linux-kernel - Re: [RESEND PATCH] sched/fair: Skip sched_balance

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Message-ID: <20251014092614.GL4067720@noisy.programming.kicks-ass.net>
Date: Tue, 14 Oct 2025 11:26:14 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: "Chen, Yu C" <yu.c.chen@...el.com>
Cc: Tim Chen <tim.c.chen@...ux.intel.com>, Ingo Molnar <mingo@...nel.org>,
	Doug Nelson <doug.nelson@...el.com>,
	Mohini Narkhede <mohini.narkhede@...el.com>,
	linux-kernel@...r.kernel.org,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Shrikanth Hegde <sshegde@...ux.ibm.com>,
	K Prateek Nayak <kprateek.nayak@....com>
Subject: Re: [RESEND PATCH] sched/fair: Skip sched_balance_running cmpxchg
 when balance is not due

On Tue, Oct 14, 2025 at 12:32:57AM +0800, Chen, Yu C wrote:
> On 10/13/2025 10:26 PM, Peter Zijlstra wrote:
> > On Thu, Oct 02, 2025 at 04:00:12PM -0700, Tim Chen wrote:
> > 
> > > During load balancing, balancing at the LLC level and above must be
> > > serialized.
> > 
> > I would argue the wording here, there is no *must*, they *are*. Per the
> > current rules SD_NUMA and up get SD_SERIALIZE.
> > 
> > This is a *very* old thing, done by Christoph Lameter back when he was
> > at SGI. I'm not sure this default is still valid or not. Someone would
> > have to investigate. An alternative would be moving it into
> > node_reclaim_distance or somesuch.
> > 
> 
> Do you mean the following:
> 
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 444bdfdab731..436c899d8da2 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -1697,11 +1697,16 @@ sd_init(struct sched_domain_topology_level *tl,
>                 sd->cache_nice_tries = 2;
> 
>                 sd->flags &= ~SD_PREFER_SIBLING;
> -               sd->flags |= SD_SERIALIZE;
>                 if (sched_domains_numa_distance[tl->numa_level] >
> node_reclaim_distance) {
>                         sd->flags &= ~(SD_BALANCE_EXEC |
>                                        SD_BALANCE_FORK |
>                                        SD_WAKE_AFFINE);
> +                       /*
> +                        * Nodes that are far away need to be serialized to
> +                        * reduce the overhead of long-distance task
> migration
> +                        * caused by load balancing.
> +                        */
> +                       sd->flags |= SD_SERIALIZE;
>                 }
> 
> We can launch some tests to see if removing SD_SERIALIZE would
> bring any impact.

Yeah, something like that. But lets first get this other thing sorted. I
agree that the SD_SERIALIZE thing is in the wrong place.