From 47a3075109978d9684f7fea59de7c639fcba7145 Mon Sep 17 00:00:00 2001 From: Keisuke Nishimura Date: Tue, 31 Oct 2023 14:38:22 +0100 Subject: [PATCH] sched/fair: Fix the decision for load balance [ Upstream commit 6d7e4782bcf549221b4ccfffec2cf4d1a473f1a3 ] should_we_balance is called for the decision to do load-balancing. When sched ticks invoke this function, only one CPU should return true. However, in the current code, two CPUs can return true. The following situation, where b means busy and i means idle, is an example, because CPU 0 and CPU 2 return true. [0, 1] [2, 3] b b i b This fix checks if there exists an idle CPU with busy sibling(s) after looking for a CPU on an idle core. If some idle CPUs with busy siblings are found, just the first one should do load-balancing. Fixes: b1bfeab9b002 ("sched/fair: Consider the idle state of the whole core for load balance") Signed-off-by: Keisuke Nishimura Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Chen Yu Reviewed-by: Shrikanth Hegde Reviewed-by: Vincent Guittot Link: https://lkml.kernel.org/r/20231031133821.1570861-1-keisuke.nishimura@inria.fr Signed-off-by: Sasha Levin --- kernel/sched/fair.c | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 0351320..fa9fff0 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -11121,12 +11121,16 @@ static int should_we_balance(struct lb_env *env) continue; } - /* Are we the first idle CPU? */ + /* + * Are we the first idle core in a non-SMT domain or higher, + * or the first idle CPU in a SMT domain? + */ return cpu == env->dst_cpu; } - if (idle_smt == env->dst_cpu) - return true; + /* Are we the first idle CPU with busy siblings? */ + if (idle_smt != -1) + return idle_smt == env->dst_cpu; /* Are we the first CPU of this group ? */ return group_balance_cpu(sg) == env->dst_cpu; -- 2.7.4