[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [PATCH 3/4] mwait-idle: add 'preferred_cstates' module argument
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> 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. Thanks, Roger.
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