linux-kernel - Re: [PATCH] SCHED: scatter nohz idle balance target cpus

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Message-ID: <20250318063847.GA25930@noisy.programming.kicks-ass.net>
Date: Tue, 18 Mar 2025 07:38:47 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Jianyong Wu <wujianyong@...on.cn>
Cc: mingo@...hat.com, vincent.guittot@...aro.org, jianyong.wu@...look.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] SCHED: scatter nohz idle balance target cpus

On Tue, Mar 18, 2025 at 02:23:58AM +0000, Jianyong Wu wrote:

Re subject; if you look at other patches for sched, you'll note that we
do not capitalize it. Also, what you're doing is not scatter, it is
rotation.

> Currently, cpu selection logic for nohz idle balance lacks history info
> that leads to cpu0 is always chosen if it's in nohz cpu mask. It's not
> fair fot the tasks reside in numa node0. It's worse in the machine with
> large cpu number, nohz idle balance may be very heavy.

Since you seem to care about ilb and numa; there is this _very_ old
patch set that never got finished:

	https://lore.kernel.org/all/20091211013056.305998000@intel.com/

IIRC there was a problem where it would simply stop running the per-node
ilb when the node went idle, leading to node level imbalances. This
should be curable by picking one such idle node and keeping its ILB
active or somesuch.

Something to poke at if you're interested..