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

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Message-ID: <20241030175552.22cza6t7dfe3xy3p@treble.attlocal.net>
Date: Wed, 30 Oct 2024 10:55:52 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
	Steven Rostedt <rostedt@...dmis.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.kernel.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 Wed, Oct 30, 2024 at 10:47:55AM -0700, Josh Poimboeuf wrote:
> > Moving from per-cpu to per-task makes this cookie task-specific and not
> > global anymore, I don't think we want this for a stack walking
> > infrastructure meant to be used by various tracers. Also a global cookie
> > is more robust and does not depend on guaranteeing that all the
> > trace data is present to guarantee current thread ID accuracy and
> > thus that cookies match between deferred unwind request and their
> > fulfillment.
> 
> I don't disagree.  What I meant was, on entry (or exit), increment the
> task cookie *with* the CPU bits included.
> 
> Or as you suggested previously, the "cookie" just be a struct with two
> fields: CPU # and per-task entry counter.

Never mind, that wouldn't work...  Two tasks could have identical
per-task counters while being scheduled on the same CPU.

-- 
Josh