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

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Date:   Sun, 22 Oct 2017 13:10:19 +0900
From:   Koichiro Den <den@...ipeden.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: [net-next] tcp: do tcp_mstamp_refresh before retransmits on TSQ
 handler

On Sat, 2017-10-21 at 20:52 -0700, Eric Dumazet wrote:
> On Sun, 2017-10-22 at 12:38 +0900, Koichiro Den wrote:
> > When retransmission on TSQ handler was introduced in the commit
> > f9616c35a0d7 ("tcp: implement TSQ for retransmits"), the retransmitted
> > skbs' timestamps were updated on the actual transmission. In the later
> > commit 385e20706fac ("tcp: use tp->tcp_mstamp in output path"), it stops
> > being done so. In the commit, the comment says "We try to refresh
> > tp->tcp_mstamp only when necessary", and at present tcp_tsq_handler and
> > tcp_v4_mtu_reduced applies to this. About the latter, it's okay since
> > it's rare enough.
> > 
> > About the former, even though possible retransmissions on the tasklet
> > comes just after the destructor run in NET_RX softirq handling, the time
> > between them could be nonnegligibly large to the extent that
> > tcp_rack_advance or rto rearming be affected if other (remaining) RX,
> > BLOCK and (preceding) TASKLET sofirq handlings are unexpectedly heavy.
> > 
> > So in the same way as tcp_write_timer_handler does, doing tcp_mstamp_refresh
> > ensures the accuracy of algorithms relying on it.
> > 
> > Signed-off-by: Koichiro Den <den@...ipeden.com>
> > ---
> >  net/ipv4/tcp_output.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> Very nice catch, thanks a lot Koichiro.
> 
> This IMO would target net tree, since it is a bug fix.
> 
> Fixes: 385e20706fac ("tcp: use tp->tcp_mstamp in output path")
Ok I will submit it to net tree. Thanks!
> 
> Thanks !
> 
> We should have caught that in our regression packetdrill tests...
In its "remote" mode testing not relying on tun xmit, I agree it could be
caught. If it's better to write, test and attach the script, please let me know.

Thank you.