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sched/fair: Fix the decision for load balance

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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: b1bfeab9b0 ("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
This commit is contained in:
Keisuke Nishimura 2023-10-31 14:38:22 +01:00 committed by Peter Zijlstra
parent 8b39d20ece
commit 6d7e4782bc
1 changed files with 7 additions and 3 deletions
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10
kernel/sched/fair.c
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@ -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;