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Date:   Sat, 28 Oct 2023 08:37:59 +0200 (CEST)
From:   Julia Lawall <julia.lawall@inria.fr>
To:     Chen Yu <yu.c.chen@intel.com>
cc:     Keisuke Nishimura <keisuke.nishimura@inria.fr>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        linux-kernel@vger.kernel.org,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Mel Gorman <mgorman@suse.de>,
        Valentin Schneider <vschneid@redhat.com>,
        Julia Lawall <julia.lawall@inria.fr>,
        Ricardo Neri <ricardo.neri-calderon@linux.intel.com>,
        Tim Chen <tim.c.chen@linux.intel.com>
Subject: Re: [PATCH] sched/fair: Fix the decision for load balance
In-Reply-To: <ZTyfoIBpm3lxd8Dy@chenyu5-mobl2.ccr.corp.intel.com>
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References: <20231027171742.1426070-1-keisuke.nishimura@inria.fr> <ZTyfoIBpm3lxd8Dy@chenyu5-mobl2.ccr.corp.intel.com>
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On Sat, 28 Oct 2023, Chen Yu wrote:

> On 2023-10-27 at 19:17:43 +0200, 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@inria.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;
> >  	}
> >
> > -	if (idle_smt == env->dst_cpu)
> > -		return true;
> > +	/* Is there an 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;
>
> Looks reasonable to me, if there is other idle SMT(from half-busy core)
> in the system, we should leverage that SMT to do the periodic lb.
> Per my understanding,

That's not the goal of this patch.  The goal of this patch is to avoid
doing return group_balance_cpu(sg) == env->dst_cpu; when a half-busy core
has been identified that is different from env->dst_cpu.

julia

>
> Reviewed-by: Chen Yu <yu.c.chen@intel.com>
>
> thanks,
> Chenyu
>