Messages in this thread | | | From | "Song Bao Hua (Barry Song)" <> | Subject | RE: [RFC] sched/topology: NUMA topology limitations | Date | Fri, 4 Sep 2020 02:02:25 +0000 |
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> -----Original Message----- > From: Valentin Schneider [mailto:valentin.schneider@arm.com] > Sent: Tuesday, September 1, 2020 9:41 PM > To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com> > Cc: linux-kernel@vger.kernel.org; Ingo Molnar <mingo@kernel.org>; Peter > Zijlstra <peterz@infradead.org>; vincent.guittot@linaro.org; > dietmar.eggemann@arm.com; morten.rasmussen@arm.com; Linuxarm > <linuxarm@huawei.com> > Subject: Re: [RFC] sched/topology: NUMA topology limitations > > > On 31/08/20 11:45, Barry Song wrote: > >> From: Valentin Schneider [mailto:valentin.schneider@arm.com] > >> > >> 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
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