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From:   Eric Dumazet <edumazet@google.com>
Date:   Wed, 30 Nov 2022 19:39:02 +0100
Message-ID: <CANn89iKifFXDpF8sZXd+rXPhF+3ajVLTuEj6n2Z4H9f27_K0kA@mail.gmail.com>
Subject: Re: [PATCH rcu 15/16] net: Use call_rcu_hurry() for dst_release()
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     "Paul E. McKenney" <paulmck@kernel.org>, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel-team@meta.com,
        rostedt@goodmis.org, David Ahern <dsahern@kernel.org>,
        "David S. Miller" <davem@davemloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>, netdev@vger.kernel.org
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Sure, thanks.

Reviewed-by: Eric Dumazet <edumazet@google.com>

I think we can work later to change how dst are freed/released to
avoid using call_rcu_hurry()

On Wed, Nov 30, 2022 at 7:17 PM Joel Fernandes <joel@joelfernandes.org> wrote:
>
> Hi Eric,
>
> Could you give your ACK for this patch for this one as well? This is
> the other networking one.
>
> 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 6:14 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> >
> > From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
> >
> > In a networking test on ChromeOS, kernels built with the new
> > CONFIG_RCU_LAZY=y Kconfig option fail a networking test in the teardown
> > phase.
> >
> > This failure may be reproduced as follows: ip netns del <name>
> >
> > The CONFIG_RCU_LAZY=y Kconfig option was introduced by earlier commits
> > in this series for the benefit of certain battery-powered systems.
> > 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.
> >
> > Returning to the test failure, use of ftrace showed that this failure
> > cause caused by the aadded delays due to this new lazy behavior of
> > call_rcu() in kernels built with CONFIG_RCU_LAZY=y.
> >
> > Therefore, make dst_release() use call_rcu_hurry() in order to revert
> > to the old test-failure-free behavior.
> >
> > [ paulmck: Apply s/call_rcu_flush/call_rcu_hurry/ feedback from Tejun Heo. ]
> >
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > Cc: David Ahern <dsahern@kernel.org>
> > Cc: "David S. Miller" <davem@davemloft.net>
> > Cc: Eric Dumazet <edumazet@google.com>
> > Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
> > Cc: Jakub Kicinski <kuba@kernel.org>
> > Cc: Paolo Abeni <pabeni@redhat.com>
> > Cc: <netdev@vger.kernel.org>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > ---
> >  net/core/dst.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/net/core/dst.c b/net/core/dst.c
> > index bc9c9be4e0801..a4e738d321ba2 100644
> > --- a/net/core/dst.c
> > +++ b/net/core/dst.c
> > @@ -174,7 +174,7 @@ void dst_release(struct dst_entry *dst)
> >                         net_warn_ratelimited("%s: dst:%p refcnt:%d\n",
> >                                              __func__, dst, newrefcnt);
> >                 if (!newrefcnt)
> > -                       call_rcu(&dst->rcu_head, dst_destroy_rcu);
> > +                       call_rcu_hurry(&dst->rcu_head, dst_destroy_rcu);
> >         }
> >  }
> >  EXPORT_SYMBOL(dst_release);
> > --
> > 2.31.1.189.g2e36527f23
> >