Re: [PATCH net-next] net: tcp: refresh tcp_mstamp for compressed ack in timer

[Menglong Dong]

On Thu, Oct 3, 2024 at 4:47 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> On Thu, Oct 3, 2024 at 10:23 AM Menglong Dong <menglong8.dong@xxxxxxxxx> wrote:
> >
> > For now, we refresh the tcp_mstamp for delayed acks and keepalives, but
> > not for the compressed ack in tcp_compressed_ack_kick().
> >
> > I have not found out the effact of the tcp_mstamp when sending ack, but
> > we can still refresh it for the compressed ack to keep consistent.
>
> This was a choice I made for the following reason :
>
> delayed ack timer can happen sometime 40ms later. Thus the
> tcp_mstamp_refresh(tp) was probably welcome.
>
> Compressed ack timer is scheduled for min( 5% of RTT, 1ms). It is
> usually in the 200 usec range.
>

Thanks for the explanation! I'm writing a tool, which tries to capture
the latency from __tcp_transmit_skb() to dev_hard_start_xmit() according
to the skb->tstamp, and the compressed ack case confuses my
application.

Maybe someone else can do similar things, and they can benefit from
this patch too.

Thanks!
Menglong Dong

> So sending the prior tsval (for flow using TCP TS) was ok (and right
> most of the time), and not changing PAWS or EDT logic.
>
> Although I do not object to your patch, there is no strong argument
> for it or against it.
>
> Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
>
> >
> > Signed-off-by: Menglong Dong <dongml2@xxxxxxxxxxxxxxx>
> > ---
> >  net/ipv4/tcp_timer.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> > index 79064580c8c0..1f37a37f9c82 100644
> > --- a/net/ipv4/tcp_timer.c
> > +++ b/net/ipv4/tcp_timer.c
> > @@ -851,6 +851,7 @@ static enum hrtimer_restart tcp_compressed_ack_kick(struct hrtimer *timer)
> >                          * LINUX_MIB_TCPACKCOMPRESSED accurate.
> >                          */
> >                         tp->compressed_ack--;
> > +                       tcp_mstamp_refresh(tp);
> >                         tcp_send_ack(sk);
> >                 }
> >         } else {
> > --
> > 2.39.5
> >