Re: [RFC PATCH v3 0/2] scheduler: expose the topology of clusters and add cluster scheduler

[Dietmar Eggemann]

On 25/01/2021 11:50, Song Bao Hua (Barry Song) wrote:
> 
>> -----Original Message-----
>> From: Dietmar Eggemann [mailto:dietmar.eggemann@xxxxxxx]
>> Sent: Wednesday, January 13, 2021 12:00 AM
>> To: Morten Rasmussen <morten.rasmussen@xxxxxxx>; Tim Chen
>> <tim.c.chen@xxxxxxxxxxxxxxx>
>> Cc: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>;
>> valentin.schneider@xxxxxxx; catalin.marinas@xxxxxxx; will@xxxxxxxxxx;
>> rjw@xxxxxxxxxxxxx; vincent.guittot@xxxxxxxxxx; lenb@xxxxxxxxxx;
>> gregkh@xxxxxxxxxxxxxxxxxxx; Jonathan Cameron <jonathan.cameron@xxxxxxxxxx>;
>> mingo@xxxxxxxxxx; peterz@xxxxxxxxxxxxx; juri.lelli@xxxxxxxxxx;
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>> mark.rutland@xxxxxxx; sudeep.holla@xxxxxxx; aubrey.li@xxxxxxxxxxxxxxx;
>> linux-arm-kernel@xxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx;
>> linux-acpi@xxxxxxxxxxxxxxx; linuxarm@xxxxxxxxxxxxx; xuwei (O)
>> <xuwei5@xxxxxxxxxx>; Zengtao (B) <prime.zeng@xxxxxxxxxxxxx>; tiantao (H)
>> <tiantao6@xxxxxxxxxxxxx>
>> Subject: Re: [RFC PATCH v3 0/2] scheduler: expose the topology of clusters and
>> add cluster scheduler
>>
>> On 11/01/2021 10:28, Morten Rasmussen wrote:
>>> On Fri, Jan 08, 2021 at 12:22:41PM -0800, Tim Chen wrote:
>>>>
>>>>
>>>> On 1/8/21 7:12 AM, Morten Rasmussen wrote:
>>>>> On Thu, Jan 07, 2021 at 03:16:47PM -0800, Tim Chen wrote:
>>>>>> On 1/6/21 12:30 AM, Barry Song wrote:

[...]

>> wake_wide() switches between packing (select_idle_sibling(), llc_size
>> CPUs) and spreading (find_idlest_cpu(), all CPUs).
>>
>> AFAICS, since none of the sched domains set SD_BALANCE_WAKE, currently
>> all wakeups are (llc-)packed.
> 
> Sorry for late response. I was struggling with some other topology
> issues recently.
> 
> For "all wakeups are (llc-)packed",
> it seems you mean current want_affine is only affecting the new_cpu,
> and for wake-up path, we will always go to select_idle_sibling() rather
> than find_idlest_cpu() since nobody sets SD_WAKE_BALANCE in any
> sched_domain ?
> 
>>
>>  select_task_rq_fair()
>>
>>    for_each_domain(cpu, tmp)
>>
>>      if (tmp->flags & sd_flag)
>>        sd = tmp;
>>
>>
>> In case we would like to further distinguish between llc-packing and
>> even narrower (cluster or MC-L2)-packing, we would introduce a 2. level
>> packing vs. spreading heuristic further down in sis().
> 
> I didn't get your point on "2 level packing". Would you like
> to describe more? It seems you mean we need to have separate
> calculation for avg_scan_cost and sched_feat(SIS_) for cluster
> (or MC-L2) since cluster and llc are not in the same level
> physically?

By '1. level packing' I meant going sis() (i.e. sd=per_cpu(sd_llc,
target)) instead of routing WF_TTWU through find_idlest_cpu() which uses
a broader sd span (in case all sd's (or at least up to an sd > llc)
would have SD_BALANCE_WAKE set).
wake_wide() (wakee/waker flip heuristic) is currently used to make this
decision. But since no sd sets SD_BALANCE_WAKE we always go sis() for
WF_TTWU.

'2. level packing' would be the decision between cluster- and
llc-packing. The question was which heuristic could be used here.

>> IMHO, Barry's current implementation doesn't do this right now. Instead
>> he's trying to pack on cluster first and if not successful look further
>> among the remaining llc CPUs for an idle CPU.
> 
> Yes. That is exactly what the current patch is doing.

And this will be favoring cluster- over llc-packing for each task instead.