linux-kernel - Re: [PATCH rcu 14/16] rxrpc: Use call_rcu_hurry() instead of call

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Date:   Wed, 30 Nov 2022 13:45:52 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...a.com,
        rostedt@...dmis.org, David Howells <dhowells@...hat.com>,
        Marc Dionne <marc.dionne@...istor.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, linux-afs@...ts.infradead.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH rcu 14/16] rxrpc: Use call_rcu_hurry() instead of
 call_rcu()

On Wed, Nov 30, 2022 at 07:37:07PM +0100, Eric Dumazet wrote:
> Ah, I see a slightly better name has been chosen ;)

call_rcu_vite()?  call_rcu_tres_grande_vitesse()?  call_rcu_tgv()?

Sorry, couldn't resist!  ;-)

							Thanx, Paul

> Reviewed-by: Eric Dumazet <edumazet@...gle.com>
> 
> On Wed, Nov 30, 2022 at 7:16 PM Joel Fernandes <joel@...lfernandes.org> wrote:
> >
> > Hi Eric,
> >
> > Could you give your ACK for this patch?
> >
> > The networking testing passed on ChromeOS and it has been in -next for
> > some time so has gotten testing there. The CONFIG option is default
> > disabled.
> >
> > Thanks a lot,
> >
> > - Joel
> >
> > On Wed, Nov 30, 2022 at 6:13 PM Paul E. McKenney <paulmck@...nel.org> wrote:
> > >
> > > From: "Joel Fernandes (Google)" <joel@...lfernandes.org>
> > >
> > > Earlier commits in this series allow battery-powered systems to build
> > > their kernels with the default-disabled CONFIG_RCU_LAZY=y Kconfig option.
> > > This Kconfig option causes call_rcu() to delay its callbacks in order
> > > to batch them.  This means that a given RCU grace period covers more
> > > callbacks, thus reducing the number of grace periods, in turn reducing
> > > the amount of energy consumed, which increases battery lifetime which
> > > can be a very good thing.  This is not a subtle effect: In some important
> > > use cases, the battery lifetime is increased by more than 10%.
> > >
> > > This CONFIG_RCU_LAZY=y option is available only for CPUs that offload
> > > callbacks, for example, CPUs mentioned in the rcu_nocbs kernel boot
> > > parameter passed to kernels built with CONFIG_RCU_NOCB_CPU=y.
> > >
> > > Delaying callbacks is normally not a problem because most callbacks do
> > > nothing but free memory.  If the system is short on memory, a shrinker
> > > will kick all currently queued lazy callbacks out of their laziness,
> > > thus freeing their memory in short order.  Similarly, the rcu_barrier()
> > > function, which blocks until all currently queued callbacks are invoked,
> > > will also kick lazy callbacks, thus enabling rcu_barrier() to complete
> > > in a timely manner.
> > >
> > > However, there are some cases where laziness is not a good option.
> > > For example, synchronize_rcu() invokes call_rcu(), and blocks until
> > > the newly queued callback is invoked.  It would not be a good for
> > > synchronize_rcu() to block for ten seconds, even on an idle system.
> > > Therefore, synchronize_rcu() invokes call_rcu_hurry() instead of
> > > call_rcu().  The arrival of a non-lazy call_rcu_hurry() callback on a
> > > given CPU kicks any lazy callbacks that might be already queued on that
> > > CPU.  After all, if there is going to be a grace period, all callbacks
> > > might as well get full benefit from it.
> > >
> > > Yes, this could be done the other way around by creating a
> > > call_rcu_lazy(), but earlier experience with this approach and
> > > feedback at the 2022 Linux Plumbers Conference shifted the approach
> > > to call_rcu() being lazy with call_rcu_hurry() for the few places
> > > where laziness is inappropriate.
> > >
> > > And another call_rcu() instance that cannot be lazy is the one
> > > in rxrpc_kill_connection(), which sometimes does a wakeup
> > > that should not be unduly delayed.
> > >
> > > Therefore, make rxrpc_kill_connection() use call_rcu_hurry() in order
> > > to revert to the old behavior.
> > >
> > > [ paulmck: Apply s/call_rcu_flush/call_rcu_hurry/ feedback from Tejun Heo. ]
> > >
> > > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > > Cc: David Howells <dhowells@...hat.com>
> > > Cc: Marc Dionne <marc.dionne@...istor.com>
> > > Cc: "David S. Miller" <davem@...emloft.net>
> > > Cc: Eric Dumazet <edumazet@...gle.com>
> > > Cc: Jakub Kicinski <kuba@...nel.org>
> > > Cc: Paolo Abeni <pabeni@...hat.com>
> > > Cc: <linux-afs@...ts.infradead.org>
> > > Cc: <netdev@...r.kernel.org>
> > > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > > ---
> > >  net/rxrpc/conn_object.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/net/rxrpc/conn_object.c b/net/rxrpc/conn_object.c
> > > index 22089e37e97f0..9c5fae9ca106c 100644
> > > --- a/net/rxrpc/conn_object.c
> > > +++ b/net/rxrpc/conn_object.c
> > > @@ -253,7 +253,7 @@ void rxrpc_kill_connection(struct rxrpc_connection *conn)
> > >          * must carry a ref on the connection to prevent us getting here whilst
> > >          * it is queued or running.
> > >          */
> > > -       call_rcu(&conn->rcu, rxrpc_destroy_connection);
> > > +       call_rcu_hurry(&conn->rcu, rxrpc_destroy_connection);
> > >  }
> > >
> > >  /*
> > > --
> > > 2.31.1.189.g2e36527f23
> > >