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

Joel Fernandes
2023-03-11T17:19:29Z

On Sat, Mar 11, 2023 at 1:24 AM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> 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.

The kthreads could be undergoing scheduler contention too especially
since the workload is launching an app if I understand Vlad's usecase.
Hence my desire for a rcutop one-stop tool which shows all these
things (rcu kthread scheduler delays, callback latencies, etc etc).
;-) The more and more I run into issues, the more that tool becomes
urgent which I'm working on...

thanks,

- Joel