Re: [PATCH net-next 1/2] tcp: measure rwnd-limited time

[Yuchung Cheng]

On Wed, Sep 7, 2016 at 5:27 PM, David Miller  wrote:
>
> From: "Francis Y. Yan" 
> Date: Tue,  6 Sep 2016 18:32:40 -0700
>
> > diff --git a/include/linux/tcp.h b/include/linux/tcp.h
> > index 7be9b12..f5b588e 100644
> > --- a/include/linux/tcp.h
> > +++ b/include/linux/tcp.h
> > @@ -176,6 +176,7 @@ struct tcp_sock {
> >* were acked.
> >*/
> >   struct u64_stats_sync syncp; /* protects 64bit vars (cf 
> > tcp_get_info()) */
> > + seqcount_t seqcnt;  /* proctects rwnd-limited-related vars, etc. 
> > */
> >
> >   u32 snd_una;/* First byte we want an ack for*/
> >   u32 snd_sml;/* Last byte of the most recently transmitted 
> > small packet */
> > @@ -204,6 +205,8 @@ struct tcp_sock {
> >
> >   u32 window_clamp;   /* Maximal window to advertise  */
> >   u32 rcv_ssthresh;   /* Current window clamp */
> > + struct skb_mstamp rwnd_limited_ts; /* Last timestamp limited by rwnd 
> > */
> > + u64 rwnd_limited;   /* Total time (us) limited by rwnd */
> >
> >   /* Information of the most recently (s)acked skb */
> >   struct tcp_rack {
>
> I understand completely the usefulness of this change, but wow that is a lot 
> of
> new TCP socket space taken up just to export some time values in tcp_info for
> debugging and statistics.
Thanks for raising the concern. The space requirement and timing
resolution should indeed be justified better on the motivation and the
dev plan. We would resubmit a v2 with a cover letter detailing those.

Re: [PATCH net-next 1/2] tcp: measure rwnd-limited time

[David Miller]

From: "Francis Y. Yan" 
Date: Tue,  6 Sep 2016 18:32:40 -0700

> diff --git a/include/linux/tcp.h b/include/linux/tcp.h
> index 7be9b12..f5b588e 100644
> --- a/include/linux/tcp.h
> +++ b/include/linux/tcp.h
> @@ -176,6 +176,7 @@ struct tcp_sock {
>* were acked.
>*/
>   struct u64_stats_sync syncp; /* protects 64bit vars (cf tcp_get_info()) 
> */
> + seqcount_t seqcnt;  /* proctects rwnd-limited-related vars, etc. */
>  
>   u32 snd_una;/* First byte we want an ack for*/
>   u32 snd_sml;/* Last byte of the most recently transmitted 
> small packet */
> @@ -204,6 +205,8 @@ struct tcp_sock {
>  
>   u32 window_clamp;   /* Maximal window to advertise  */
>   u32 rcv_ssthresh;   /* Current window clamp */
> + struct skb_mstamp rwnd_limited_ts; /* Last timestamp limited by rwnd */
> + u64 rwnd_limited;   /* Total time (us) limited by rwnd */
>  
>   /* Information of the most recently (s)acked skb */
>   struct tcp_rack {

I understand completely the usefulness of this change, but wow that is a lot of
new TCP socket space taken up just to export some time values in tcp_info for
debugging and statistics.

Re: [PATCH net-next 1/2] tcp: measure rwnd-limited time

[Eric Dumazet]

On Tue, 2016-09-06 at 18:32 -0700, Francis Y. Yan wrote:
> This patch measures the total time when TCP transmission is limited
> by receiver's advertised window (rwnd), and exports it in tcp_info as
> tcpi_rwnd_limited.
> 
> The rwnd-limited time is defined as the period when the next segment
> to send by TCP cannot fit into rwnd. To measure it, we record the last
> timestamp when limited by rwnd (rwnd_limited_ts) and the total
> rwnd-limited time (rwnd_limited) in tcp_sock.
> 
> Then we export the total rwnd-limited time so far in tcp_info, where
> by so far, we mean that if TCP transmission is still being limited by
> rwnd, the time interval since rwnd_limited_ts needs to be counted as
> well; otherwise, we simply export rwnd_limited.
> 
> It is worth noting that we also have to add a new sequence counter
> (seqcnt) in tcp_sock to carefully handle tcp_info's reading of
> rwnd_limited_ts and rwnd_limited in order to get a consistent snapshot
> of both variables together.
> 
> Signed-off-by: Francis Y. Yan 
> Signed-off-by: Yuchung Cheng 
> ---

Acked-by: Eric Dumazet 

Thanks Francis !

Re: [PATCH net-next 1/2] tcp: measure rwnd-limited time

[Soheil Hassas Yeganeh]

On Tue, Sep 6, 2016 at 9:32 PM, Francis Y. Yan  wrote:
> This patch measures the total time when TCP transmission is limited
> by receiver's advertised window (rwnd), and exports it in tcp_info as
> tcpi_rwnd_limited.
>
> The rwnd-limited time is defined as the period when the next segment
> to send by TCP cannot fit into rwnd. To measure it, we record the last
> timestamp when limited by rwnd (rwnd_limited_ts) and the total
> rwnd-limited time (rwnd_limited) in tcp_sock.
>
> Then we export the total rwnd-limited time so far in tcp_info, where
> by so far, we mean that if TCP transmission is still being limited by
> rwnd, the time interval since rwnd_limited_ts needs to be counted as
> well; otherwise, we simply export rwnd_limited.
>
> It is worth noting that we also have to add a new sequence counter
> (seqcnt) in tcp_sock to carefully handle tcp_info's reading of
> rwnd_limited_ts and rwnd_limited in order to get a consistent snapshot
> of both variables together.
>
> Signed-off-by: Francis Y. Yan 
> Signed-off-by: Yuchung Cheng 

Acked-by: Soheil Hassas Yeganeh