Re: [PATCH v2 5/7] x86/sched: Remove SD_ASYM_PACKING from the "SMT" domain

[Valentin Schneider]

On 29/12/22 11:02, Ricardo Neri wrote:
> On Thu, Dec 22, 2022 at 04:56:51PM +0000, Valentin Schneider wrote:
>> diff --git a/include/linux/sched/sd_flags.h b/include/linux/sched/sd_flags.h
>> index 57bde66d95f7a..8dc16942135b4 100644
>> --- a/include/linux/sched/sd_flags.h
>> +++ b/include/linux/sched/sd_flags.h
>> @@ -132,12 +132,12 @@ SD_FLAG(SD_SERIALIZE, SDF_SHARED_PARENT | SDF_NEEDS_GROUPS)
>>  /*
>>   * Place busy tasks earlier in the domain
>>   *
>> - * SHARED_CHILD: Usually set on the SMT level. Technically could be set further
>> - *               up, but currently assumed to be set from the base domain
>> - *               upwards (see update_top_cache_domain()).
>> + * SHARED_PARENT: Usually set on the SMT level. Can be set further up if all
>> + *                siblings of an SMT core are identical, but SMT cores themselves
>> + *                have different priorites.
>>   * NEEDS_GROUPS: Load balancing flag.
>>   */
>> -SD_FLAG(SD_ASYM_PACKING, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
>> +SD_FLAG(SD_ASYM_PACKING, SDF_SHARED_PARENT | SDF_NEEDS_GROUPS)
>
> But this would not work for Power7. It only has SD_ASYM_PACKING in the SMT
> sched domain. Must it have either of these flags?
>

It's not mandatory, but making sure SD flags conform to either of them
means the topology debugging infra can help spot misshapen topologies...

> In Power7 SMT siblings have the different priority but, IIUC, physical
> cores are identical.
>


...But you're right, this doesn't work with Power7 as it would need
SD_ASYM_PACKING all the way up the topology to conform with
SDF_SHARED_PARENT, which clearly doesn't work with how Power7 uses
asym_packing.

> It seems to me that asym_packing is specific to a domain.
>

For Power7 it is, since the asymmetry is only between siblings of a given
core. For other systems where the asymmetry is between cores, that could
theoretically affect several levels. Consider:

  DIE [                      ]
  MC  [          ][          ]
  SMT [    ][    ][    ][    ]
  CPU  0  1  2  3  4  5  6  7
  prio 3  3  2  2  1  1  0  0

As done in your patch, here asym_packing doesn't make sense for SMT, but it
does for MC and DIE.

Anywho, I think what this means if we should drop the SDF_SHARED_* metaflag
for SD_ASYM_PACKING, unless we can think of a nice way to programmatically
describe how SD_ASYM_PACKING should be set.

> Thanks and BR,
> Ricardo