Messages in this thread Patch in this message | | | From | Yihao Wu <> | Subject | [PATCH] sched/fair: Again ignore percpu threads for imbalance pulls | Date | Sat, 11 Dec 2021 17:48:08 +0800 |
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commit 2f5f4cce496e ("sched/fair: Ignore percpu threads for imbalance pulls") was meant to fix a performance issue, when load balance tries to migrate pinned kernel threads at MC domain level. This was destined to fail. After it fails, it further makes wakeup balance at NUMA domain level messed up. The most severe case that I noticed and frequently occurs: |sum_nr_running(node1) - sum_nr_running(node2)| > 100
However the original bugfix failed, because it covers only case 1) below. 1) Created by create_kthread 2) Created by kernel_thread No kthread is assigned to task_struct in case 2 (Please refer to comments in free_kthread_struct) so it simply won't work.
The easist way to cover both cases is to check nr_cpus_allowed, just as discussed in the mailing list of the v1 version of the original fix.
* lmbench3.lat_proc -P 104 fork (2 NUMA, and 26 cores, 2 threads)
w/out patch w/ patch fork+exit latency 1660 ms 1520 ms ( 8.4%)
Fixes: 2f5f4cce496e ("sched/fair: Ignore percpu threads for imbalance pulls") Signed-off-by: Yihao Wu <wuyihao@linux.alibaba.com> --- kernel/kthread.c | 6 +----- 1 file changed, 1 insertion(+), 5 deletions(-)
diff --git a/kernel/kthread.c b/kernel/kthread.c index 4a4d7092a2d8..cb05d3ff2de4 100644 --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -543,11 +543,7 @@ void kthread_set_per_cpu(struct task_struct *k, int cpu) bool kthread_is_per_cpu(struct task_struct *p) { - struct kthread *kthread = __to_kthread(p); - if (!kthread) - return false; - - return test_bit(KTHREAD_IS_PER_CPU, &kthread->flags); + return (p->flags & PF_KTHREAD) && p->nr_cpus_allowed == 1; } /** -- 2.32.0.604.gb1f3e1269
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