Re: [PATCH v3 5/6] sched/fair: Consider SMT in ASYM_PACKING load balance

[Nicholas Piggin <npiggin@gmail.com>]

Excerpts from Peter Zijlstra's message of May 19, 2021 7:59 pm:
> On Tue, May 18, 2021 at 12:07:40PM -0700, Ricardo Neri wrote:
>> On Fri, May 14, 2021 at 07:14:15PM -0700, Ricardo Neri wrote:
>> > On Fri, May 14, 2021 at 11:47:45AM +0200, Peter Zijlstra wrote:
> 
>> > > So I'm thinking that this is a property of having ASYM_PACKING at a core
>> > > level, rather than some arch special. Wouldn't something like this be
>> > > more appropriate?
> 
>> > Thanks Peter for the quick review! This makes sense to me. The only
>> > reason we proposed arch_asym_check_smt_siblings() is because we were
>> > about breaking powerpc (I need to study how they set priorities for SMT,
>> > if applicable). If you think this is not an issue I can post a
>> > v4 with this update.
>> 
>> As far as I can see, priorities in powerpc are set by the CPU number.
>> However, I am not sure how CPUs are enumerated? If CPUs in brackets are
>> SMT sibling, Does an enumeration looks like A) [0, 1], [2, 3] or B) [0, 2],
>> [1, 3]? I guess B is the right answer. Otherwise, both SMT siblings of a
>> core would need to be busy before a new core is used.
>> 
>> Still, I think the issue described in the cover letter may be
>> reproducible in powerpc as well. If CPU3 is offlined, and [0, 2] pulled
>> tasks from [1, -] so that both CPU0 and CPU2 become busy, CPU1 would not be
>> able to help since CPU0 has the highest priority.
>> 
>> I am cc'ing the linuxppc list to get some feedback.
> 
> IIRC the concern with Power is that their Cores can go faster if the
> higher SMT siblings are unused.
> 
> That is, suppose you have an SMT4 Core with only a single active task,
> then if only SMT0 is used it can reach max performance, but if the
> active sibling is SMT1 it can not reach max performance, and if the only
> active sibling is SMT2 it goes slower still.
> 
> So they need to pack the tasks to the lowest SMT siblings, and have the
> highest SMT siblings idle (where possible) in order to increase
> performance.

That's correct.

Thanks,
Nick