On 16/03/21 16:49, Dietmar Eggemann wrote: > On 11/03/2021 13:05, Valentin Schneider wrote: >> From: Lingutla Chandrasekhar <clingu...@codeaurora.org> >> >> In load balancing, when balancing group is unable to pull task >> due to ->cpus_ptr constraints from busy group, then it sets >> LBF_SOME_PINNED to lb env flags, as a consequence, sgc->imbalance >> is set for its parent domain level. which makes the group >> classified as imbalance to get help from another balancing cpu. >> >> Consider a 4-CPU big.LITTLE system with CPUs 0-1 as LITTLEs and > > Does it have to be a big.LITTLE system? I assume this issue also happens > on an SMP system. >
Aye, though the consequences are "worse" on asym CPU capacity systems. >> CPUs 2-3 as Bigs with below scenario: >> - CPU0 doing newly_idle balancing >> - CPU1 running percpu kworker and RT task (small tasks) > > What's the role of the small RT task here in the story? > I don't think it matters much here. >> - CPU2 running 2 big tasks >> - CPU3 running 1 medium task >> >> While CPU0 is doing newly_idle load balance at MC level, it fails to >> pull percpu kworker from CPU1 and sets LBF_SOME_PINNED to lb env flag >> and set sgc->imbalance at DIE level domain. As LBF_ALL_PINNED not cleared, >> it tries to redo the balancing by clearing CPU1 in env cpus, but it don't >> find other busiest_group, so CPU0 stops balacing at MC level without >> clearing 'sgc->imbalance' and restart the load balacing at DIE level. >> >> And CPU0 (balancing cpu) finds LITTLE's group as busiest_group with group >> type as imbalance, and Bigs that classified the level below imbalance type >> would be ignored to pick as busiest, and the balancing would be aborted >> without pulling any tasks (by the time, CPU1 might not have running tasks). >> >> It is suboptimal decision to classify the group as imbalance due to >> percpu threads. So don't use LBF_SOME_PINNED for per cpu threads. > > This sentence mentioned per-cpu threads (and so does the patch name) but > the implementation (only) deals with per-cpu kernel threads. IMHO, it > would be good to align this. > Tell you what, I'll go for: 1) how can pcpu kthreads cause LBF_SOME_PINNED 2) why we may not want this, but still ignore !kthread pcpu tasks 3) why this is even more important for big.LITTLE