6.6-stable review patch. If anyone has any objections, please let me know.
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From: Keisuke Nishimura keisuke.nishimura@inria.fr
[ 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 keisuke.nishimura@inria.fr Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Reviewed-by: Chen Yu yu.c.chen@intel.com Reviewed-by: Shrikanth Hegde sshegde@linux.vnet.ibm.com Reviewed-by: Vincent Guittot vincent.guittot@linaro.org Link: https://lkml.kernel.org/r/20231031133821.1570861-1-keisuke.nishimura@inria.f... Signed-off-by: Sasha Levin sashal@kernel.org --- 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 0351320148177..fa9fff0f9620d 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;