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

[Uladzislau Rezki <urezki@gmail.com>]

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.

--
Vlad Rezki