linux-kernel - Re: Performance regression: thread wakeup time (latency) increased up to 3x

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Message-ID: <20211107155616.GA269390@lothringen>
Date:   Sun, 7 Nov 2021 16:56:16 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Norbert <nbrtt01@...il.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yunfeng Ye <yeyunfeng@...wei.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: Performance regression: thread wakeup time (latency) increased
 up to 3x

Hi Norbert,

On Tue, Oct 19, 2021 at 12:01:32AM -0700, Norbert wrote:
> 
> On the thought that it might enter deeper idle/wait/sleep states:
> 
> The benchmark executes this test in a quite tight loop, except that so far
> it waited 1000 ns (with a mix of pause and rdtsc) before calling futex-wake,
> to make sure the other thread fully enters the futex-wait without taking any
> shortcuts.
> 
> Except when this "prepare time" is reduced to less than even 350 ns or so,
> the timings remain the same (they go up before they start going down).
> Surely in this situation the thread is at least not supposed to enter deeper
> states for such short waiting times.

Is it possible for you to share this benchmark so that I can try to reproduce?

Thanks!