linux-kernel - Re: [PATCH] sched/fair: Fix the decision for load balance

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Date:   Mon, 30 Oct 2023 11:02:13 +0100
From:   Keisuke Nishimura <keisuke.nishimura@...ia.fr>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Mel Gorman <mgorman@...e.de>,
        Valentin Schneider <vschneid@...hat.com>,
        Julia Lawall <julia.lawall@...ia.fr>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] sched/fair: Fix the decision for load balance



On 30/10/2023 09:05, Vincent Guittot wrote:
> On Mon, 30 Oct 2023 at 05:03, Shrikanth Hegde
> <sshegde@...ux.vnet.ibm.com> wrote:
>>
>>
>>
>> On 10/27/23 10:47 PM, 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 | 5 +++--
>>>   1 file changed, 3 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index 2048138ce54b..eff0316d6c7d 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -11083,8 +11083,9 @@ static int should_we_balance(struct lb_env *env)
>>>                return cpu == env->dst_cpu;
>>>        }
>>>
>>
>>
>> There is comment above this /* Are we the first idle CPU? */
>> Maybe update that comment as /* Are we the first idle core */
> 
> I was about to say the same but it's not always true. If we are at SMT
> level, we look for an idle CPU in the core
> 

Maybe I should update the comment with the additional contexts:

/*
  * Are we the first idle core in a sched_domain not-sharing capacity,
  * or the first idle CPU in a sched_domain sharing capacity?
  */


>>
>>> -     if (idle_smt == env->dst_cpu)
>>> -             return true;
>>> +     /* Is there an idle CPU with busy siblings? */
>> nit: We can keep the comment style fixed in this function.
>> /* Are we the first idle CPU with busy siblings */
>>

OK, agreed. Should I create version 2?

thanks,
Keisuke

>>> +     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;
>>
>> code changes LGTM
>> Reviewed-by: Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>