From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To: Peter Zijlstra <peterz@infradead.org>,
Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>,
x86@kernel.org, Ingo Molnar <mingo@kernel.org>,
Arnaldo Carvalho de Melo <acme@kernel.org>,
linux-kernel@vger.kernel.org,
Indu Bhagat <indu.bhagat@oracle.com>,
Mark Rutland <mark.rutland@arm.com>,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
Jiri Olsa <jolsa@kernel.org>, Namhyung Kim <namhyung@kernel.org>,
Ian Rogers <irogers@google.com>,
Adrian Hunter <adrian.hunter@intel.com>,
linux-perf-users@vger.kernel.org, Mark Brown <broonie@kernel.org>,
linux-toolchains@vger.kernel.org, Jordan Rome <jordalgo@meta.com>,
Sam James <sam@gentoo.org>
Subject: Re: [PATCH v2 00/11] unwind, perf: sframe user space unwinding, deferred perf callchains
Date: Mon, 16 Sep 2024 02:15:01 +0200 [thread overview]
Message-ID: <36f32c27-3cab-48ea-b8de-8df91b91836d@efficios.com> (raw)
In-Reply-To: <20240916181545.GD4723@noisy.programming.kicks-ass.net>
On 2024-09-16 20:15, Peter Zijlstra wrote:
[...]
> The point being that it is possible to wrap one CPU into the id space of
> another CPU. It is not trivial, but someone who wants to can make it
> happen.
I agree that the overflow of the free-running counter bleeding into the
CPU numbers is something we want to prevent. We don't care if this
counter overflows after day/months/years for sake of correlation
within a system call, but we do care about the fact that this
free-running counter could be made to overflow after a very long
time while the system runs, and then we reach a state where the
CPU numbers are mixed up, which leads to short-term correlation
issues.
I would recommend this layout for this 64-bit value instead:
low-bits: cpu number
high-bits: free-running counter
This way, we eliminate any carry from overflow into the cpu number bits.
Thanks,
Mathieu
--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com
next prev parent reply other threads:[~2024-09-17 0:15 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-09-13 23:02 Josh Poimboeuf
2024-09-13 23:02 ` [PATCH v2 01/11] unwind: Introduce generic user space unwinding interface Josh Poimboeuf
2024-09-13 23:02 ` [PATCH v2 02/11] unwind/x86: Add HAVE_USER_UNWIND Josh Poimboeuf
2024-09-16 11:46 ` Peter Zijlstra
2024-10-20 8:09 ` Josh Poimboeuf
2024-09-13 23:02 ` [PATCH v2 03/11] unwind: Introduce SFrame user space unwinding Josh Poimboeuf
2024-09-14 11:23 ` Steven Rostedt
2024-10-01 18:20 ` Indu Bhagat
2024-10-01 18:36 ` Steven Rostedt
2024-10-02 8:18 ` Florian Weimer
2024-10-02 14:05 ` Steven Rostedt
2024-10-23 13:59 ` Jens Remus
2024-10-27 17:49 ` Josh Poimboeuf
2024-09-13 23:02 ` [PATCH v2 04/11] unwind/x86/64: Add HAVE_USER_UNWIND_SFRAME Josh Poimboeuf
2024-09-13 23:02 ` [PATCH v2 05/11] perf/x86: Use user_unwind interface Josh Poimboeuf
2024-09-16 6:48 ` kernel test robot
2024-09-17 22:01 ` Namhyung Kim
2024-09-13 23:02 ` [PATCH v2 06/11] perf: Remove get_perf_callchain() 'init_nr' argument Josh Poimboeuf
2024-09-13 23:02 ` [PATCH v2 07/11] perf: Remove get_perf_callchain() 'crosstask' argument Josh Poimboeuf
2024-09-13 23:02 ` [PATCH v2 08/11] perf: Simplify get_perf_callchain() user logic Josh Poimboeuf
2024-09-13 23:02 ` [PATCH v2 09/11] perf: Introduce deferred user callchains Josh Poimboeuf
2024-09-17 22:07 ` Namhyung Kim
2024-09-13 23:02 ` [PATCH v2 10/11] perf/x86: Add HAVE_PERF_CALLCHAIN_DEFERRED Josh Poimboeuf
2024-09-13 23:02 ` [PATCH v2 11/11] perf/x86: Enable SFrame unwinding for deferred user callchains Josh Poimboeuf
2024-09-14 12:12 ` [PATCH v2 00/11] unwind, perf: sframe user space unwinding, deferred perf callchains Steven Rostedt
2024-09-15 11:11 ` Josh Poimboeuf
2024-09-15 11:38 ` Steven Rostedt
2024-09-16 14:08 ` Peter Zijlstra
2024-09-16 15:39 ` Josh Poimboeuf
2024-09-16 18:15 ` Peter Zijlstra
2024-09-16 0:15 ` Mathieu Desnoyers [this message]
2024-09-16 0:33 ` Mathieu Desnoyers
2024-09-17 0:37 ` Mathieu Desnoyers
2024-09-16 22:46 ` Steven Rostedt
2024-09-17 21:58 ` Namhyung Kim
2024-09-18 5:14 ` Mathieu Desnoyers
2024-10-03 2:31 ` Steven Rostedt
2024-10-03 2:37 ` Josh Poimboeuf
2024-10-03 14:56 ` Steven Rostedt
2024-09-16 16:03 ` Steven Rostedt
2024-09-14 19:37 ` Namhyung Kim
2024-10-23 13:22 ` Jens Remus
2024-10-24 2:22 ` Steven Rostedt
2024-10-27 17:24 ` Josh Poimboeuf
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