linux-kernel - Re: [PATCH v4 3/6] perf: Shrink the size of the recursion counter.

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Message-ID: <20240701131040.GJ20127@noisy.programming.kicks-ass.net>
Date: Mon, 1 Jul 2024 15:10:40 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	Adrian Hunter <adrian.hunter@...el.com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Daniel Bristot de Oliveira <bristot@...nel.org>,
	Frederic Weisbecker <frederic@...nel.org>,
	Ian Rogers <irogers@...gle.com>, Ingo Molnar <mingo@...hat.com>,
	Jiri Olsa <jolsa@...nel.org>, Kan Liang <kan.liang@...ux.intel.com>,
	Marco Elver <elver@...gle.com>, Mark Rutland <mark.rutland@....com>,
	Namhyung Kim <namhyung@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4 3/6] perf: Shrink the size of the recursion counter.

On Mon, Jul 01, 2024 at 02:56:43PM +0200, Sebastian Andrzej Siewior wrote:
> On 2024-07-01 14:31:37 [+0200], Peter Zijlstra wrote:
> > On Mon, Jun 24, 2024 at 05:15:16PM +0200, Sebastian Andrzej Siewior wrote:
> > > There are four recursion counter, one for each context. The type of the
> > > counter is `int' but the counter is used as `bool' since it is only
> > > incremented if zero.
> > > 
> > > Reduce the type of the recursion counter to an unsigned char, keep the
> > > increment/ decrement operation.
> > 
> > Does this actually matter? Aren't u8 memops encoded by longer
> > instructions etc..
> 
> The goal here isn't to reduce the opcodes but to add it to task_struct
> without making it larger by filling a hole.

Changelog failed to mention this crucial fact.