linux-kernel - Re: [PATCH v2 00/11] unwind, perf: sframe user space unwinding, deferred perf callchains

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Message-ID: <Zun7nKFzWz2J2rSz@google.com>
Date: Tue, 17 Sep 2024 14:58:52 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-kernel@...r.kernel.org, Indu Bhagat <indu.bhagat@...cle.com>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	linux-perf-users@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	linux-toolchains@...r.kernel.org, Jordan Rome <jordalgo@...a.com>,
	Sam James <sam@...too.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH v2 00/11] unwind, perf: sframe user space unwinding,
 deferred perf callchains

Hello,

On Mon, Sep 16, 2024 at 06:46:45PM -0400, Steven Rostedt wrote:
> On Mon, 16 Sep 2024 20:15:45 +0200
> Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Mon, Sep 16, 2024 at 05:39:53PM +0200, Josh Poimboeuf wrote:
> > 
> > > The cookie is incremented once per entry from userspace, when needed.
> > > 
> > > It's a unique id used by the tracers which is emitted with both kernel
> > > and user stacktrace samples so the userspace tool can stitch them back
> > > together.  Even if you have multiple tracers triggering at the same time
> > > they can all share a single user trace.  
> > 
> > But perf don't need this at all, right? It knows that the next deferred
> > trace for that tid will be the one.

Well.. technically you can sample without tid.  But I'm not sure how
much it'd be useful if you collect callchains without tid.

> 
> Is that because perf uses per task buffers? Will perf know this if it
> uses a global buffer? What does perf do with "-a"?

Then it'd use per-cpu ring buffers.  But each sample would have pid/tid
pair and time so perf tools can match it with a deferred callchian.

Thanks,
Namhyung