linux-kernel - Re: RCU vs NOHZ

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Message-ID: <YzXtOi7rjjWI0ea0@hirez.programming.kicks-ass.net>
Date:   Thu, 29 Sep 2022 21:08:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Boqun Feng <boqun.feng@...il.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: RCU vs NOHZ

On Thu, Sep 29, 2022 at 09:36:24AM -0700, Paul E. McKenney wrote:

> > How has this been tried; and why did the energy cost go up? Is this
> > because the offload thread ends up waking up the CPU we just put to
> > sleep?
> 
> Because doing the additional work consumes energy.  I am not clear on
> exactly what you are asking for here, given the limitations of the tools
> that measure energy consumption.

What additional work? Splicing the cpu pending list onto another list
with or without atomic op barely qualifies for work. The main point is
making sure the pending list isn't in the way of going (deep) idle.