On 25/01/2021 11:50, Song Bao Hua (Barry Song) wrote:

quoted

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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:

quoted

On Fri, Jan 08, 2021 at 12:22:41PM -0800, Tim Chen wrote:

quoted

On 1/8/21 7:12 AM, Morten Rasmussen wrote:

quoted

On Thu, Jan 07, 2021 at 03:16:47PM -0800, Tim Chen wrote:

quoted

On 1/6/21 12:30 AM, Barry Song wrote:

[...]

quoted

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 ?

quoted

 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.

quoted

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.

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