Message-ID: <CAL+tcoDonudsr800HmhDir7f0B6cx0RPwmnrsRmQF=yDUJUszg@mail.gmail.com>
Date: Wed, 30 Oct 2024 11:04:58 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: Martin KaFai Lau <martin.lau@...ux.dev>, willemb@...gle.com, davem@...emloft.net, 
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, dsahern@...nel.org, 
	ast@...nel.org, daniel@...earbox.net, andrii@...nel.org, eddyz87@...il.com, 
	song@...nel.org, yonghong.song@...ux.dev, john.fastabend@...il.com, 
	kpsingh@...nel.org, sdf@...ichev.me, haoluo@...gle.com, jolsa@...nel.org, 
	shuah@...nel.org, ykolal@...com, bpf@...r.kernel.org, netdev@...r.kernel.org, 
	Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next v3 02/14] net-timestamp: allow two features to
 work parallelly

On Wed, Oct 30, 2024 at 10:47 AM Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
> Jason Xing wrote:
> > On Wed, Oct 30, 2024 at 9:45 AM Willem de Bruijn
> > <willemdebruijn.kernel@...il.com> wrote:
> > >
> > > Jason Xing wrote:
> > > > On Wed, Oct 30, 2024 at 7:00 AM Martin KaFai Lau <martin.lau@...ux.dev> wrote:
> > > > >
> > > > > On 10/28/24 4:05 AM, Jason Xing wrote:
> > > > > > From: Jason Xing <kernelxing@...cent.com>
> > > > > >
> > > > > > This patch has introduced a separate sk_tsflags_bpf for bpf
> > > > > > extension, which helps us let two feature work nearly at the
> > > > > > same time.
> > > > > >
> > > > > > Each feature will finally take effect on skb_shinfo(skb)->tx_flags,
> > > > > > say, tcp_tx_timestamp() for TCP or skb_setup_tx_timestamp() for
> > > > > > other types, so in __skb_tstamp_tx() we are unable to know which
> > > > > > feature is turned on, unless we check each feature's own socket
> > > > > > flag field.
> > > > > >
> > > > > > Signed-off-by: Jason Xing <kernelxing@...cent.com>
> > > > > > ---
> > > > > >   include/net/sock.h |  1 +
> > > > > >   net/core/skbuff.c  | 39 +++++++++++++++++++++++++++++++++++++++
> > > > > >   2 files changed, 40 insertions(+)
> > > > > >
> > > > > > diff --git a/include/net/sock.h b/include/net/sock.h
> > > > > > index 7464e9f9f47c..5384f1e49f5c 100644
> > > > > > --- a/include/net/sock.h
> > > > > > +++ b/include/net/sock.h
> > > > > > @@ -445,6 +445,7 @@ struct sock {
> > > > > >       u32                     sk_reserved_mem;
> > > > > >       int                     sk_forward_alloc;
> > > > > >       u32                     sk_tsflags;
> > > > > > +     u32                     sk_tsflags_bpf;
> > > > > >       __cacheline_group_end(sock_write_rxtx);
> > > > > >
> > > > > >       __cacheline_group_begin(sock_write_tx);
> > > > > > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > > > > > index 1cf8416f4123..39309f75e105 100644
> > > > > > --- a/net/core/skbuff.c
> > > > > > +++ b/net/core/skbuff.c
> > > > > > @@ -5539,6 +5539,32 @@ void skb_complete_tx_timestamp(struct sk_buff *skb,
> > > > > >   }
> > > > > >   EXPORT_SYMBOL_GPL(skb_complete_tx_timestamp);
> > > > > >
> > > > > > +/* This function is used to test if application SO_TIMESTAMPING feature
> > > > > > + * or bpf SO_TIMESTAMPING feature is loaded by checking its own socket flags.
> > > > > > + */
> > > > > > +static bool sk_tstamp_tx_flags(struct sock *sk, u32 tsflags, int tstype)
> > > > > > +{
> > > > > > +     u32 testflag;
> > > > > > +
> > > > > > +     switch (tstype) {
> > > > > > +     case SCM_TSTAMP_SCHED:
> > > > > > +             testflag = SOF_TIMESTAMPING_TX_SCHED;
> > > > > > +             break;
> > > > > > +     case SCM_TSTAMP_SND:
> > > > > > +             testflag = SOF_TIMESTAMPING_TX_SOFTWARE;
> > > > > > +             break;
> > > > > > +     case SCM_TSTAMP_ACK:
> > > > > > +             testflag = SOF_TIMESTAMPING_TX_ACK;
> > > > > > +             break;
> > > > > > +     default:
> > > > > > +             return false;
> > > > > > +     }
> > > > > > +     if (tsflags & testflag)
> > > > > > +             return true;
> > > > > > +
> > > > > > +     return false;
> > > > > > +}
> > > > > > +
> > > > > >   static void skb_tstamp_tx_output(struct sk_buff *orig_skb,
> > > > > >                                const struct sk_buff *ack_skb,
> > > > > >                                struct skb_shared_hwtstamps *hwtstamps,
> > > > > > @@ -5549,6 +5575,9 @@ static void skb_tstamp_tx_output(struct sk_buff *orig_skb,
> > > > > >       u32 tsflags;
> > > > > >
> > > > > >       tsflags = READ_ONCE(sk->sk_tsflags);
> > > > > > +     if (!sk_tstamp_tx_flags(sk, tsflags, tstype))
> > > > >
> > > > > I still don't get this part since v2. How does it work with cmsg only
> > > > > SOF_TIMESTAMPING_TX_*?
> > > > >
> > > > > I tried with "./txtimestamp -6 -c 1 -C -N -L ::1" and it does not return any tx
> > > > > time stamp after this patch.
> > > > >
> > > > > I am likely missing something
> > > > > or v2 concluded that this behavior change is acceptable?
> > > >
> > > > Sorry, I submitted this series accidentally removing one important
> > > > thing which is similar to what Vadim Fedorenko mentioned in the v1
> > > > [1]:
> > > > adding another member like sk_flags_bpf to handle the cmsg case.
> > > >
> > > > Willem, would it be acceptable to add another field in struct sock to
> > > > help us recognise the case where BPF and cmsg works parallelly?
> > > >
> > > > [1]: https://lore.kernel.org/all/662873cb-a897-464e-bdb3-edf01363c3b2@linux.dev/
> > >
> > > The current timestamp flags don't need a u32. Maybe just reserve a bit
> > > for this purpose?
> >
> > Sure. Good suggestion.
> >
> > But I think only using one bit to reflect whether the sk->sk_tsflags
> > is used by normal or cmsg features is not enough. The existing
> > implementation in tcp_sendmsg_locked() doesn't override the
> > sk->sk_tsflags even the normal and cmsg features enabled parallelly.
> > It only overrides sockc.tsflags in tcp_sendmsg_locked(). Based on
> > that, even if at some point users suddenly remove the cmsg use and
> > then the prior normal SO_TIMESTAMPING continues to work.
> >
> > How about this, please see below:
> > For now, sk->sk_tsflags only uses 17 bits (see the last one
> > SOF_TIMESTAMPING_OPT_RX_FILTER). The cmsg feature only uses 4 flags
> > (see SOF_TIMESTAMPING_TX_RECORD_MASK in __sock_cmsg_send()). With that
> > said, we could reserve the highest four bits for cmsg use for the
> > moment. Four bits represents four points where we can record the
> > timestamp in the tx case.
> >
> > Do you agree on this point?
>
> I don't follow.
>
> I probably miss the entire point.
>
> The goal for sockcm fields is to start with the sk field and
> optionally override based on cmsg. This is what sockcm_init does for
> tsflags.
>
> This information is for the skb, so these are recording flags.
>
> Why does the new datapath need to know whether features are enabled
> through setsockopt or on a per-call basis with a cmsg?
>
> The goal was always to keep the reporting flags per socket, but make
> the recording flag per packet, mainly for sampling.

If a user uses 1) cmsg feature, 2) bpf feature at the same time, we
allow each feature to work independently.

How could it work? It relies on sk_tstamp_tx_flags() function in the
current patch: when we are in __skb_tstamp_tx(), we cannot know which
flags in each feature are set without fetching sk->sk_tsflags and
sk->sk_tsflags_bpf. Then we are able to know what timestamp we want to
record. To put it in a simple way, we're not sure if the user wants to
see a SCHED timestamp by using the cmsg feature in __skb_tstamp_tx()
if we hit this test statement "skb_shinfo(skb)->tx_flags &
SKBTX_SCHED_TSTAMP)". So we need those two socket tsflag fields to
help us.

Thanks,
Jason

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