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

[Florian Weimer]

* 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,
Florian