linux-kernel - Re: [RFC PATCH] preempt: Add a generic function to return the preemption string.

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Message-ID: <20250109093702.5fe3400d@gandalf.local.home>
Date: Thu, 9 Jan 2025 09:37:02 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev, Ben Segall
 <bsegall@...gle.com>, Catalin Marinas <catalin.marinas@....com>, Dietmar
 Eggemann <dietmar.eggemann@....com>, Ingo Molnar <mingo@...hat.com>, Juri
 Lelli <juri.lelli@...hat.com>, Mel Gorman <mgorman@...e.de>, Thomas
 Gleixner <tglx@...utronix.de>, Valentin Schneider <vschneid@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>, Will Deacon
 <will@...nel.org>, x86@...nel.org
Subject: Re: [RFC PATCH] preempt: Add a generic function to return the
 preemption string.

On Thu, 9 Jan 2025 12:43:39 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> Hurmph.. a bunch of those combinations are nonsense :/
> 
> Also, we have a string thing in sched_dynamic_show().
> 
> Can't we do something like:
> 
> const char *preempt_model_str(void)
> {
> 	static char buf[128];
> 	size_t off = 0, len = sizeof(buf), r;
> 	bool brace = IS_ENABLED(CONFIG_PREEMPT_RT) &&
> 		     (IS_ENABLED(CONFIG_PREEMPT_DYNAMIC) ||
> 		      IS_ENABLED(CONFIG_PREEMPT_LAZY));
> 
> 	if (IS_ENABLED(CONFIG_PREEMPT_BUILD)) {
> 		r = snprintf(buf + off, len, "PREEMPT");
> 		off += r;
> 		len -= r;
> 
> 		if (IS_ENABLED(CONFIG_PREEMPT_RT)) {
> 			r = snprintf(buf + off, len, "%sRT%s", 
> 					brace ? "_{" : "_",
> 					brace ? "," : "");
> 			off += r;
> 			len -= r;
> 		}
> 
> 		if (IS_ENABLED(CONFIG_PREEMPT_DYNAMIC)) {
> 			snprintf(buf + off, len, "(%s)%s", 
> 				preempt_modes[preempt_dynamic_mode],
> 				brace ? "}" : "");
> 			retun buf;
> 		}
> 
> 		if (IS_ENABLED(CONFIG_PREEMPT_LAZY)) {
> 			snprintf(buf + off, len, "LAZY%s", 
> 				brace ? "}" : "");
> 			retun buf;
> 		}

I'm sorry, but I can't even tell what the above is doing without my brain
hurting. Why make code that was easy to read into a cryptic obfuscation? I
can't see this as an optimization as IS_ENABLED() is determined at compile
time.

-- Steve

> 
> 		return buf;
> 	}
> 
> 	if (IS_ENABLED(CONFIG_VOLUNTARY_BUILD))
> 		return "VOLUNTARY";
> 
> 	return "NONE";
> }