linux-kernel - Re: [patch V6 01/37] tracing/hwlat: Use ktime_get_mono_fast

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Date:   Wed, 20 May 2020 21:51:11 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Wei Liu <wei.liu@...nel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Jason Chen CJ <jason.cj.chen@...el.com>,
        Zhao Yakui <yakui.zhao@...el.com>,
        "Peter Zijlstra \(Intel\)" <peterz@...radead.org>
Subject: Re: [patch V6 01/37] tracing/hwlat: Use ktime_get_mono_fast_ns()

Steven Rostedt <rostedt@...dmis.org> writes:

> On Tue, 19 May 2020 23:45:10 +0200
> Thomas Gleixner <tglx@...utronix.de> wrote:
>
>> >> @@ -165,20 +155,22 @@ void trace_hwlat_callback(bool enter)
>> >>   * Used to repeatedly capture the CPU TSC (or similar), looking for potential
>> >>   * hardware-induced latency. Called with interrupts disabled and with
>> >>   * hwlat_data.lock held.
>> >> + *
>> >> + * Use ktime_get_mono_fast() here as well because it does not wait on the
>> >> + * timekeeping seqcount like ktime_get_mono().  
>> >
>> > When doing a "git grep ktime_get_mono" I only find
>> > ktime_get_mono_fast_ns() (and this comment), so I don't know what to compare
>> > that to. Did you mean another function?  
>> 
>> Yeah. I fatfingered the comment. The code uses ktime_get_mono_fast_ns().
>
> Well, I assumed that's what you meant with "ktime_get_mono_fast()" but I
> don't know what function you are comparing it to that waits on the seqcount
> like "ktime_get_mono()" as there is no such function.

Gah. ktime_get_mono_fast_ns() and ktime_get() of course.