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

[Joel Fernandes]

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!

thanks,

 - Joel