Re: [PATCH v4 00/14] Implement call_rcu_lazy() and miscellaneous fixes

[Paul E. McKenney]

On Thu, Sep 01, 2022 at 01:59:10PM +0200, Uladzislau Rezki wrote:
> On Thu, Sep 01, 2022 at 01:29:47PM +0200, Frederic Weisbecker wrote:
> > On Tue, Aug 30, 2022 at 06:44:51PM +0200, Uladzislau Rezki wrote:
> > > Hello, Frederic.
> > > 
> > > > 
> > > > Although who knows, may be some periodic file operation while idle are specific
> > > > to Android. I'll try to trace lazy callbacks while idle and the number of grace
> > > > periods associated.
> > > > 
> > > > 
> > > Everything related to lazy call-backs is about not waking "nocb"
> > > kthreads in order to offload one or i should say few callbacks
> > > because it is more or less useless. Currently if incoming callback
> > > is the only one, it will kick a GP whereas a GP will kick nocb_kthread
> > > to offload.
> > 
> > Not sure this is only about not waking "nocb" kthreads. The grace period
> > kthread is also awaken in !NOCB and has quite some work to do. And there,
> > having a server expands the issue because you may have a lot of CPUs's extended
> > quiescent states to check.
> > 
> I mean here the following combination: NOCB + call_rcu_lazy() tandem.
> The !NOCB is not about power save, IMHO. Because it implies callbacks
> to be processed on CPUs they are landed.
> 
> In this scenario you can not let the EAS scheduler to find a more
> efficient CPU for further handling.

Just to follow up, Uladzislau and others did some detailed performance
analysis of NOCB on Android.  Of course, this analysis might or might
not carry over to servers, but it was pretty detailed.

							Thanx, Paul

> > Also in !NOCB, pending callbacks retain the timer tick of a CPU (see
> > rcu_needs_cpu()), and cpuidle relies on the tick to be stopped before
> > allowing the CPU into low power mode. So a lazy callback may delay a CPU from
> > entering into low power mode for a few milliseconds.
> > 
> > And I can observe those retained ticks on my idle box.
> >
> Maybe !NOCB is more about performance. But i have no clue about
> workloads and if such workloads exist nowadays.
> 
> --
> Uladzislau Rezki