sched/fair: Fix the decision for load balance

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.fr

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 025d90925bf631ef415bb83539c965374e9d4a64..d7a3c63a2171a8569abc8fe7c3997dc1d22e001e 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11184,12 +11184,16 @@ 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;