netdev - Re: [PATCH net-next] gro_cells: avoid using synchronize_rcu() in gro_cells

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Date:   Sat, 19 Feb 2022 20:06:32 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] gro_cells: avoid using synchronize_rcu() in gro_cells_destroy()

On Sat, Feb 19, 2022 at 7:57 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
>
> Another thing making netns dismantles potentially very slow is located
> in gro_cells_destroy(),
> whenever cleanup_net() has to remove a device using gro_cells framework.
>
> RTNL is not held at this stage, so synchronize_net()
> is calling synchronize_rcu():
>
> netdev_run_todo()
>  ip_tunnel_dev_free()
>   gro_cells_destroy()
>    synchronize_net()
>     synchronize_rcu() // Ouch.
>
> This patch uses call_rcu(), and gave me a 25x performance improvement
> in my tests.
>
> cleanup_net() is no longer blocked ~10 ms per synchronize_rcu()
> call.
>
> In the case we could not allocate the memory needed to queue the
> deferred free, use synchronize_rcu_expedited()
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  net/core/gro_cells.c | 36 +++++++++++++++++++++++++++++++-----
>  1 file changed, 31 insertions(+), 5 deletions(-)
>
> diff --git a/net/core/gro_cells.c b/net/core/gro_cells.c
> index 6eb2e5ec2c5068e1d798557e55d084b785187a9b..46fa7d93fd9696755efd56b72731f08e821042e1 100644
> --- a/net/core/gro_cells.c
> +++ b/net/core/gro_cells.c
> @@ -89,8 +89,23 @@ int gro_cells_init(struct gro_cells *gcells, struct net_device *dev)
>  }
>  EXPORT_SYMBOL(gro_cells_init);
>
> +struct percpu_free_defer {
> +       struct rcu_head rcu;
> +       void __percpu   *ptr;
> +};
> +
> +void percpu_free_defer_callback(struct rcu_head *head)

This will get a static in v2

> +{
> +       struct percpu_free_defer *defer;
> +
> +       defer = container_of(head, struct percpu_free_defer, rcu);
> +       free_percpu(defer->ptr);
> +       kfree(defer);
> +}
> +
>  void gro_cells_destroy(struct gro_cells *gcells)
>  {
> +       struct percpu_free_defer *defer;
>         int i;
>
>         if (!gcells->cells)
> @@ -102,12 +117,23 @@ void gro_cells_destroy(struct gro_cells *gcells)
>                 __netif_napi_del(&cell->napi);
>                 __skb_queue_purge(&cell->napi_skbs);
>         }
> -       /* This barrier is needed because netpoll could access dev->napi_list
> -        * under rcu protection.
> +       /* We need to observe an rcu grace period before freeing ->cells,
> +        * because netpoll could access dev->napi_list under rcu protection.
> +        * Try hard using call_rcu() instead of synchronize_rcu(),
> +        * because we might be called from cleanup_net(), and we
> +        * definitely do not want to block this critical task.
>          */
> -       synchronize_net();
> -
> -       free_percpu(gcells->cells);
> +       defer = kmalloc(sizeof(*defer), GFP_KERNEL | __GFP_NOWARN);
> +       if (likely(defer)) {
> +               defer->ptr = gcells->cells;
> +               call_rcu(&defer->rcu, percpu_free_defer_callback);
> +       } else {
> +               /* We do not hold RTNL at this point, synchronize_net()
> +                * would not be able to expedite this sync.
> +               */

I also will fix the comment alignment.

> +               synchronize_rcu_expedited();
> +               free_percpu(gcells->cells);
> +       }
>         gcells->cells = NULL;
>  }
>  EXPORT_SYMBOL(gro_cells_destroy);
> --
> 2.35.1.473.g83b2b277ed-goog
>