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

Joel Fernandes
2023-03-14T22:45:15Z

On Mon, Mar 13, 2023 at 11:32 AM Uladzislau Rezki <urezki@gmail.com> wrote:
>
> On Mon, Mar 13, 2023 at 06:58:30AM -0700, Joel Fernandes wrote:
> >
> >
> > > On Mar 13, 2023, at 2:51 AM, Uladzislau Rezki <urezki@gmail.com> wrote:
> > >
> > > ﻿On Fri, Mar 10, 2023 at 10:24:34PM -0800, Paul E. McKenney 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.
> > >>
> > > I hove done a small and quick prototype:
> > >
> > > <snip>
> > > diff --git a/include/linux/rcupdate_wait.h b/include/linux/rcupdate_wait.h
> > > index 699b938358bf..e1a4cca9a208 100644
> > > --- a/include/linux/rcupdate_wait.h
> > > +++ b/include/linux/rcupdate_wait.h
> > > @@ -9,6 +9,8 @@
> > > #include <linux/rcupdate.h>
> > > #include <linux/completion.h>
> > >
> > > +extern struct llist_head gp_wait_llist;
> > > +
> > > /*
> > > * Structure allowing asynchronous waiting on RCU.
> > > */
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index ee27a03d7576..50b81ca54104 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -113,6 +113,9 @@ int rcu_num_lvls __read_mostly = RCU_NUM_LVLS;
> > > int num_rcu_lvl[] = NUM_RCU_LVL_INIT;
> > > int rcu_num_nodes __read_mostly = NUM_RCU_NODES; /* Total # rcu_nodes in use. */
> > >
> > > +/* Waiters for a GP kthread. */
> > > +LLIST_HEAD(gp_wait_llist);
> > > +
> > > /*
> > > * The rcu_scheduler_active variable is initialized to the value
> > > * RCU_SCHEDULER_INACTIVE and transitions RCU_SCHEDULER_INIT just before the
> > > @@ -1776,6 +1779,14 @@ static noinline void rcu_gp_cleanup(void)
> > > on_each_cpu(rcu_strict_gp_boundary, NULL, 0);
> > > }
> > >
> > > +static void rcu_notify_gp_end(struct llist_node *llist)
> > > +{
> > > + struct llist_node *rcu, *next;
> > > +
> > > + llist_for_each_safe(rcu, next, llist)
> > > + complete(&((struct rcu_synchronize *) rcu)->completion);
> >
> > This looks broken to me, so the synchronize will complete even
> > if it was called in the middle of an ongoing GP?
> >
> Do you mean before replacing the list(and after rcu_gp_cleanup()) a new
> GP sequence can be initiated?

It looks interesting, I am happy to try it on ChromeOS once you
provide a patch, in case it improves something, even if that is
suspend or boot time.

I think the main concern I had was if you did not wait for a full
grace period (which as you indicated, you would fix), you are not
really measuring the long delays that the full grace period can cause
so IMHO it is important to only measure once correctness is preserved
by the modification. To that end, perhaps having rcutorture pass with
your modification could be a vote of confidence before proceeding to
performance tests.

- Joel