Re: [PATCH] Created tunable to force small pages on stack allocation.

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 13/03/23 07:07, Cupertino Miranda wrote:
> 
> Hi Florian
> 
>> * Cupertino Miranda via Libc-alpha:
>>
>>> +static void
>>> +TUNABLE_CALLBACK (set_stack_hugetlb) (tunable_val_t *valp)
>>> +{
>>> +  enum malloc_thp_mode_t thp_mode = __malloc_thp_mode ();
>>> +  /*
>>> +     Only allow to change the default of this tunable if the system
>>> +     does support and has either 'madvise' or 'always' mode. Otherwise
>>> +     the madvise() call is wasteful.
>>> +  */
>>> +  switch(thp_mode)
>>> +    {
>>> +    case malloc_thp_mode_always:
>>> +    case malloc_thp_mode_madvise:
>>> +      __nptl_stack_hugetlb = (int32_t) valp->numval;
>>> +      break;
>>> +    default:
>>> +      break;
>>> +    }
>>> +}
>>
>> I suspect that quite a few failing madvise calls are cheaper than the
>> those three system calls in __malloc_thp_mode.  In addition,
>> __malloc_thp_mode may fail due to future kernel changes, disabling the
>> tunable by accident.
> Thanks for your review. Ok, I did not check the inner workings of
> malloc_thp_mode. I used it by suggestion from the RFC thread.
> I will prepare a version removing it.

It only make sense to madvise iff __malloc_thp_mode is set as 'always',
and tunable is a opt-in feature.  So I think it would make sense to
check the sysfs is tunable is used; the startup code will be amortized
on high threads workloads.