[PATCH] powerpc: Warn about use of smt_snooze_delay

[Tyrel Datwyler]

On 6/25/20 3:29 AM, Christophe Leroy wrote:
> 
> 
> Le 25/06/2020 à 12:03, Joel Stanley a écrit :
>> 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.
> 
> Why not just drop the file entirely  if it is useless ?

Userspace tooling that hasn't learned its useless yet. Joel has also submitted a
pull request for the ppc64_util tool in question to drop using this interface.

-Tyrel

> 
> Christophe
> 
>>
>> Signed-off-by: Joel Stanley <joel at jms.id.au>
>> ---
>>   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);
>>