Re: [PATCH v3] rcu: Add a minimum time for marking boot as completed

Paul E. McKenney
2023-03-11T06:24:43Z

On Fri, Mar 10, 2023 at 09:55:02AM +0100, Uladzislau Rezki wrote:
> On Thu, Mar 09, 2023 at 10:10:56PM +0000, Joel Fernandes wrote:
> > On Thu, Mar 09, 2023 at 01:57:42PM +0100, Uladzislau Rezki wrote:
> > [..]
> > > > > > > > See this commit:
> > > > > > > >
> > > > > > > > 3705b88db0d7cc ("rcu: Add a module parameter to force use of
> > > > > > > > expedited RCU primitives")
> > > > > > > >
> > > > > > > > Antti provided this commit precisely in order to allow Android
> > > > > > > > devices to expedite the boot process and to shut off the
> > > > > > > > expediting at a time of Android userspace's choosing. So Android
> > > > > > > > has been making this work for about ten years, which strikes me
> > > > > > > > as an adequate proof of concept. ;-)
> > > > > > >
> > > > > > > Thanks for the pointer. That's true. Looking at Android sources, I
> > > > > > > find that Android Mediatek devices at least are setting
> > > > > > > rcu_expedited to 1 at late stage of their userspace boot (which is
> > > > > > > weird, it should be set to 1 as early as possible), and
> > > > > > > interestingly I cannot find them resetting it back to 0!. Maybe
> > > > > > > they set rcu_normal to 1? But I cannot find that either. Vlad? :P
> > > > > >
> > > > > > Interesting. Though this is consistent with Antti's commit log,
> > > > > > where he talks about expediting grace periods but not unexpediting
> > > > > > them.
> > > > > >
> > > > > Do you think we need to unexpedite it? :))))
> > > >
> > > > Android runs on smallish systems, so quite possibly not!
> > > >
> > > We keep it enabled and never unexpedite it. The reason is a performance. I
> > > have done some app-launch time analysis with enabling and disabling of it.
> > >
> > > An expedited case is much better when it comes to app launch time. It
> > > requires ~25% less time to run an app comparing with unexpedited variant.
> > > So we have a big gain here.
> >
> > Wow, that's huge. I wonder if you can dig deeper and find out why that is so
> > as the callbacks may need to be synchronize_rcu_expedited() then, as it could
> > be slowing down other usecases! I find it hard to believe, real-time
> > workloads will run better without those callbacks being always-expedited if
> > it actually gives back 25% in performance!
> >
> I can dig further, but on a high level i think there are some spots
> which show better performance if expedited is set. I mean synchronize_rcu()
> becomes as "less blocking a context" from a time point of view.
>
> The problem of a regular synchronize_rcu() is - it can trigger a big latency
> delays for a caller. For example for nocb case we do not know where in a list
> our callback is located and when it is invoked to unblock a caller.

True, expedited RCU grace periods do not have this callback-invocation
delay that normal RCU does.

> I have already mentioned somewhere. Probably it makes sense to directly wake-up
> callers from the GP kthread instead and not via nocb-kthread that invokes our callbacks
> one by one.

Makes sense, but it is necessary to be careful. Wakeups are not fast,
so making the RCU grace-period kthread do them all sequentially is not
a strategy to win. For example, note that the next expedited grace
period can start before the previous expedited grace period has finished
its wakeups.

Thanx, Paul