netdev - Re: [PATCH v6 net-next 10/13] net: Postpone skb_clear_delivery_time() until knowing the skb is delivered locally

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Date:   Wed, 2 Mar 2022 16:47:17 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     bpf <bpf@...r.kernel.org>, netdev <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        kernel-team <kernel-team@...com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH v6 net-next 10/13] net: Postpone skb_clear_delivery_time()
 until knowing the skb is delivered locally

On Wed, Mar 2, 2022 at 4:20 PM Martin KaFai Lau <kafai@...com> wrote:
>
> On Wed, Mar 02, 2022 at 03:41:59PM -0800, Eric Dumazet wrote:

> > Name was a bit confusing :)
> A few names were attempted in the early version and
> then concluded on delivery_time. :p
>
> >
> > And it seems you have a big opportunity to not call ktime_get_real()
> > when skb->sk is known at this point (early demux)
> > because few sockets actually enable timestamping ?
> iiuc, you are suggesting to also check the skb->sk (if early demux)
> and check for SK_FLAGS_TIMESTAMP.
>
> Without checking skb->sk here, it should not be worse than the
> current ktime_get_real() done in dev.c where it also does not have sk
> available?  netstamp_needed_key should have been enabled as
> long as there is one sk asked for it.

Yes, but typically there is often one active timestamp consumer,
enabling the static key.

This is a corner case anyway, not sure if hosts running VM would have
slow  ktime_get_real()