RE: [RFC] sched/topology: NUMA topology limitations

[Song Bao Hua (Barry Song)]

> -----Original Message-----
> From: Valentin Schneider [mailto:valentin.schneider@xxxxxxx]
> Sent: Tuesday, September 1, 2020 9:41 PM
> To: Song Bao Hua (Barry Song) <song.bao.hua@xxxxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx; Ingo Molnar <mingo@xxxxxxxxxx>; Peter
> Zijlstra <peterz@xxxxxxxxxxxxx>; vincent.guittot@xxxxxxxxxx;
> dietmar.eggemann@xxxxxxx; morten.rasmussen@xxxxxxx; Linuxarm
> <linuxarm@xxxxxxxxxx>
> Subject: Re: [RFC] sched/topology: NUMA topology limitations
> 
> 
> On 31/08/20 11:45, Barry Song wrote:
> >> From: Valentin Schneider [mailto:valentin.schneider@xxxxxxx]
> >>
> >> Ignoring corner cases where task affinity gets in the way, load balance
> >> will always pull tasks to the local CPU (i.e. the CPU who's sched_domain we
> >> are working on).
> >>
> >> If we're balancing load for CPU0-domain1, we would be looking at which
> CPUs
> >> in [0-2] (i.e. the domain's span) we could (if we should) pull tasks from
> >> to migrate them over to CPU0.
> >>
> >> We'll first try to figure out which sched_group has the more load (see
> >> find_busiest_group() & friends), and that's where we may hit issues.
> >>
> >> Consider a scenario where CPU3 is noticeably busier than the other
> >> CPUs. We'll end up marking CPU0-domain1-group2 (1-3) as the busiest
> group,
> >> and compute an imbalance (i.e. amount of load to pull) mostly based on the
> >> status of CPU3.
> >>
> >> We'll then go to find_busiest_queue(); the mask of CPUs we iterate over is
> >> restricted by the sched_domain_span (i.e. doesn't include CPU3 here), so
> >> we'll pull things from either CPU1 or CPU2 based on stats we built looking
> >> at CPU3, which is bound to be pretty bogus.
> >>
> >> To summarise: we won't pull from the "outsider" node(s) (i.e., nodes
> >> included in the sched_groups but not covered by the sched_domain), but
> they
> >> will influence the stats and heuristics of the load balance.
> >
> > Hi Valentin,
> > Thanks for your clarification. For many scenarios, to achieve good
> performance, people would
> > pin processes in numa node. So the priority to pin would be local node first,
> then domain0 with one hop. Domain1
> > with two hops is actually too far. Domain2 with three hops would be a
> disaster. If cpu0 pulls task from cpu2,
> > but memory is still one CPU2's node, 3 hops would be a big problem for
> memory access and page migration.
> >
> 
> Did you mean CPU3 here?

Yep. I meant cpu3 here.

> 
> > However, for automatic numa balance, I would agree we need to fix the
> groups layout to make groups
> > stay in the span of sched_domain. Otherwise, it seems the scheduler is
> running incorrectly to find the right
> > cpu to pull task.
> >
> > In case we have
> > 0 task on cpu0
> > 1 task on cpu1
> > 1 task on cpu2
> > 4 task on cpu3
> >
> > In sched_domain1, cpu1+cpu3 is busy, so cpu0 would try to pull task from
> cpu2 of the group(1-3) because cpu3 is busy,
> > meanwhile, it is an outsider.
> >
> 
> Right, we'd pull from either CPU1 or CPU2 (in this case via a tentative
> active load balance) because they are in the same group as CPU3 which
> inflates the sched_group load stats, but we can't pull from it at this
> domain because it's not included in the domain span.
> 
Thanks
Barry