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Date:   Fri, 3 Nov 2023 11:06:26 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Z qiang <qiang.zhang1211@gmail.com>
Cc:     frederic@kernel.org, joel@joelfernandes.org, boqun.feng@gmail.com,
        rcu@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] rcu: Force quiescent states only for ongoing grace period
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On Fri, Nov 03, 2023 at 03:14:11PM +0800, Z qiang wrote:
> >
> > On Wed, Nov 01, 2023 at 11:35:07AM +0800, Zqiang wrote:
> > > Currently, when running the rcutorture testing, if the fqs_task
> > > kthread was created, the periodic fqs operations will be performed,
> > > regardless of whether the grace-period is ongoing. however, if there
> > > is no ongoing grace-period, invoke the rcu_force_quiescent_state() has
> > > no effect, because when the new grace-period starting, will clear all
> > > flags int rcu_state.gp_flags in rcu_gp_init(). this commit therefore add
> > > rcu_gp_in_progress() check in rcu_force_quiescent_state(), if there is
> > > no ongoing grace-period, return directly.
> > >
> > > Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
> >
> > Nice optimization, but one question below.
> >
> >                                                 Thanx, Paul
> >
> > > ---
> > >  kernel/rcu/tree.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > >
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index aa4c808978b8..5b4279ef66da 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -2338,6 +2338,8 @@ void rcu_force_quiescent_state(void)
> > >       struct rcu_node *rnp;
> > >       struct rcu_node *rnp_old = NULL;
> > >
> > > +     if (!rcu_gp_in_progress())
> > > +             return;
> >
> > Suppose that the grace period that was in progress above ends right
> > at this point in the code.  We will still do the useless grace
> > forcing of quiescent states.  Which means that this code path
> > does need to be tested.
> >
> > So, when you run rcutorture with this change, how often has the
> > grace period ended before this function returns?  If that happens
> > reasonably often, say more than once per minute or so, then this
> > works nicely.  If not, we do need to do something to make sure
> > that that code path gets tested.
> >
> > Thoughts?
> 
> Thanks for the suggestion, I will add some debug information to test again.

Very good, and I look forward to seeing what you come up with!

							Thanx, Paul

> Thanks
> Zqiang
> 
> >
> > >       /* Funnel through hierarchy to reduce memory contention. */
> > >       rnp = raw_cpu_read(rcu_data.mynode);
> > >       for (; rnp != NULL; rnp = rnp->parent) {
> > > --
> > > 2.17.1
> > >