From: "Paul E. McKenney" <paulmck@kernel.org>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>, rcu <rcu@vger.kernel.org>,
Uladzislau Rezki <urezki@gmail.com>,
Neeraj Upadhyay <quic_neeraju@quicinc.com>,
Boqun Feng <boqun.feng@gmail.com>,
Joel Fernandes <joel@joelfernandes.org>
Subject: Re: [PATCH 1/4] rcu/nocb: Protect lazy shrinker against concurrent (de-)offloading
Date: Wed, 29 Mar 2023 13:45:54 -0700 [thread overview]
Message-ID: <15ba9151-16ad-4c23-a236-918a061bd893@paulmck-laptop> (raw)
In-Reply-To: <ZCRiXrQQRNy2aJAS@lothringen>
On Wed, Mar 29, 2023 at 06:07:58PM +0200, Frederic Weisbecker wrote:
> On Sun, Mar 26, 2023 at 02:45:18PM -0700, Paul E. McKenney wrote:
> > On Sun, Mar 26, 2023 at 10:01:34PM +0200, Frederic Weisbecker wrote:
> > > > > > > /* Snapshot count of all CPUs */
> > > > > > > for_each_possible_cpu(cpu) {
> > > > > > > struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> > > > > > > - int _count = READ_ONCE(rdp->lazy_len);
> > > > > > > + int _count;
> > > > > > > +
> > > > > > > + if (!rcu_rdp_is_offloaded(rdp))
> > > > > > > + continue;
> > > > > >
> > > > > > If the CPU is offloaded, isn't ->lazy_len guaranteed to be zero?
> > > > > >
> > > > > > Or can it contain garbage after a de-offloading operation?
> > > > >
> > > > > If it's deoffloaded, ->lazy_len is indeed (supposed to be) guaranteed to be zero.
> > > > > Bypass is flushed and disabled atomically early on de-offloading and the
> > > > > flush resets ->lazy_len.
> > > >
> > > > Whew! At the moment, I don't feel strongly about whether or not
> > > > the following code should (1) read the value, (2) warn on non-zero,
> > > > (3) assume zero without reading, or (4) some other option that is not
> > > > occurring to me. Your choice!
> > >
> > > (2) looks like a good idea!
> >
> > Sounds good to me!
>
> So since we now iterate rcu_nocb_mask after the patchset, there is no more
> deoffloaded rdp to check. Meanwhile I put a WARN in the new series making
> sure that an rdp in rcu_nocb_mask is also offloaded (heh!)
Sounds good, thank you!
Thanx, Paul
next prev parent reply other threads:[~2023-03-29 20:45 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-22 19:44 [PATCH 0/4] rcu/nocb: Shrinker related boring fixes Frederic Weisbecker
2023-03-22 19:44 ` [PATCH 1/4] rcu/nocb: Protect lazy shrinker against concurrent (de-)offloading Frederic Weisbecker
2023-03-22 23:18 ` Paul E. McKenney
2023-03-24 0:55 ` Joel Fernandes
2023-03-24 1:06 ` Paul E. McKenney
2023-03-24 22:09 ` Frederic Weisbecker
2023-03-24 22:51 ` Paul E. McKenney
2023-03-26 20:01 ` Frederic Weisbecker
2023-03-26 21:45 ` Paul E. McKenney
2023-03-29 16:07 ` Frederic Weisbecker
2023-03-29 20:45 ` Paul E. McKenney [this message]
2023-03-22 19:44 ` [PATCH 2/4] rcu/nocb: Fix shrinker race against callback enqueuer Frederic Weisbecker
2023-03-22 23:19 ` Paul E. McKenney
2023-03-22 19:44 ` [PATCH 3/4] rcu/nocb: Recheck lazy callbacks under the ->nocb_lock from shrinker Frederic Weisbecker
2023-03-22 23:21 ` Paul E. McKenney
2023-03-22 19:44 ` [PATCH 4/4] rcu/nocb: Make shrinker to iterate only NOCB CPUs Frederic Weisbecker
2023-03-24 0:41 ` Joel Fernandes
2023-03-29 16:01 [PATCH 0/4 v2] rcu/nocb: Shrinker related boring fixes Frederic Weisbecker
2023-03-29 16:02 ` [PATCH 1/4] rcu/nocb: Protect lazy shrinker against concurrent (de-)offloading Frederic Weisbecker
2023-03-29 20:44 ` Paul E. McKenney
2023-03-29 21:18 ` Frederic Weisbecker
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