Re: RCU vs NOHZ

[Paul E. McKenney]

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