On 31/08/20 11:45, Barry Song wrote: >> From: Valentin Schneider [mailto:[email protected]] >> >> 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? > 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
