linux-kernel - Re: [PATCH v4] time/tick-sched: idle load balancing when nohz

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Date: Thu, 16 May 2024 09:20:08 +0100
From: Yun Levi <ppbuk5246@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Frederic Weisbecker <frederic@...nel.org>, Joel Fernandes <joel@...lfernandes.org>, 
	Vineeth Pillai <vineeth@...byteword.org>, Vincent Guittot <vincent.guittot@...aro.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, anna-maria@...utronix.de, mingo@...nel.org, 
	tglx@...utronix.de, Markus.Elfring@....de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] time/tick-sched: idle load balancing when nohz_full
 cpu becomes idle.

> None of that HK nonsense is relevant. The NOHZ_FULL nonsense implies
> single CPU partitions, and *that* should be avoiding any and all
> load-balancing.

Do you mean.. tick_nohz_full cpu (non-HK-ticked cpu) shouldn't belong
to any sched_domain?

>
> If there still is, that's a bug, but that's not related to HK goo.
>
> As such, I don't think the HK_TYPE_SCHED check in
> nohz_balance_enter_idle() actually makes sense, the on_null_omain()
> check a little below that should already take care of things, no?

IIUC,
currently, whether cpu belongs on domain or null is determined by
HK_DOMAIN_FLAGS
However, when "nohz_full=" is used, it still on HK_DOMAIN, so it
belongs to sched_domain
so, it couldn't be filtered out by on_null_domain().

unless "isolcpus=domain" or "isolcpus={cpu_list}", it's on null domain.
with "isolcpus=tick", it participates sched_domain.