linux-kernel - Re: [PATCH] sched/nohz: Fix NOHZ imbalance by adding options for ILB CPU

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Message-ID: <a968dba3-0acb-f175-fb73-aa384a2745a9@gentwo.org>
Date: Thu, 4 Sep 2025 09:18:42 -0700 (PDT)
From: "Christoph Lameter (Ampere)" <cl@...two.org>
To: Frederic Weisbecker <frederic@...nel.org>
cc: Valentin Schneider <vschneid@...hat.com>, 
    Adam Li <adamli@...amperecomputing.com>, mingo@...hat.com, 
    peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org, 
    dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com, 
    mgorman@...e.de, linux-kernel@...r.kernel.org, patches@...erecomputing.com
Subject: Re: [PATCH] sched/nohz: Fix NOHZ imbalance by adding options for
 ILB CPU

On Thu, 4 Sep 2025, Frederic Weisbecker wrote:

> > > That's an argument _not_ in favour of dynamic balancing such as ILB, even for
> > > this usecase in nohz_full (all the other usecases of nohz_full I know really
> > > want static affinity and no balancing at all).
> > >
> > > So I have to ask, what would be wrong with static affinities to these tasks?
> >
> > Static affinities are great but they keep the tick active and thus the
> > rendevous can be off off one or the other compute thread.
>
> How do static affinities keep the tick active?

If you dont use hohz_full and only do static affinities then the tick is
active.

My wish for the future would be that nohz_full would be the default and
that the scheduler does correct load balancing regardless of the cpu being
in tick mode or not. It should automatically switch the tick off if a cpu
goes into 100% compute and ideally there would be no performance
regression if the tick is off.