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Date:   Wed, 12 Oct 2022 12:23:58 +0200
From:   Frederic Weisbecker <frederic@kernel.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     "Paul E . McKenney" <paulmck@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/2] rcu/nocb: Spare bypass locking upon normal enqueue
Message-ID: <20221012102358.GA1074896@lothringen>
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On Tue, Oct 11, 2022 at 07:47:07PM -0400, Joel Fernandes wrote:
> On Tue, Oct 11, 2022 at 3:21 PM Frederic Weisbecker <frederic@kernel.org> wrote:
> >
> > On Tue, Oct 11, 2022 at 02:00:40AM +0000, Joel Fernandes wrote:
> > > On Tue, Oct 11, 2022 at 12:39:56AM +0200, Frederic Weisbecker wrote:
> > > > When a callback is to be enqueued to the normal queue and not the bypass
> > > > one, a flush to the bypass queue is always tried anyway. This attempt
> > > > involves locking the bypass lock unconditionally. Although it is
> > > > guaranteed not to be contended at this point, because only call_rcu()
> > > > can lock the bypass lock without holding the nocb lock, it's still not
> > > > free and the operation can easily be spared most of the time by just
> > > > checking if the bypass list is empty. The check is safe as nobody can
> > > > queue nor flush the bypass concurrently.
> > > >
> > > > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> > > > ---
> > > >  kernel/rcu/tree_nocb.h | 6 ++++--
> > > >  1 file changed, 4 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > > > index 094fd454b6c3..30c3d473ffd8 100644
> > > > --- a/kernel/rcu/tree_nocb.h
> > > > +++ b/kernel/rcu/tree_nocb.h
> > > > @@ -423,8 +423,10 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> > > >             if (*was_alldone)
> > > >                     trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
> > > >                                         TPS("FirstQ"));
> > > > -           WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, j));
> > > > -           WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
> > > > +           if (rcu_cblist_n_cbs(&rdp->nocb_bypass)) {
> > > > +                   WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, j));
> > > > +                   WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
> > > > +           }
> > > >             return false; // Caller must enqueue the callback.
> > > >     }
> > >
> > > Instead of this, since as you mentioned that the bypass lock is not contended
> > > in this path, isn't it unnecessary to even check or attempt to acquire the
> > > lock in call_rcu() path? So how about something like the following, or would
> > > this not work for some reason?
> >
> > You're right. But it's a bit error prone and it adds quite some code complication
> > just for a gain on a rare event (bypass is supposed to be flushed on rare
> > occasions by the caller).
> 
> But the "checking of whether to flush" which leads to "acquiring the
> bypass lock first" , is not a rare event as you pointed out (can be
> spared most of the time as you said). The alternative I proposed
> removes the need for the frequent locking (which is another way of
> implementing what you suggested).

It's not rare as a whole but this quick-check patch addresses the fast path.
What you propose is to extend the API to also cover the other flushes in
rcu_nocb_try_bypass() that are slower path.

I think this makes the API more error prone (users may get it easily wrong)
and complicated for tiny, if measurable, gains.