linux-kernel - Re: [RFC][PATCH] ring-buffer: Have nested events still record running time stamp

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Message-ID: <126813531.12266.1593093195147.JavaMail.zimbra@efficios.com>
Date:   Thu, 25 Jun 2020 09:53:15 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     rostedt <rostedt@...dmis.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Yordan Karadzhov <y.karadz@...il.com>,
        Tzvetomir Stoyanov <tz.stoyanov@...il.com>,
        Tom Zanussi <zanussi@...nel.org>,
        Jason Behmer <jbehmer@...gle.com>,
        Julia Lawall <julia.lawall@...ia.fr>,
        Clark Williams <williams@...hat.com>,
        bristot <bristot@...hat.com>, Daniel Wagner <wagi@...om.org>,
        Darren Hart <dvhart@...are.com>,
        Jonathan Corbet <corbet@....net>,
        "Suresh E. Warrier" <warrier@...ux.vnet.ibm.com>
Subject: Re: [RFC][PATCH] ring-buffer: Have nested events still record
 running time stamp

----- On Jun 25, 2020, at 9:44 AM, rostedt rostedt@...dmis.org wrote:

> From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> 
> [ SEVEN YEAR PROBLEM SOLVED! ]
> 
> Up until now, if an event is interrupted while it is recorded by an
> interrupt, and that interrupt records events, the time of those events will
> all be the same. This is because events only record the delta of the time
> since the previous event (or beginning of a page), and to handle updating
> the time keeping for that of nested events is extremely racy. After years of
> thinking about this and several failed attempts, I finally have a solution
> to solve this puzzle.

Out of curiosity, considering that LTTng has solved this problem 10+ years ago
with a simpler concurrency-friendly time-stamping model, why not simply use it
rather than add complexity to the current ftrace timestamp scheme ?

Thanks,

Mathieu


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