linux-kernel - Re: [PATCH v3 11/19] unwind: Add deferred user space unwinding API

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Message-ID: <20241030021722.2d1fe6d3@rorschach.local.home>
Date: Wed, 30 Oct 2024 02:17:22 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 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>, Namhyung Kim <namhyung@...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>, linux-trace-kernel@...r.kerne.org, Andrii Nakryiko
 <andrii.nakryiko@...il.com>, Jens Remus <jremus@...ux.ibm.com>, Florian
 Weimer <fweimer@...hat.com>, Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v3 11/19] unwind: Add deferred user space unwinding API

On Tue, 29 Oct 2024 19:20:32 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> The 48:16 bit split gives you uniqueness for around 78 hours at 1GHz.

Are you saying that there will be one system call per nanosecond? This
number is incremented only when a task enters the kernel from user
spaces *and* requests a stack trace. If that happens 1000 times a
second, that would still be around 9000 years.

> 
> But seriously, perf doesn't need this. It really only needs a sequence
> number if you care to stitch over a LOST packet (and I can't say I care
> about that case much) -- and doing that right doesn't really take much
> at all.

Perf may not care because it has a unique descriptor per task, right?
Where it can already know what events are associated to a task. But
that's just a unique characteristic of perf. The unwinder should give a
identifier for every user space stack trace that it will produce and
pass that back to the tracer when it requests a stack trace but it
cannot yet be performed. This identifier is what we are calling a
context cookie. Then when it wants the stack trace, the unwinder will
give the tracer the stack trace along with the identifier
(context-cookie) that this stack trace was for in the past.

It definitely belongs with the undwinder logic.

-- Steve