From: Uladzislau Rezki <urezki@gmail.com>
To: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc: "Joel Fernandes (Google)" <joel@joelfernandes.org>,
linux-kernel@vger.kernel.org,
Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
Josh Triplett <josh@joshtriplett.org>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
"Paul E. McKenney" <paulmck@kernel.org>,
rcu@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH rcu/dev -fixes 2/4] rcu/tree: Skip entry into the page allocator for PREEMPT_RT
Date: Wed, 22 Apr 2020 13:45:06 +0200 [thread overview]
Message-ID: <20200422114506.GA17108@pc636> (raw)
In-Reply-To: <20200422103536.GA16263@pc636>
On Wed, Apr 22, 2020 at 12:35:36PM +0200, Uladzislau Rezki wrote:
> On Mon, Apr 20, 2020 at 11:38:35AM -0400, Joel Fernandes (Google) wrote:
> > To keep kfree_rcu() path working on raw non-preemptible sections,
> > prevent the optional entry into the allocator as it uses sleeping locks.
> > In fact, even if the caller of kfree_rcu() is preemptible, this path
> > still is not, as krcp->lock is a raw spinlock as done in previous
> > patches. With additional page pre-allocation in the works, hitting this
> > return is going to be much less likely soon so just prevent it for now
> > so that PREEMPT_RT does not break. Note that page allocation here is an
> > optimization and skipping it still makes kfree_rcu() work.
> >
> > Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > Co-developed-by: Uladzislau Rezki <urezki@gmail.com>
> > Signed-off-by: Uladzislau Rezki <urezki@gmail.com>
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > ---
> > kernel/rcu/tree.c | 12 ++++++++++++
> > 1 file changed, 12 insertions(+)
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index cf68d3d9f5b81..cd61649e1b001 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3092,6 +3092,18 @@ kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp,
> > if (!bnode) {
> > WARN_ON_ONCE(sizeof(struct kfree_rcu_bulk_data) > PAGE_SIZE);
> >
> > + /*
> > + * To keep this path working on raw non-preemptible
> > + * sections, prevent the optional entry into the
> > + * allocator as it uses sleeping locks. In fact, even
> > + * if the caller of kfree_rcu() is preemptible, this
> > + * path still is not, as krcp->lock is a raw spinlock.
> > + * With additional page pre-allocation in the works,
> > + * hitting this return is going to be much less likely.
> > + */
> > + if (IS_ENABLED(CONFIG_PREEMPT_RT))
> > + return false;
> > +
> > bnode = (struct kfree_rcu_bulk_data *)
> > __get_free_page(GFP_NOWAIT | __GFP_NOWARN);
> > }
> This will not be correctly working by just reverting everything to the
> "rcu_head path" for CONFIG_PREEMPT_RT kernel. We need to preallocate at
> least once. I can add caching on top of this series to address it.
>
Forgot to add: Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Caching should be added as separate patch.
--
Vlad Rezki
next prev parent reply other threads:[~2020-04-22 11:45 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-20 15:38 [PATCH rcu/dev -fixes 0/4] Joel Fernandes (Google)
2020-04-20 15:38 ` [PATCH rcu/dev -fixes 1/4] rcu/tree: Keep kfree_rcu() awake during lock contention Joel Fernandes (Google)
2020-04-21 13:12 ` Uladzislau Rezki
2020-04-20 15:38 ` [PATCH rcu/dev -fixes 2/4] rcu/tree: Skip entry into the page allocator for PREEMPT_RT Joel Fernandes (Google)
2020-04-22 10:35 ` Uladzislau Rezki
2020-04-22 11:45 ` Uladzislau Rezki [this message]
2020-04-22 13:18 ` joel
2020-04-22 13:28 ` Paul E. McKenney
2020-04-20 15:38 ` [PATCH rcu/dev -fixes 3/4] rcu/tree: Avoid using xchg() in kfree_call_rcu_add_ptr_to_bulk() Joel Fernandes (Google)
2020-04-20 17:18 ` Uladzislau Rezki
2020-04-20 18:19 ` Joel Fernandes
2020-04-20 15:38 ` [PATCH rcu/dev -fixes 4/4] rcu/tree: Use consistent style for comments Joel Fernandes (Google)
2020-04-21 13:08 ` Uladzislau Rezki
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