Re: fast path cycle muncher (vmstat: make vmstat_updater deferrable again and shut down on idle)

[Mike Galbraith]

On Wed, 2016-01-27 at 04:12 +0100, Mike Galbraith wrote:
> Good morning,
> 
> On Tue, 2016-01-26 at 13:20 -0600, Christoph Lameter wrote:
> > On Tue, 26 Jan 2016, Mike Galbraith wrote:
> > 
> > > > The system going idle is a fastpath. Mind boogling.
> > > 
> > > Hohum, noted.  Now what about those cycles, and the sleeping lock you
> > > injected for -rt?
> > 
> > Since we (the NOHZ people) care mostly about NOHZ then lets restrict
> > that to the NOHZ mode. Then it should not affect your load.
> 
> Tons of folks do have NO_HZ enabled (including me).  Isn't there a spot
> somewhere in NO_HZ_FULL code where it can take up residence?  (one with
> a tad lower maximum call frequency would be good, a nohz_full cpu isn't
> necessarily being used for pure compute exclusively)

I forgot to mention that the spot you picked is called with irqs
disabled.

> > Subject: Move quiet_vmstat() to NOHZ code
> > 
> > quiet_vmstat() seems to cause regressions for some load because
> > the cpu going idle is a "fastpath". Mind boogling. Strange claim.
> > If the system goes idle then it has nothing to do after all

Hm, seems I also forgot to say "Hohum, noted..." again.

	-Mike