netdev - Re: [net-next] tcp: do tcp_mstamp_refresh before retransmits on TSQ handler

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Date:   Mon, 23 Oct 2017 12:26:17 +0900
From:   Koichiro Den <den@...ipeden.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, ncardwell@...gle.com
Subject: Re: [net-next] tcp: do tcp_mstamp_refresh before retransmits on TSQ
 handler

On Sun, 2017-10-22 at 10:11 -0700, Eric Dumazet wrote:
> On Sun, 2017-10-22 at 09:49 -0700, Eric Dumazet wrote:
> 
> > Here is the test I cooked.
> > 
> > Note that it actually passes just fine, do I am wondering if your patch
> > is needed after all..
> > 
> 
> Oh this is because tcp_mstamp_refresh() is called from tcp_write_xmit()
> and tcp_write_xmit() is directly called from tcp_tsq_handler()

Thanks for all you help. I guess we should focus on retransmit call path
which comes just before that, or not?
> 
> So it looks your patch has no observable effect.
> 
> (Even with usec TS as we use here at Google, it has no effect since TCP
> stack does not consider RTT samples being valid if skb was ever
> retransmitted)
> 
> 

Is the point about tcp_rack_advance part or in general?

Btw, perf probe makes the original point described in my commit message apparent
as follows:

$ perf probe 'T1=tcp_write_xmit:25 tm=+1464(%bx):u64'
$ perf probe 'T2=tcp_xmit_retransmit_queue+0
              l_o=+1664(%di):u32  # tp->lost_out
              r_o=+1520(%di):u32  # tp->retrans_out
              tm=+1464(%di):u64'  # skb_mstamp will be set to this.
$ perf trace -aT --no-syscalls --call-graph dwarf \
  -e probe:T1/max-stack=4/
  -e probe:T2/max-stack=1/ |& \
  sed 's/..kernel.kallsyms..//;s/probe://' # to make it a bit more readable :p

$ # run fq-pacing-tsq-rtx-ts.pkt Eric provided:

[before patch applied]
-----8<------8<-----
2560581.604 T2:(ffffffff817da5e0) l_o=25 r_o=0 tm=2560581404)
                                       tcp_xmit_retransmit_queue.part.33
                                       tcp_xmit_recovery.part.53
2560581.614 T1:(ffffffff817d7e74) tm=2560581612)  # <--- (1)
                                       tcp_write_xmit
                                       __tcp_push_pending_frames
                                       tcp_rcv_established
                                       tcp_v4_do_rcv

/*
 * ^
 * | could be unexpectedly long, e.g. due to BLOCK_SOFTIRQ
 * v
 */
2560582.215 T2:(ffffffff817da5e0) l_o=25 r_o=5 tm=2560581612)
/* <- same as (1) above */
                                       tcp_xmit_retransmit_queue.part.33
                                       tcp_tasklet_func
----->8------->8-----


[after patch applied]
-----8<------8<-----
 97846.150 T2:(ffffffff817da5e0) l_o=25 r_o=0 tm=97846132)
                                       tcp_xmit_retransmit_queue.part.33
                                       tcp_xmit_recovery.part.53
 97846.159 T1:(ffffffff817d7e74) tm=97846158)    # <--- (1)
                                       tcp_write_xmit
                                       __tcp_push_pending_frames
                                       tcp_rcv_established
                                       tcp_v4_do_rcv
/*
 * ^
 * | could be unexpectedly long, e.g. due to BLOCK_SOFTIRQ
 * v
 */
 97847.144 T2:(ffffffff817da5e0) l_o=25 r_o=5 tm=97847134)
/* <- not the same as (1).
 * never affected now that we ensure it's refreshed */
                                       tcp_xmit_retransmit_queue.part.33
                                       tcp_tasklet_func
----->8------->8-----

Now I wonder this is more of a theoretical one rather than a patch to fix one
specific bug.