Re: [PATCH net-next] openvswitch: reduce stack usage in do_execute_actions

[Eric Dumazet]

On Thu, Sep 21, 2023 at 9:03 PM Ilya Maximets <i.maximets@xxxxxxx> wrote:
>
> do_execute_actions() function can be called recursively multiple
> times while executing actions that require pipeline forking or
> recirculations.  It may also be re-entered multiple times if the packet
> leaves openvswitch module and re-enters it through a different port.
>
> Currently, there is a 256-byte array allocated on stack in this
> function that is supposed to hold NSH header.  Compilers tend to
> pre-allocate that space right at the beginning of the function:
>
>      a88:       48 81 ec b0 01 00 00    sub    $0x1b0,%rsp
>
> NSH is not a very common protocol, but the space is allocated on every
> recursive call or re-entry multiplying the wasted stack space.
>
> Move the stack allocation to push_nsh() function that is only used
> if NSH actions are actually present.  push_nsh() is also a simple
> function without a possibility for re-entry, so the stack is returned
> right away.
>
> With this change the preallocated space is reduced by 256 B per call:
>
>      b18:       48 81 ec b0 00 00 00    sub    $0xb0,%rsp
>
> Signed-off-by: Ilya Maximets <i.maximets@xxxxxxx>
> ---
>  net/openvswitch/actions.c | 20 +++++++++-----------
>  1 file changed, 9 insertions(+), 11 deletions(-)
>
> diff --git a/net/openvswitch/actions.c b/net/openvswitch/actions.c
> index 5f8094acd056..80cc5c512d7b 100644
> --- a/net/openvswitch/actions.c
> +++ b/net/openvswitch/actions.c
> @@ -312,10 +312,16 @@ static int push_eth(struct sk_buff *skb, struct sw_flow_key *key,
>  }
>
>  static int push_nsh(struct sk_buff *skb, struct sw_flow_key *key,
> -                   const struct nshhdr *nh)
> +                   const struct nlattr *a)

Presumably this function should be inlined. (one caller only)

I would add noinline_for_stack to make sure the compiler will not play
games with this attempt.

>  {
> +       u8 buffer[NSH_HDR_MAX_LEN];
> +       struct nshhdr *nh = (struct nshhdr *)buffer;
>         int err;
>
> +       err = nsh_hdr_from_nlattr(a, nh, NSH_HDR_MAX_LEN);
> +       if (err)
> +               return err;
> +
>         err = nsh_push(skb, nh);
>         if (err)
>                 return err;
> @@ -1439,17 +1445,9 @@ static int do_execute_actions(struct datapath *dp, struct sk_buff *skb,
>                         err = pop_eth(skb, key);
>                         break;
>
> -               case OVS_ACTION_ATTR_PUSH_NSH: {
> -                       u8 buffer[NSH_HDR_MAX_LEN];
> -                       struct nshhdr *nh = (struct nshhdr *)buffer;
> -
> -                       err = nsh_hdr_from_nlattr(nla_data(a), nh,
> -                                                 NSH_HDR_MAX_LEN);
> -                       if (unlikely(err))
> -                               break;
> -                       err = push_nsh(skb, key, nh);
> +               case OVS_ACTION_ATTR_PUSH_NSH:
> +                       err = push_nsh(skb, key, nla_data(a));
>                         break;
> -               }
>
>                 case OVS_ACTION_ATTR_POP_NSH:
>                         err = pop_nsh(skb, key);
> --
> 2.41.0
>