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

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Date: Thu, 16 May 2024 06:29:51 +0100
From: Yun Levi <ppbuk5246@...il.com>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Joel Fernandes <joel@...lfernandes.org>, Vineeth Pillai <vineeth@...byteword.org>, 
	Vincent Guittot <vincent.guittot@...aro.org>, Peter Zijlstra <peterz@...radead.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.

HI. Frederic!

Thanks for your reply.

> 1) Is Idle load balancing actually relevant for nohz_full? HK_TYPE_MISC already
>    prevent those CPUs from becoming idle load balancer. They can still be
>    targets for load balancing but nohz_full CPUs are supposed to run only one
>    task.

I'm not sure nohz_full cpu is really relevant with Idle load balancing.
However, I couldn't find any reason it shouldn't participate Idle load
balancing.
And about HK_TYPE_MISC, this is half true when it sets with
"nohz_full=" options,
if nohz_full cpu is set via "isolcpus=nohz," it's still in HK_TYPE_MISC mask
so find_ilb() doesn't prevent entering idle load balance.

> 2) This is related to previous point: HK_TYPE_SCHED is never activated. It would
>    prevent the CPU from even beeing part of idle load balancing. Should we
>    remove it or plug it?

IMHO,  If we want to prevent nohz_full cpu entering, I should set with
HK_TYPE_TICK | HK_TYPE_SCHED together.
Because, find_new_ilb()'s comment say
" HK_TYPE_MISC CPUs are used for this task, because HK_TYPE_SCHED is
not set anywhere yet."
or integrate with HK_TYPE_TICK....? removing HK_TYPE_SCHED in the hk list...

It depends on ilb relevant for nohz_full...
If it is, Idle load balance could be controlled with HK_TYPE_SCHED.
If it doesn't ,IMHO, It seems better to remove HK_TYPE_SCHED and check
with HK_TYPE_SCHED.

> 4) Why is nohz_balance_exit_idle() called from the tick and not from the idle
>    exit path? Is it to avoid overhead?

> I'm adding some scheduler people in Cc who might help answer some of those
> questions.
..

Thanks :)