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Message-ID: <ZUCXwmzaoNbRbNpR@chenyu5-mobl2.ccr.corp.intel.com>
Date: Tue, 31 Oct 2023 13:59:30 +0800
From: Chen Yu <yu.c.chen@...el.com>
To: Keisuke Nishimura <keisuke.nishimura@...ia.fr>
CC: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Vincent Guittot <vincent.guittot@...aro.org>,
<linux-kernel@...r.kernel.org>,
Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Mel Gorman <mgorman@...e.de>,
Valentin Schneider <vschneid@...hat.com>,
Julia Lawall <julia.lawall@...ia.fr>
Subject: Re: [PATCH v2] sched/fair: Fix the decision for load balance
On 2023-10-30 at 18:29:46 +0100, Keisuke Nishimura wrote:
> 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@...ia.fr>
> ---
> 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 2048138ce54b..69d63fae34f4 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11079,12 +11079,16 @@ static int should_we_balance(struct lb_env *env)
> continue;
> }
>
> - /* Are we the first idle CPU? */
> + /*
> + * Are we the first idle core in a MC or higher domain
It is possible that the Cluster domain is lower than a MC.
cluser domain: CPUs share the same L2
MC domain: CPUs share the same LLC
grep . domain*/{name,flags}
domain0/name:CLS
domain1/name:MC
domain2/name:NUMA
domain0/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING
domain1/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SHARE_PKG_RESOURCES SD_PREFER_SIBLING
domain2/flags:SD_BALANCE_NEWIDLE SD_BALANCE_EXEC SD_BALANCE_FORK SD_WAKE_AFFINE SD_SERIALIZE SD_OVERLAP SD_NUMA
So, maybe:
Are we the first idle core in a non-SMT domain or higher,
thanks,
Chenyu
> + * 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;
> --
> 2.34.1
>
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