linux-kernel - Re: [PATCH v5 13/17] perf: Support deferred user callchains

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Message-ID: <20250509115553.0811747e@gandalf.local.home>
Date: Fri, 9 May 2025 11:55:53 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org, Masami
 Hiramatsu <mhiramat@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Andrew Morton <akpm@...ux-foundation.org>, Josh Poimboeuf
 <jpoimboe@...nel.org>, x86@...nel.org, Peter Zijlstra
 <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, Arnaldo Carvalho de
 Melo <acme@...nel.org>, Indu Bhagat <indu.bhagat@...cle.com>, Alexander
 Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa
 <jolsa@...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>, Andrii Nakryiko
 <andrii.nakryiko@...il.com>, Jens Remus <jremus@...ux.ibm.com>, Florian
 Weimer <fweimer@...hat.com>, Andy Lutomirski <luto@...nel.org>, Weinan Liu
 <wnliu@...gle.com>, Blake Jones <blakejones@...gle.com>, Beau Belgrave
 <beaub@...ux.microsoft.com>, "Jose E. Marchesi" <jemarch@....org>
Subject: Re: [PATCH v5 13/17] perf: Support deferred user callchains

On Fri, 9 May 2025 08:45:46 -0700
Namhyung Kim <namhyung@...nel.org> wrote:

> > Is it possible to have many threads writing into the same
> > ring buffer in that scenario ? Are all event records stamped
> > with their associated PID ? As long as we have enough information
> > to know which thread was associated with the timestamp cookie
> > on both ends (request for callchain and saving the user callchain
> > on return to userspace), we should be OK.  
> 
> Yep, basically perf sets PERF_SAMPLE_TID (and sample_id_all) which makes
> every records come with PID/TID.. 

If LTTng is fine with this too, then we should be OK as perf and ftrace
both record TID in the events. I'm assuming that BPF would be OK too.

-- Steve