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

[Uladzislau Rezki <urezki@gmail.com>]

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.

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.

--
Uladzislau Rezki