linux-kernel - Re: [PATCH v3 2/2] tracing/preemptirq: Optimize 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: <20250708184656.GB477119@noisy.programming.kicks-ass.net>
Date: Tue, 8 Jul 2025 20:46:56 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Wander Lairson Costa <wander@...hat.com>
Cc: Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	David Woodhouse <dwmw@...zon.co.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	Boqun Feng <boqun.feng@...il.com>,
	open list <linux-kernel@...r.kernel.org>,
	"open list:TRACING" <linux-trace-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Clark Williams <williams@...hat.com>,
	Gabriele Monaco <gmonaco@...hat.com>
Subject: Re: [PATCH v3 2/2] tracing/preemptirq: Optimize
 preempt_disable/enable() tracepoint overhead

On Tue, Jul 08, 2025 at 10:09:45AM -0300, Wander Lairson Costa wrote:
> On Mon, Jul 07, 2025 at 01:26:22PM +0200, Peter Zijlstra wrote:
> > On Fri, Jul 04, 2025 at 02:07:43PM -0300, Wander Lairson Costa wrote:
> > > +#if defined(CONFIG_DEBUG_PREEMPT) || defined(CONFIG_TRACE_PREEMPT_TOGGLE)
> > > +#define preempt_count_dec_and_test() \
> > > +	({ preempt_count_sub(1); should_resched(0); })
> > > +#endif
> > 
> > Also this is terrible. Surely you can do better.
> > 
> 
> Thank you for pointing this out. I'm not sure I've fully understood the
> concern here. My understanding was that this logic was pre-existing and
> my patch only reorganized it.
> 
> I'm clearly missing something. Could you please elaborate a bit on the
> issue you've spotted?

The normal (!DEBUG) case uses __preempt_count_dec_and_test(), which is
significantly better.