netdev - Re: [PATCH net] net: gro: do not keep too many GRO packets in napi->rx

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Date:   Thu, 04 Feb 2021 14:14:51 -0800
From:   Saeed Mahameed <saeed@...nel.org>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        John Sperbeck <jsperbeck@...gle.com>,
        Jian Yang <jianyang@...gle.com>,
        Maxim Mikityanskiy <maximmi@...lanox.com>,
        Alexander Lobakin <alobakin@...nk.ru>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Edward Cree <ecree@...arflare.com>
Subject: Re: [PATCH net] net: gro: do not keep too many GRO packets in
 napi->rx_list

On Thu, 2021-02-04 at 13:31 -0800, Eric Dumazet wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> 
> Commit c80794323e82 ("net: Fix packet reordering caused by GRO and
> listified RX cooperation") had the unfortunate effect of adding
> latencies in common workloads.
> 
> Before the patch, GRO packets were immediately passed to
> upper stacks.
> 
> After the patch, we can accumulate quite a lot of GRO
> packets (depdending on NAPI budget).
> 

Why napi budget ? looking at the code it seems to be more related to
MAX_GRO_SKBS * gro_normal_batch, since we are counting GRO SKBs as 1

but maybe i am missing some information about the actual issue you are
hitting.

> My fix is counting in napi->rx_count number of segments
> instead of number of logical packets.
> 
> Fixes: c80794323e82 ("net: Fix packet reordering caused by GRO and
> listified RX cooperation")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Bisected-by: John Sperbeck <jsperbeck@...gle.com>
> Tested-by: Jian Yang <jianyang@...gle.com>
> Cc: Maxim Mikityanskiy <maximmi@...lanox.com>
> Cc: Alexander Lobakin <alobakin@...nk.ru>
> Cc: Saeed Mahameed <saeedm@...lanox.com>
> Cc: Edward Cree <ecree@...arflare.com>
> ---
>  net/core/dev.c | 11 ++++++-----
>  1 file changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index
> a979b86dbacda9dfe31dd8b269024f7f0f5a8ef1..449b45b843d40ece7dd1e2ed6a5
> 996ee1db9f591 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -5735,10 +5735,11 @@ static void gro_normal_list(struct
> napi_struct *napi)
>  /* Queue one GRO_NORMAL SKB up for list processing. If batch size
> exceeded,
>   * pass the whole batch up to the stack.
>   */
> -static void gro_normal_one(struct napi_struct *napi, struct sk_buff
> *skb)
> +static void gro_normal_one(struct napi_struct *napi, struct sk_buff
> *skb, int segs)
>  {
>         list_add_tail(&skb->list, &napi->rx_list);
> -       if (++napi->rx_count >= gro_normal_batch)
> +       napi->rx_count += segs;
> +       if (napi->rx_count >= gro_normal_batch)
>                 gro_normal_list(napi);
>  }
>  
> @@ -5777,7 +5778,7 @@ static int napi_gro_complete(struct napi_struct
> *napi, struct sk_buff *skb)
>         }
>  
>  out:
> -       gro_normal_one(napi, skb);
> +       gro_normal_one(napi, skb, NAPI_GRO_CB(skb)->count);

Seems correct to me,

Reviewed-by: Saeed Mahameed <saeedm@...dia.com>