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

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Message-ID: <20241029171752.4y67p3ob24riogpi@treble.attlocal.net>
Date: Tue, 29 Oct 2024 10:17:52 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: 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.kerne.org,
	Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Jens Remus <jremus@...ux.ibm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.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, Oct 29, 2024 at 02:56:17PM +0100, Peter Zijlstra wrote:
> On Mon, Oct 28, 2024 at 02:47:38PM -0700, Josh Poimboeuf wrote:
> 
> > + * The only exception is when the task has migrated to another CPU, *and* this
> > + * is called while the task work is running (or has already run).  Then a new
> > + * cookie will be generated and the callback will be called again for the new
> > + * cookie.
> 
> So that's a bit crap. The user stack won't change for having been
> migrated.
> 
> So perf can readily use the full u64 cookie value as a sequence number,
> since the whole perf record will already have the TID of the task in.
> Mixing in this CPU number for no good reason and causing trouble like
> this just doesn't make sense to me.
> 
> If ftrace needs brain damage like this, can't we push this to the user?
> 
> That is, do away with the per-cpu sequence crap, and add a per-task
> counter that is incremented for every return-to-userspace.

That would definitely make things easier for me, though IIRC Steven and
Mathieu had some concerns about TID wrapping over days/months/years.

With that mindset I suppose the per-CPU counter could also wrap, though
that could be mitigated by making the cookie a struct with more bits.

-- 
Josh