netdev - Re: [PATCH net-next v1 1/2] net: core: count drops from GRO

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Message-ID: <20210108103537.00005168@intel.com>
Date:   Fri, 8 Jan 2021 10:35:37 -0800
From:   Jesse Brandeburg <jesse.brandeburg@...el.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     netdev <netdev@...r.kernel.org>, intel-wired-lan@...ts.osuosl.org,
        Jamal Hadi Salim <jhs@...atatu.com>
Subject: Re: [PATCH net-next v1 1/2] net: core: count drops from GRO

Eric Dumazet wrote:
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -6071,6 +6071,7 @@ static gro_result_t napi_skb_finish(struct napi_struct *napi,
> >                 break;
> >
> >         case GRO_DROP:
> > +               atomic_long_inc(&skb->dev->rx_dropped);
> >                 kfree_skb(skb);
> >                 break;
> >
> > @@ -6159,6 +6160,7 @@ static gro_result_t napi_frags_finish(struct napi_struct *napi,
> >                 break;
> >
> >         case GRO_DROP:
> > +               atomic_long_inc(&skb->dev->rx_dropped);
> >                 napi_reuse_skb(napi, skb);
> >                 break;
> >
> 
> 
> This is not needed. I think we should clean up ice instead.

My patch 2 already did that. I was trying to address the fact that I'm
*actually seeing* GRO_DROP return codes coming back from stack.

I'll try to reproduce that issue again that I saw. Maybe modern kernels
don't have the problem as frequently or at all.

> Drivers are supposed to have allocated the skb (using
> napi_get_frags()) before calling napi_gro_frags()

ice doesn't use napi_get_frags/napi_gro_frags, so I'm not sure how this
is relevant. 

> Only napi_gro_frags() would return GRO_DROP, but we supposedly could
> crash at that point, since a driver is clearly buggy.

seems unlikely since we don't call those functions.

> We probably can remove GRO_DROP completely, assuming lazy drivers are fixed.

This might be ok, but doesn't explain why I was seeing this return
code (which was the whole reason I was trying to count them), however I
may have been running on a distro kernel from redhat/centos 8 when I
was seeing these events. I haven't fully completed spelunking all the
different sources, but might be able to follow down the rabbit hole
further.

> diff --git a/net/core/dev.c b/net/core/dev.c
> index 8fa739259041aaa03585b5a7b8ebce862f4b7d1d..c9460c9597f1de51957fdcfc7a64ca45bce5af7c
> 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -6223,9 +6223,6 @@ gro_result_t napi_gro_frags(struct napi_struct *napi)
>         gro_result_t ret;
>         struct sk_buff *skb = napi_frags_skb(napi);
> 
> -       if (!skb)
> -               return GRO_DROP;
> -
>         trace_napi_gro_frags_entry(skb);
> 
>         ret = napi_frags_finish(napi, skb, dev_gro_receive(napi, skb));

This change (noted from your other patches is fine), and a likely
improvement, thanks for sending those!