Re: [PATCH 3/4] mwait-idle: add 'preferred_cstates' module argument

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.04.2022 10:06, Roger Pau Monné wrote:
> On Thu, Apr 28, 2022 at 08:37:58AM +0200, Jan Beulich wrote:
>> On 27.04.2022 18:12, Roger Pau Monné wrote:
>>> On Wed, Apr 27, 2022 at 05:25:35PM +0200, Jan Beulich wrote:
>>>> On 27.04.2022 17:06, Roger Pau Monné wrote:
>>>>> On Wed, Apr 27, 2022 at 03:41:24PM +0200, Jan Beulich wrote:
>>>>>> On 27.04.2022 14:45, Roger Pau Monné wrote:
>>>>>>> On Tue, Apr 26, 2022 at 12:05:28PM +0200, Jan Beulich wrote:
>>>>>>>> --- unstable.orig/xen/arch/x86/cpu/mwait-idle.c
>>>>>>>> +++ unstable/xen/arch/x86/cpu/mwait-idle.c
>>>>>>>> @@ -82,6 +82,18 @@ boolean_param("mwait-idle", opt_mwait_id
>>>>>>>>  
>>>>>>>>  static unsigned int mwait_substates;
>>>>>>>>  
>>>>>>>> +/*
>>>>>>>> + * Some platforms come with mutually exclusive C-states, so that if 
>>>>>>>> one is
>>>>>>>> + * enabled, the other C-states must not be used. Example: C1 and C1E 
>>>>>>>> on
>>>>>>>> + * Sapphire Rapids platform. This parameter allows for selecting the
>>>>>>>> + * preferred C-states among the groups of mutually exclusive C-states 
>>>>>>>> - the
>>>>>>>> + * selected C-states will be registered, the other C-states from the 
>>>>>>>> mutually
>>>>>>>> + * exclusive group won't be registered. If the platform has no 
>>>>>>>> mutually
>>>>>>>> + * exclusive C-states, this parameter has no effect.
>>>>>>>> + */
>>>>>>>> +static unsigned int __ro_after_init preferred_states_mask;
>>>>>>>> +integer_param("preferred-cstates", preferred_states_mask);
>>>>>>>> +
>>>>>>>>  #define LAPIC_TIMER_ALWAYS_RELIABLE 0xFFFFFFFF
>>>>>>>>  /* Reliable LAPIC Timer States, bit 1 for C1 etc. Default to only C1. 
>>>>>>>> */
>>>>>>>>  static unsigned int lapic_timer_reliable_states = (1 << 1);
>>>>>>>> @@ -96,6 +108,7 @@ struct idle_cpu {
>>>>>>>>        unsigned long auto_demotion_disable_flags;
>>>>>>>>        bool byt_auto_demotion_disable_flag;
>>>>>>>>        bool disable_promotion_to_c1e;
>>>>>>>> +      bool enable_promotion_to_c1e;
>>>>>>>
>>>>>>> I'm confused by those fields, shouldn't we just have:
>>>>>>> promotion_to_c1e = true | false?
>>>>>>>
>>>>>>> As one field is the negation of the other:
>>>>>>> enable_promotion_to_c1e = !disable_promotion_to_c1e
>>>>>>>
>>>>>>> I know this is code from Linux, but would like to understand why two
>>>>>>> fields are needed.
>>>>>>
>>>>>> This really is a tristate; Linux is now changing their global variable
>>>>>> to an enum, but we don't have an equivalent of that global variable.
>>>>>
>>>>> So it would be: leave default, disable C1E promotion, enable C1E
>>>>> promotion.
>>>>>
>>>>> And Linux is leaving the {disable,enable}_promotion_to_c1e in
>>>>> idle_cpu?
>>>>
>>>> Iirc they only have disable_promotion_to_c1e there (as a struct field)
>>>> and keep it, but they convert the similarly named file-scope variable
>>>> to a tristate.
>>>>
>>>>> I guess there's not much we can do unless we want to diverge from
>>>>> upstream.
>>>>
>>>> We've diverged some from Linux here already - as said, for example we
>>>> don't have their file-scope variable. I could convert our struct field
>>>> to an enum, but that would be larger code churn for (I think) little
>>>> gain.
>>>
>>> Hm, OK, could gaining the file scope variable would make sense in order
>>> to reduce divergences?  Or are the other roadblocks there?
>>
>> I don't recall. It might have originated from a change I decided to not
>> port over, or I might have dropped it while porting. To be honest I'm
>> not keen on putting time into researching this, the more that I would
>> generally try to avoid static variables.
>>
>> What I would be willing to put time in is making a more user friendly
>> command line option, but as said - I can't think of any good alternative
>> (except perhaps "preferred-cstates=c1e" or "cstates=preferred:c1e", with
>> internal translation of the strings into a bit mask, as long as (a) you
>> would think that's an improvement and (b) the further divergence from
>> Linux is not deemed a problem).
> 
> I think (b) won't be a problem as long as internally the user option
> is translated into a bitmask.
> 
> Regarding (a) I do think it would be helpful to express this in a more
> user friendly way, I'm not sure whether it would make sense to keep
> Linux format also for compatibility reasons if users already have a
> bitmask and want to use the same parameter for Xen and Linux, ie:
> 
> preferred-cstates = <string of c1e,c1,...> | <integer bitmask>

Yes, I would have been planning to accept both (but probably reject
mixing of both).

> What I think we should fix is the naming of the two booleans:
> 
> bool disable_promotion_to_c1e;
> bool enable_promotion_to_c1e;
> 
> I would rather translated this into an enum, as right now it's
> confusing IMO.

Okay, I can certainly do that. The more that I did consider doing
so already, breaking ties simply based on the "less code churn"
argument.

Jan