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

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Message-ID: <aB4jKjLsXCzcH4yd@google.com>
Date: Fri, 9 May 2025 08:45:46 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Steven Rostedt <rostedt@...dmis.org>, 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

Hi Mathieu,

On Fri, May 09, 2025 at 08:23:49AM -0400, Mathieu Desnoyers wrote:
> 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.

Yep, basically perf sets PERF_SAMPLE_TID (and sample_id_all) which makes
every records come with PID/TID.. 

Thanks,
Namhyung