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

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Date:   Sat, 28 Oct 2023 17:03:23 +0200 (CEST)
From:   Julia Lawall <julia.lawall@...ia.fr>
To:     Chen Yu <yu.c.chen@...el.com>
cc:     Keisuke Nishimura <keisuke.nishimura@...ia.fr>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel@...r.kernel.org,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Mel Gorman <mgorman@...e.de>,
        Valentin Schneider <vschneid@...hat.com>,
        Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH] sched/fair: Fix the decision for load balance



On Sat, 28 Oct 2023, Chen Yu wrote:

> On 2023-10-28 at 08:37:59 +0200, Julia Lawall wrote:
> >
> >
> > 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@...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;
> > > >  	}
> > > >
> > > > -	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;
>
> Yes, I mean, without this patch, we could incorrectly choose the current
> non idle CPU rather than that idle SMT, but actually we should let that
> idle SMT to do the idle lb.

OK, agreed.  Thanks for the feedback!

julia

>
> thanks,
> Chenyu
>
> > when a half-busy core
> > has been identified that is different from env->dst_cpu.
> >
> > julia
> >
> > >
> > > Reviewed-by: Chen Yu <yu.c.chen@...el.com>
> > >
> > > thanks,
> > > Chenyu
> > >
>