From: Lingutla Chandrasekhar clingutla@codeaurora.org
[ Upstream commit 9bcb959d05eeb564dfc9cac13a59843a4fb2edf2 ]
During load balance, LBF_SOME_PINNED will be set if any candidate task cannot be detached due to CPU affinity constraints. This can result in setting env->sd->parent->sgc->group_imbalance, which can lead to a group being classified as group_imbalanced (rather than any of the other, lower group_type) when balancing at a higher level.
In workloads involving a single task per CPU, LBF_SOME_PINNED can often be set due to per-CPU kthreads being the only other runnable tasks on any given rq. This results in changing the group classification during load-balance at higher levels when in reality there is nothing that can be done for this affinity constraint: per-CPU kthreads, as the name implies, don't get to move around (modulo hotplug shenanigans).
It's not as clear for userspace tasks - a task could be in an N-CPU cpuset with N-1 offline CPUs, making it an "accidental" per-CPU task rather than an intended one. KTHREAD_IS_PER_CPU gives us an indisputable signal which we can leverage here to not set LBF_SOME_PINNED.
Note that the aforementioned classification to group_imbalance (when nothing can be done) is especially problematic on big.LITTLE systems, which have a topology the likes of:
DIE [ ] MC [ ][ ] 0 1 2 3 L L B B
arch_scale_cpu_capacity(L) < arch_scale_cpu_capacity(B)
Here, setting LBF_SOME_PINNED due to a per-CPU kthread when balancing at MC level on CPUs [0-1] will subsequently prevent CPUs [2-3] from classifying the [0-1] group as group_misfit_task when balancing at DIE level. Thus, if CPUs [0-1] are running CPU-bound (misfit) tasks, ill-timed per-CPU kthreads can significantly delay the upgmigration of said misfit tasks. Systems relying on ASYM_PACKING are likely to face similar issues.
Signed-off-by: Lingutla Chandrasekhar clingutla@codeaurora.org [Use kthread_is_per_cpu() rather than p->nr_cpus_allowed] [Reword changelog] Signed-off-by: Valentin Schneider valentin.schneider@arm.com Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Dietmar Eggemann dietmar.eggemann@arm.com Reviewed-by: Vincent Guittot vincent.guittot@linaro.org Link: https://lkml.kernel.org/r/20210407220628.3798191-2-valentin.schneider@arm.co... Signed-off-by: Sasha Levin sashal@kernel.org --- kernel/sched/fair.c | 4 ++++ 1 file changed, 4 insertions(+)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 54ca03bacddf..d89ddacc1148 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7597,6 +7597,10 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env) if (throttled_lb_pair(task_group(p), env->src_cpu, env->dst_cpu)) return 0;
+ /* Disregard pcpu kthreads; they are where they need to be. */ + if ((p->flags & PF_KTHREAD) && kthread_is_per_cpu(p)) + return 0; + if (!cpumask_test_cpu(env->dst_cpu, p->cpus_ptr)) { int cpu;