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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 27.04.2022 14:45, Roger Pau Monné wrote:
> On Tue, Apr 26, 2022 at 12:05:28PM +0200, Jan Beulich wrote:
>> From: Artem Bityutskiy <artem.bityutskiy@xxxxxxxxxxxxxxx>
>>
>> On Sapphire Rapids Xeon (SPR) the C1 and C1E states are basically mutually
>> exclusive - only one of them can be enabled. By default, 'intel_idle' driver
>> enables C1 and disables C1E. However, some users prefer to use C1E instead of
>> C1, because it saves more energy.
>>
>> This patch adds a new module parameter ('preferred_cstates') for enabling C1E
>> and disabling C1. Here is the idea behind it.
>>
>> 1. This option has effect only for "mutually exclusive" C-states like C1 and
>>    C1E on SPR.
>> 2. It does not have any effect on independent C-states, which do not require
>>    other C-states to be disabled (most states on most platforms as of today).
>> 3. For mutually exclusive C-states, the 'intel_idle' driver always has a
>>    reasonable default, such as enabling C1 on SPR by default. On other
>>    platforms, the default may be different.
>> 4. Users can override the default using the 'preferred_cstates' parameter.
>> 5. The parameter accepts the preferred C-states bit-mask, similarly to the
>>    existing 'states_off' parameter.
>> 6. This parameter is not limited to C1/C1E, and leaves room for supporting
>>    other mutually exclusive C-states, if they come in the future.
>>
>> Today 'intel_idle' can only be compiled-in, which means that on SPR, in order
>> to disable C1 and enable C1E, users should boot with the following kernel
>> argument: intel_idle.preferred_cstates=4
>>
>> Signed-off-by: Artem Bityutskiy <artem.bityutskiy@xxxxxxxxxxxxxxx>
>> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
>> Origin: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 
>> da0e58c038e6
>>
>> Enable C1E (if requested) not only on the BSP's socket / package.
> 
> Maybe we should also add a note here that the command line option for
> Xen is preferred-cstates instead of intel_idle.preferred_cstates?
> 
> I think this is a bad interface however, we should have a more generic
> option (ie: cstate-mode = 'performance | powersave') so that users
> don't have to fiddle with model specific C state masks.

Performance vs powersave doesn't cover it imo, especially if down
the road more states would appear which can be controlled this way.
I don't think there's a way around providing _some_ way to control
things one a per-state level. When porting this over, I too didn't
like this interface very much, but I had no good replacement idea.

>> --- 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.

Jan