -
> -	per_cpu(smt_snooze_delay, cpu->dev.id) = snooze;
> +	pr_warn_ratelimited("%s (%d) used unsupported smt_snooze_delay, this has no effect\n",
> +			    current->comm, current->pid);

Can we make this:

	"%s (%d) stored to unsupported smt_snooze_delay, which has no effect.\n",

>  	return count;
>  }
>  
> @@ -62,9 +59,9 @@ static ssize_t show_smt_snooze_delay(struct device *dev,
>  				     struct device_attribute *attr,
>  				     char *buf)
>  {
> -	struct cpu *cpu = container_of(dev, struct cpu, dev);
> -
> -	return sprintf(buf, "%ld\n", per_cpu(smt_snooze_delay, cpu->dev.id));
> +	pr_warn_ratelimited("%s (%d) used unsupported smt_snooze_delay, this has no effect\n",
> +			    current->comm, current->pid);

It has as much effect as it ever did :)

So maybe:

	"%s (%d) read from unsupported smt_snooze_delay.\n",

I can do those changes when applying if you like rather than making you
do a v3.

cheers