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

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Message-ID: <8889c16b-9c3d-4ab3-b353-e00146532174@efficios.com>
Date: Fri, 9 May 2025 08:23:49 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 Namhyung Kim <namhyung@...nel.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 2025-05-08 14:54, Steven Rostedt wrote:
> On Thu, 8 May 2025 14:49:59 -0400
> Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> 
>> AFAIR, the cookie method generates the cookie by combining the cpu
>> number with a per-cpu count.
>>
>> This ensures that there are not two cookies emitted at the same time
>> from two CPUs that have the same value by accident.
>>
>> How would the timestamp method prevent this ?
> 
> Do we care? It only needs to be unique per pid doesn't it?

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.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com