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Date: Tue, 8 Jul 2025 20:46:56 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Wander Lairson Costa <wander@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>, Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Valentin Schneider <vschneid@redhat.com>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	David Woodhouse <dwmw@amazon.co.uk>,
	Thomas Gleixner <tglx@linutronix.de>,
	Boqun Feng <boqun.feng@gmail.com>,
	open list <linux-kernel@vger.kernel.org>,
	"open list:TRACING" <linux-trace-kernel@vger.kernel.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Clark Williams <williams@redhat.com>,
	Gabriele Monaco <gmonaco@redhat.com>
Subject: Re: [PATCH v3 2/2] tracing/preemptirq: Optimize
 preempt_disable/enable() tracepoint overhead
Message-ID: <20250708184656.GB477119@noisy.programming.kicks-ass.net>
References: <20250704170748.97632-1-wander@redhat.com>
 <20250704170748.97632-3-wander@redhat.com>
 <20250707112622.GZ1613376@noisy.programming.kicks-ass.net>
 <ppdduzdqnd3kwourcmocfi35i7wcbuagmzqgtgmyr55aps774t@37wucnoii7o3>
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In-Reply-To: <ppdduzdqnd3kwourcmocfi35i7wcbuagmzqgtgmyr55aps774t@37wucnoii7o3>

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.