linux-kernel - Re: [PATCH v4 28/39] unwind_user/deferred: Add deferred unwinding interface

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Message-ID: <20250205022500.lnz3jlaetzh6gxds@jpoimboe>
Date: Tue, 4 Feb 2025 18:25:00 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Peter Zijlstra <peterz@...radead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 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.kernel.org,
	Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Jens Remus <jremus@...ux.ibm.com>,
	Florian Weimer <fweimer@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Weinan Liu <wnliu@...gle.com>
Subject: Re: [PATCH v4 28/39] unwind_user/deferred: Add deferred unwinding
 interface

On Thu, Jan 30, 2025 at 03:21:36PM -0500, Steven Rostedt wrote:
> Coming back from this. It would be fine if we could do the back trace when
> we come back from the scheduler, so it should not be an issue if the task
> even has to schedule again to fault in the sframe information.

So there would be two callback hook points:

  - schedule() after enabling preemption

  - task work

and first one wins?

> I was also wondering if the unwinder doesn't keep track of who requested
> the back trace, just that someone did. Then it would just take a single
> flag in the task struct to do the back trace. Return the "cookie" to the
> tracer that requested the back trace, and when you do the back trace, just
> call all callbacks with that cookie. Let the tracer decided if it wants to
> record the back trace or ignore it based on the cookie.
> 
> That is, the tracers would need to keep track of the cookies that it cares
> about, as if there's other tracers asking for stack traces on tasks that
> this tracer doesn't care about it needs to handle being called when it
> doesn't care about the stack trace. That said, if you want to trace all
> tasks, you can just ignore the cookies and record the traces.

Easy enough for the unwinder, but IIUC each tracer would have to
maintain a global list of pending cookies (and corresponding ptrs to
perf_event, trace_array, etc)?  Would that not create a lot of
contention?

Seems like there really needs to be some kind of per-task or per-request
state.

-- 
Josh