On Thu, 2017-03-30 at 06:52 -0700, Eric Dumazet wrote:
> On Thu, 2017-03-30 at 14:03 +0200, Paolo Abeni wrote:
> > sock_recv_ts_and_drops() unconditionally set sk->sk_stamp for
> > every packet, even if the SOCK_TIMESTAMP flag is not set in the
> > related socket.
> > If selinux is enabled, this cause a cache miss for every packet
> > since sk->sk_stamp and sk->sk_security share the same cacheline.
> > With this change sk_stamp is set only if the SOCK_TIMESTAMP
> > flag is set, and is cleared for the first packet, so that the user
> > perceived behavior is unchanged.
> >
> > This gives up to 5% speed-up under udp-flood with small packets.
> >
> > Signed-off-by: Paolo Abeni <pab...@redhat.com>
> > ---
> > include/net/sock.h | 5 ++++-
> > net/core/sock.c | 2 +-
> > 2 files changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/net/sock.h b/include/net/sock.h
> > index cb241a0..8e53158 100644
> > --- a/include/net/sock.h
> > +++ b/include/net/sock.h
> > @@ -2239,6 +2239,7 @@ sock_recv_timestamp(struct msghdr *msg, struct sock
> > *sk, struct sk_buff *skb)
> > void __sock_recv_ts_and_drops(struct msghdr *msg, struct sock *sk,
> > struct sk_buff *skb);
> >
> > +#define SK_DEFAULT_STAMP (-1L * NSEC_PER_SEC)
> > static inline void sock_recv_ts_and_drops(struct msghdr *msg, struct sock
> > *sk,
> > struct sk_buff *skb)
> > {
> > @@ -2249,8 +2250,10 @@ static inline void sock_recv_ts_and_drops(struct
> > msghdr *msg, struct sock *sk,
> >
> > if (sk->sk_flags & FLAGS_TS_OR_DROPS || sk->sk_tsflags & TSFLAGS_ANY)
> > __sock_recv_ts_and_drops(msg, sk, skb);
> > - else
> > + else if (unlikely(sk->sk_flags & SOCK_TIMESTAMP))
> > sk->sk_stamp = skb->tstamp;
> > + else if (unlikely(sk->sk_stamp == SK_DEFAULT_STAMP))
> > + sk->sk_stamp = 0;
> > }
> >
>
> This looks very nice, but why using 0 here instead of skb->tstamp ?
Thank you for reviewing this.
The network stack can already mark sk->sk_stamp with 0, if the
'netstamp_needed' static key is false when the packet is received.
'0' is used as a special value by sock_get_timestamp(), providing to
the caller the current ktime.
This way the kernel is able to detect if no packets have been received
and to provide a somewhat valid timestamp for the last packet received
before that the SOCK_TIMESTAMP flag was enabled; the assumption is that
the ioctl() follows closely the read call.
This should be the same behavior the user space already observes if net
timestamping is disabled when the SOCK_TIMESTAMP flag is set.
> This might give some regression on applications reading their first
> socket timestamp in some contexts.
>
> What about
>
> if (sk->sk_flags & FLAGS_TS_OR_DROPS || sk->sk_tsflags & TSFLAGS_ANY)
> __sock_recv_ts_and_drops(msg, sk, skb);
> else if (unlikely(sk->sk_flags & SOCK_TIMESTAMP ||
> sk->sk_stamp == SK_DEFAULT_STAMP))
> sk->sk_stamp = skb->tstamp;
That way, if the net timestamp is enable, we will record the timestamp
of the first packet received by the socket (it can be far away in the
past).
I think is just a different kind of approximation.
Cheers,
Paolo
