linux-kernel - Re: RCU vs NOHZ

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Date:   Thu, 29 Sep 2022 12:56:41 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.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 09:08:42PM +0200, Peter Zijlstra wrote:
> 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.

Very good.  Send a patch.

After some time, its successor might correctly handle lock/memory
contention, CPU hotplug, presumed upcoming runtime changes in CPUs'
housekeeping status, frequent idle entry/exit, grace period begin/end,
quiet embedded systems, and so on.

Then we can see if it actually reduces power consumption.

							Thanx, Paul