netdev - Re: [PATCH net] tcp: fix TLP timer not set when CA

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Date:   Sat, 23 Jan 2021 12:25:10 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Neal Cardwell <ncardwell@...gle.com>
Cc:     Pengcheng Yang <yangpc@...gsu.com>,
        David Miller <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Netdev <netdev@...r.kernel.org>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH net] tcp: fix TLP timer not set when CA_STATE changes
 from DISORDER to OPEN

On Sat, 23 Jan 2021 13:25:23 -0500 Neal Cardwell wrote:
> On Sat, Jan 23, 2021 at 9:15 AM Pengcheng Yang <yangpc@...gsu.com> wrote:
> >
> > On Sat, Jan 23, 2021 at 9:27 AM "Jakub Kicinski" <kuba@...nel.org> wrote:  
> > >
> > > On Fri, 22 Jan 2021 11:53:46 +0100 Eric Dumazet wrote:  
> > > > On Fri, Jan 22, 2021 at 11:28 AM Pengcheng Yang <yangpc@...gsu.com> wrote:  
> > > > >
> > > > > When CA_STATE is in DISORDER, the TLP timer is not set when receiving
> > > > > an ACK (a cumulative ACK covered out-of-order data) causes CA_STATE to
> > > > > change from DISORDER to OPEN. If the sender is app-limited, it can only
> > > > > wait for the RTO timer to expire and retransmit.
> > > > >
> > > > > The reason for this is that the TLP timer is set before CA_STATE changes
> > > > > in tcp_ack(), so we delay the time point of calling tcp_set_xmit_timer()
> > > > > until after tcp_fastretrans_alert() returns and remove the
> > > > > FLAG_SET_XMIT_TIMER from ack_flag when the RACK reorder timer is set.
> > > > >
> > > > > This commit has two additional benefits:
> > > > > 1) Make sure to reset RTO according to RFC6298 when receiving ACK, to
> > > > > avoid spurious RTO caused by RTO timer early expires.
> > > > > 2) Reduce the xmit timer reschedule once per ACK when the RACK reorder
> > > > > timer is set.
> > > > >
> > > > > Link: https://lore.kernel.org/netdev/1611139794-11254-1-git-send-email-yangpc@wangsu.com
> > > > > Signed-off-by: Pengcheng Yang <yangpc@...gsu.com>
> > > > > Cc: Neal Cardwell <ncardwell@...gle.com>  
> > > >
> > > > This looks like a very nice patch, let me run packetdrill tests on it.
> > > >
> > > > By any chance, have you cooked a packetdrill test showing the issue
> > > > (failing on unpatched kernel) ?  
> > >
> > > Any guidance on backporting / fixes tag? (once the packetdrill
> > > questions are satisfied)  
> >
> > By reading the commits, we can add:
> > Fixes: df92c8394e6e ("tcp: fix xmit timer to only be reset if data ACKed/SACKed")  
> 
> Thanks for the fix and the packetdrill test!
> 
> Eric ran our internal packetdrill test suite and all the changes in
> behavior with your patch all look like fixes to me.
> 
> Acked-by: Neal Cardwell <ncardwell@...gle.com>

Thanks!

> The Fixes: tag you suggest also looks good to me. (It seems like
> patchwork did not pick it up from your email and add it to the commit
> message automatically, BTW?)

Indeed, we haven't worked that out yet in patchwork.kernel.org, but
it's no big deal, normally we can add manually. In this case seems 
like Yuchung asked for a small rewrite to the commit message so I'd
appreciate a full repost.