linux-kernel - Re: [PATCH v2 5/6] osnoise: handle quiescent states for PREEMPT

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20241114092225.lVdeX1wG@linutronix.de>
Date: Thu, 14 Nov 2024 10:22:25 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Ankur Arora <ankur.a.arora@...cle.com>
Cc: linux-kernel@...r.kernel.org, peterz@...radead.org, tglx@...utronix.de,
	paulmck@...nel.org, mingo@...nel.org, juri.lelli@...hat.com,
	vincent.guittot@...aro.org, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, frederic@...nel.org, efault@....de,
	sshegde@...ux.ibm.com, boris.ostrovsky@...cle.com,
	Daniel Bristot de Oliveira <bristot@...nel.org>
Subject: Re: [PATCH v2 5/6] osnoise: handle quiescent states for
 PREEMPT_RCU=n, PREEMPTION=y

On 2024-11-06 12:17:57 [-0800], Ankur Arora wrote:
> To reduce RCU noise for nohz_full configurations, osnoise depends
> on cond_resched() providing quiescent states for PREEMPT_RCU=n
> configurations. And, for PREEMPT_RCU=y configurations does this
> by directly calling rcu_momentary_eqs().
> 
> With PREEMPT_LAZY=y, however, we can have configurations with
> (PREEMPTION=y, PREEMPT_RCU=n), which means neither of the above
> can help.

The problem is as you say CONFIG_PREEMPT_RCU=n + CONFIG_PREEMPTION=y.
You can't select any of those two directly but get here via
 PREEMPT_LAZY=y + PREEMPT_DYNAMIC=n.

Please spell it out to make it obvious. It is not a large group of
configurations, it is exactly this combo.

 With PREEMPT_LAZY=y + PREEMPT_DYNAMIC=n however we get PREEMPT_RCU=n
 which means no direct rcu_momentary_eqs() invocations and
 cond_resched() is an empty stub.

> Handle that by fallback to the explicit quiescent states via
> rcu_momentary_eqs().

> Cc: Paul E. McKenney <paulmck@...nel.org>
> Cc: Daniel Bristot de Oliveira <bristot@...nel.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Suggested-by: Paul E. McKenney <paulmck@...nel.org>
> Acked-by: Daniel Bristot de Oliveira <bristot@...nel.org>
> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>

Sebastian