If load_balance() fails to migrate any tasks because all tasks were affined, load_balance() removes the source cpu from consideration and attempts to redo and balance among the new subset of cpus.
There is a bug in this code path where the algorithm considers all active cpus in the system (minus the source that was just masked out). This is not valid for two reasons: some active cpus may not be in the current scheduling domain and one of the active cpus is dst_cpu. These cpus should not be considered, as we cannot pull load from them. Instead of failing out of load_balance(), we may end up redoing the search with no valid cpus and incorrectly concluding the domain is balanced. Additionally, if the group_imbalance flag was just set, it may also be incorrectly unset, thus the flag will not be seen by other cpus in future load_balance() runs as that algorithm intends. Fix the check by removing cpus not in the current domain and the dst_cpu from considertation, thus limiting the evaluation to valid remaining cpus from which load might be migrated. Co-authored-by: Austin Christ <austi...@codeaurora.org> Co-authored-by: Dietmar Eggemann <dietmar.eggem...@arm.com> Signed-off-by: Jeffrey Hugo <jh...@codeaurora.org> Tested-by: Tyler Baicar <tbai...@codeaurora.org> --- kernel/sched/fair.c | 19 ++++++++++++++++++- 1 file changed, 18 insertions(+), 1 deletion(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index d711093..a5d41b1 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -8220,7 +8220,24 @@ static int load_balance(int this_cpu, struct rq *this_rq, /* All tasks on this runqueue were pinned by CPU affinity */ if (unlikely(env.flags & LBF_ALL_PINNED)) { cpumask_clear_cpu(cpu_of(busiest), cpus); - if (!cpumask_empty(cpus)) { + /* + * dst_cpu is not a valid busiest cpu in the following + * check since load cannot be pulled from dst_cpu to be + * put on dst_cpu. + */ + cpumask_clear_cpu(env.dst_cpu, cpus); + /* + * Go back to "redo" iff the load-balance cpumask + * contains other potential busiest cpus for the + * current sched domain. + */ + if (cpumask_intersects(cpus, sched_domain_span(env.sd))) { + /* + * Now that the check has passed, reenable + * dst_cpu so that load can be calculated on + * it in the redo path. + */ + cpumask_set_cpu(env.dst_cpu, cpus); env.loop = 0; env.loop_break = sched_nr_migrate_break; goto redo; -- Qualcomm Datacenter Technologies as an affiliate of Qualcomm Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.