linux-kernel - Re: [RESEND][PATCH v15 0/4] perf: Support the deferred unwinding infrastructure

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Message-ID: <20250908132106.435b0e6f@gandalf.local.home>
Date: Mon, 8 Sep 2025 13:21:06 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 bpf@...r.kernel.org, x86@...nel.org, Masami Hiramatsu
 <mhiramat@...nel.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Josh Poimboeuf <jpoimboe@...nel.org>, Ingo Molnar <mingo@...nel.org>, Jiri
 Olsa <jolsa@...nel.org>, Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Andrii Nakryiko <andrii@...nel.org>, Indu Bhagat <indu.bhagat@...cle.com>,
 "Jose E. Marchesi" <jemarch@....org>, Beau Belgrave
 <beaub@...ux.microsoft.com>, Jens Remus <jremus@...ux.ibm.com>, Linus
 Torvalds <torvalds@...ux-foundation.org>, Andrew Morton
 <akpm@...ux-foundation.org>, Florian Weimer <fweimer@...hat.com>, Sam James
 <sam@...too.org>, Kees Cook <kees@...nel.org>, "Carlos O'Donell"
 <codonell@...hat.com>
Subject: Re: [RESEND][PATCH v15 0/4] perf: Support the deferred unwinding
 infrastructure


Peter, can you take a look at these patches please. I believe you're the
only one that really maintains this code today.

-- Steve


On Mon, 08 Sep 2025 13:14:12 -0400
Steven Rostedt <rostedt@...nel.org> wrote:

> [
>   This is simply a resend of version 15 of this patch series
>   but with only the kernel changes. I'm separating out the user space
>   changes to their own series.
>   The original v15 is here:
>     https://lore.kernel.org/linux-trace-kernel/20250825180638.877627656@kernel.org/
> ]
> 
> This patch set is based off of perf/core of the tip tree:
>   git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git
> 
> To run this series, you can checkout this repo that has this series as well as the above:
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace.git  unwind/perf-test
> 
> This series implements the perf interface to use deferred user space stack
> tracing.
> 
> Patch 1 adds a new API interface to the user unwinder logic to allow perf to
> get the current context cookie for it's task event tracing. Perf's task event
> tracing maps a single task per perf event buffer and it follows the task
> around, so it only needs to implement its own task_work to do the deferred
> stack trace. Because it can still suffer not knowing which user stack trace
> belongs to which kernel stack due to dropped events, having the cookie to
> create a unique identifier for each user space stack trace to know which
> kernel stack to append it to is useful.
> 
> Patch 2 adds the per task deferred stack traces to perf. It adds a new event
> type called PERF_RECORD_CALLCHAIN_DEFERRED that is recorded when a task is
> about to go back to user space and happens in a location that pages may be
> faulted in. It also adds a new callchain context called PERF_CONTEXT_USER_DEFERRED
> that is used as a place holder in a kernel callchain to append the deferred
> user space stack trace to.
> 
> Patch 3 adds the user stack trace context cookie in the kernel callchain right
> after the PERF_CONTEXT_USER_DEFERRED context so that the user space side can
> map the request to the deferred user space stack trace.
> 
> Patch 4 adds support for the per CPU perf events that will allow the kernel to
> associate each of the per CPU perf event buffers to a single application. This
> is needed so that when a request for a deferred stack trace happens on a task
> that then migrates to another CPU, it will know which CPU buffer to use to
> record the stack trace on. It is possible to have more than one perf user tool
> running and a request made by one perf tool should have the deferred trace go
> to the same perf tool's perf CPU event buffer. A global list of all the
> descriptors representing each perf tool that is using deferred stack tracing
> is created to manage this.
> 
> 
> Josh Poimboeuf (1):
>       perf: Support deferred user callchains
> 
> Steven Rostedt (3):
>       unwind deferred: Add unwind_user_get_cookie() API
>       perf: Have the deferred request record the user context cookie
>       perf: Support deferred user callchains for per CPU events
> 
> ----
>  include/linux/perf_event.h            |  11 +-
>  include/linux/unwind_deferred.h       |   5 +
>  include/uapi/linux/perf_event.h       |  25 +-
>  kernel/bpf/stackmap.c                 |   4 +-
>  kernel/events/callchain.c             |  14 +-
>  kernel/events/core.c                  | 421 +++++++++++++++++++++++++++++++++-
>  kernel/unwind/deferred.c              |  21 ++
>  tools/include/uapi/linux/perf_event.h |  25 +-
>  8 files changed, 518 insertions(+), 8 deletions(-)