linux-kernel - Re: [RFC PATCH 3/3] softirq: Avoid waking up ksoftirqd from flush_smp_call_function

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Message-ID: <7acc5dd0-8cc0-be08-b08a-6f36c1964961@amd.com>
Date: Wed, 10 Jul 2024 23:50:13 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Peter Zijlstra <peterz@...radead.org>
CC: Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>, <linux-kernel@...r.kernel.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>, Steven Rostedt
	<rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Mel Gorman
	<mgorman@...e.de>, Daniel Bristot de Oliveira <bristot@...hat.com>, "Valentin
 Schneider" <vschneid@...hat.com>, "Paul E. McKenney" <paulmck@...nel.org>,
	Imran Khan <imran.f.khan@...cle.com>, Leonardo Bras <leobras@...hat.com>,
	"Guo Ren" <guoren@...nel.org>, Rik van Riel <riel@...riel.com>, Tejun Heo
	<tj@...nel.org>, Cruz Zhao <CruzZhao@...ux.alibaba.com>, Lai Jiangshan
	<jiangshanlai@...il.com>, Joel Fernandes <joel@...lfernandes.org>, Zqiang
	<qiang.zhang1211@...il.com>, Julia Lawall <julia.lawall@...ia.fr>, "Gautham
 R. Shenoy" <gautham.shenoy@....com>
Subject: Re: [RFC PATCH 3/3] softirq: Avoid waking up ksoftirqd from
 flush_smp_call_function_queue()

Hello Peter,

Thank you for the feedback.

On 7/10/2024 8:35 PM, Peter Zijlstra wrote:
> On Wed, Jul 10, 2024 at 09:02:10AM +0000, K Prateek Nayak wrote:
> 
>> [..snip..]
> 
> On first reading I wonder why you've not re-used and hooked into the
> PREEMPT_RT variant of should_wake_ksoftirqd(). That already has a per
> CPU variable to do exactly this.

With this RFC, I intended to check if everyone was onboard with the idea
and of the use-case. One caveat with re-using the existing
"softirq_ctrl.cnt" hook that PREEMPT_RT uses is that we'll need to
expose the functions that increment and decrement it, for it to be used
in kernel/smp.c. I'll make those changes in v2 and we can see if there
are sufficient WARN_ON() to catch any incorrect usage in !PREEMPT_RT
case.

-- 
Thanks and Regards,
Prateek