Re: [PATCH tip/core/rcu 03/17] rcu/tree: Skip entry into the page allocator for PREEMPT_RT

From: Uladzislau Rezki <urezki@gmail.com>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: paulmck@kernel.org, joel@joelfernandes.org, rcu@vger.kernel.org,
	linux-kernel@vger.kernel.org, kernel-team@fb.com,
	mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com,
	akpm@linux-foundation.org, mathieu.desnoyers@efficios.com,
	josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org,
	rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com,
	fweisbec@gmail.com, oleg@redhat.com,
	Uladzislau Rezki <urezki@gmail.com>
Subject: Re: [PATCH tip/core/rcu 03/17] rcu/tree: Skip entry into the page allocator for PREEMPT_RT
Date: Wed, 15 Jul 2020 15:38:08 +0200	[thread overview]
Message-ID: <20200715133808.GA24778@pc636> (raw)
In-Reply-To: <20200630164543.4mdcf6zb4zfclhln@linutronix.de>

On Tue, Jun 30, 2020 at 06:45:43PM +0200, Sebastian Andrzej Siewior wrote:
> On 2020-06-24 13:12:12 [-0700], paulmck@kernel.org wrote:
> > From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
> > 
> > To keep the kfree_rcu() code working in purely atomic sections on RT,
> > such as non-threaded IRQ handlers and raw spinlock sections, avoid
> > calling into the page allocator which uses sleeping locks on RT.
> > 
> > In fact, even if the  caller is preemptible, the kfree_rcu() code is
> > not, as the krcp->lock is a raw spinlock.
> > 
> > Calling into the page allocator is optional and avoiding it should be
> > Ok, especially with the page pre-allocation support in future patches.
> > Such pre-allocation would further avoid the a need for a dynamically
> > allocated page in the first place.
> > 
> > Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > Reviewed-by: Uladzislau Rezki <urezki@gmail.com>
> > 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>
> > Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> > Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> > ---
> >  kernel/rcu/tree.c | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 64592b4..dbdd509 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3184,6 +3184,18 @@ 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;
> 
> This is not going to work together with the "wait context validator"
> (CONFIG_PROVE_RAW_LOCK_NESTING).

>
> As of -rc3 it should complain about printk() which is why it is still disabled by default.
>
Have you tried to trigger a "complain" you are talking about?
I suspect to get some trace dump when CONFIG_PROVE_RAW_LOCK_NESTING=y.

Thank you.

--
Vlad Rezki