Re: [PATCH v2 2/3] x86/smp: Allow forcing the mwait hint for play dead loop

[Dave Hansen <dave.hansen@intel.com>]

On 10/29/24 03:15, Patryk Wlazlyn wrote:
> +void smp_set_mwait_play_dead_hint(unsigned int hint)
> +{
> +	WRITE_ONCE(play_dead_mwait_hint, hint);
> +}

This all feels a bit hacky and unstructured to me.

Could we at least set up a few rules here?  Like, say what the hints
are, what values can they have?  Where do they come from?  Can this get
called more than once?  Does it _need_ to be set?  What's the behavior
when it is not set?  Who is responsible for calling this?

What good does the smp_ prefix do?  I don't think _callers_ care whether
this is getting optimized out or not.

> -	hint = get_deepest_mwait_hint();
> +	hint = READ_ONCE(play_dead_mwait_hint);
> +	if (hint == PLAY_DEAD_MWAIT_HINT_UNSET)
> +		hint = get_deepest_mwait_hint();

This is also rather opaque.

Why are there two hints?  What makes one better than the other one?