linux-kernel - Re: [PATCH rcu/dev -fixes 2/4] rcu/tree: Skip entry into the page allocator for PREEMPT

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Date:   Wed, 22 Apr 2020 06:28:53 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     joel@...lfernandes.org
Cc:     Uladzislau Rezki <urezki@...il.com>, linux-kernel@...r.kernel.org,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH rcu/dev -fixes 2/4] rcu/tree: Skip entry into the page
 allocator for PREEMPT_RT

On Wed, Apr 22, 2020 at 09:18:41AM -0400, joel@...lfernandes.org wrote:
> 
> 
> On April 22, 2020 6:35:36 AM EDT, Uladzislau Rezki <urezki@...il.com> 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@...utronix.de>
> >> Co-developed-by: Uladzislau Rezki <urezki@...il.com>
> >> Signed-off-by: Uladzislau Rezki <urezki@...il.com>
> >> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.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.
> 
> I discussed with Vlad offline, this patch is fine for mainline as we
> don't have headless support yet. So this patch is good. Future patches
> adding caching will also add the headless support after additional
> caching, so skipping the allocation here is ok.

Sounds good!

But would one of the realtime guys be willing to give a Tested-by?

							Thanx, Paul

> Thanks.
> 
> - Joel
> 
> 
> 
> 
> >--
> >Vlad Rezki
> 
> -- 
> Sent from my Android device with K-9 Mail. Please excuse my brevity.