linux-kernel - Re: [PATCH v2] sched/fair: Fix the decision for load balance

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Message-ID: <ZUCcL28IrCCiLPx2@chenyu5-mobl2.ccr.corp.intel.com>
Date:   Tue, 31 Oct 2023 14:18:23 +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 2023-10-31 at 13:59:30 +0800, Chen Yu wrote:
> 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,
>

I suppose you can also carry the Reviewed-by tags in V1(Shrikanth and Vincent's)
as there is no code change.

Reviewed-by: Chen Yu <yu.c.chen@...el.com>

thanks,
Chenyu