linux-kernel - Re: [PATCH 1/1] rcu/tree: add emergency pool for headless case

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Date:   Fri, 3 Apr 2020 12:14:19 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        RCU <rcu@...r.kernel.org>, linux-mm@...ck.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>
Subject: Re: [PATCH 1/1] rcu/tree: add emergency pool for headless case

On Fri, Apr 03, 2020 at 07:30:51PM +0200, Uladzislau Rezki (Sony) wrote:
> Maintain an emergency pool for each CPU with some
> extra objects. There is read-only sysfs attribute,
> the name is "rcu_nr_emergency_objs". It reflects
> the size of the pool. As for now the default value
> is 3.
> 
> The pool is populated when low memory condition is
> detected. Please note it is only for headless case
> it means when the regular SLAB is not able to serve
> any request, the pool is used.
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> ---
>  kernel/rcu/tree.c | 133 +++++++++++++++++++++++++++++++++-------------
>  1 file changed, 97 insertions(+), 36 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 5e26145e9ead..f9f1f935ab0b 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -114,6 +114,14 @@ int rcu_num_lvls __read_mostly = RCU_NUM_LVLS;
>  int rcu_kfree_nowarn;
>  module_param(rcu_kfree_nowarn, int, 0444);
>  
> +/*
> + * For headless variant. Under memory pressure an
> + * emergency pool can be used if the regular SLAB
> + * is not able to serve some memory for us.
> + */
> +int rcu_nr_emergency_objs = 3;
> +module_param(rcu_nr_emergency_objs, int, 0444);

Please document this in Documentation/admin-guide/kernel-parameters.txt.

							Thanx, Paul