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

[Gautham R Shenoy <ego@linux.vnet.ibm.com>]

On Thu, Jun 25, 2020 at 07:33:49PM +0930, Joel Stanley wrote:
> 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.
> 
> Signed-off-by: Joel Stanley <joel@jms.id.au>

The only known user of "smt_snooze_delay" is the "ppc64_cpu" which
uses the presence of this file to assume that the system is SMT
capable.

Since we have "/sys/devices/system/cpu/smt/" these days, perhaps the
userspace utility can use that and we can get rid of the file
altogether ?

FWIW,
Acked-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
> ---
>  arch/powerpc/kernel/sysfs.c | 41 +++++++++++++------------------------
>  1 file changed, 14 insertions(+), 27 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/sysfs.c b/arch/powerpc/kernel/sysfs.c
> index 571b3259697e..530ae92bc46d 100644
> --- a/arch/powerpc/kernel/sysfs.c
> +++ b/arch/powerpc/kernel/sysfs.c
> @@ -32,29 +32,25 @@
>  
>  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;
> +	WARN_ON_ONCE("smt_snooze_delay sysfs file has no effect\n");
>  	return count;
>  }
>  
> @@ -62,9 +58,9 @@ 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);
> +	WARN_ON_ONCE("smt_snooze_delay sysfs file has no effect\n");
>  
> -	return sprintf(buf, "%ld\n", per_cpu(smt_snooze_delay, cpu->dev.id));
> +	return sprintf(buf, "100\n");
>  }
>  
>  static DEVICE_ATTR(smt_snooze_delay, 0644, show_smt_snooze_delay,
> @@ -72,16 +68,7 @@ static DEVICE_ATTR(smt_snooze_delay, 0644, show_smt_snooze_delay,
>  
>  static int __init setup_smt_snooze_delay(char *str)
>  {
> -	unsigned int cpu;
> -	long snooze;
> -
> -	if (!cpu_has_feature(CPU_FTR_SMT))
> -		return 1;
> -
> -	snooze = simple_strtol(str, NULL, 10);
> -	for_each_possible_cpu(cpu)
> -		per_cpu(smt_snooze_delay, cpu) = snooze;
> -
> +	WARN_ON_ONCE("smt-snooze-delay command line option has no effect\n");
>  	return 1;
>  }
>  __setup("smt-snooze-delay=", setup_smt_snooze_delay);
> -- 
> 2.27.0
>