linux-kernel - Re: [PATCH 2/2] rcu/nocb: Spare bypass locking upon normal enqueue

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Date:   Tue, 11 Oct 2022 21:21:50 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Paul E . McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] rcu/nocb: Spare bypass locking upon normal enqueue

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@...nel.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).

Thanks.