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

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Message-ID: <3873b6cd-a983-41fe-8618-a5e5635a7c82@linux.vnet.ibm.com>
Date:   Mon, 30 Oct 2023 19:43:36 +0530
From:   Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
To:     Keisuke Nishimura <keisuke.nishimura@...ia.fr>
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>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH] sched/fair: Fix the decision for load balance



On 10/30/23 3:32 PM, Keisuke Nishimura wrote:
> 
> 
> 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?
>  */
> 


/*
 * Are we the first idle core in a MC or higher domain
 * or the first idle CPU in a SMT domain
 */


> 
>>>
>>>> -     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?

Yes. That would be good.

> 
> 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>