This is a note to let you know that I've just added the patch titled
sched/fair: Fix the decision for load balance
to the 6.6-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
sched-fair-fix-the-decision-for-load-balance.patch
and it can be found in the queue-6.6 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 5206e1ddba8d16a1a2d3e279b1af233dc7c3f1b8
Author: Keisuke Nishimura <keisuke.nishimura@xxxxxxxx>
Date: Tue Oct 31 14:38:22 2023 +0100
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 <keisuke.nishimura@xxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Chen Yu <yu.c.chen@xxxxxxxxx>
Reviewed-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20231031133821.1570861-1-keisuke.nishimura@xxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
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;
