linux-kernel - Re: [PATCH v3 01/17] tools/rv: Do not skip idle in trace

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Message-ID: <20250716130846.GC3429938@noisy.programming.kicks-ass.net>
Date: Wed, 16 Jul 2025 15:08:46 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Gabriele Monaco <gmonaco@...hat.com>
Cc: linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	linux-trace-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Nam Cao <namcao@...utronix.de>, Tomas Glozar <tglozar@...hat.com>,
	Juri Lelli <jlelli@...hat.com>,
	Clark Williams <williams@...hat.com>,
	John Kacur <jkacur@...hat.com>
Subject: Re: [PATCH v3 01/17] tools/rv: Do not skip idle in trace

On Wed, Jul 16, 2025 at 03:05:50PM +0200, Gabriele Monaco wrote:
> On Wed, 2025-07-16 at 14:41 +0200, Peter Zijlstra wrote:
> > On Wed, Jul 16, 2025 at 02:18:28PM +0200, Gabriele Monaco wrote:
> > > On Wed, 2025-07-16 at 13:50 +0200, Peter Zijlstra wrote:
> > > > On Tue, Jul 15, 2025 at 09:14:18AM +0200, Gabriele Monaco wrote:
> > > > > Currently, the userspace RV tool skips trace events triggered by the RV
> > > > > tool itself, this can be changed by passing the parameter -s, which
> > > > > sets the variable config_my_pid to 0 (instead of the tool's PID). The
> > > > > current condition for per-task monitors (config_has_id) does not check
> > > > > that config_my_pid isn't 0 to skip. In case we pass -s, we show events
> > > > > triggered by RV but don't show those triggered by idle (PID 0).
> > > > 
> > > > The distinction between !my_pid and has_id is that you can in fact trace
> > > > pid-0 if you want?
> > > > 
> > > Yes pretty much, no flag is meant to skip events from pid-0.
> > 
> > > > > -	if (config_has_id && (config_my_pid == id))
> > > > > +	if (config_my_pid && config_has_id && (config_my_pid == id))
> > 
> > But should we then not write:
> > 
> > 	if (config_has_id && (config_my_pid == id))
> 
> Sorry, got a bit confused, I flipped the two while describing:
> * -s shows traces from RV but skips from pid-0 (unintended)
> * omitting -s skips events from RV (correct)
> 
> If we are running a per-task monitor config_has_id is always true, we pass -s,
> which makes config_my_pid = 0 (intended /not/ to skip RV).
> Now when we are about to trace an event from idle (id=0), we skip it, although
> we really shouldn't.
> That's why we also needs to check for config_my_pid not being 0.
> 
> Does it make sense?

Sorta, but would it not make sense to use has_pid := -1 for the invalid
case, instead of 0, which is a valid pid?