netdev - RE: [PATCH net 2/4] tcp: tcp_fragment() should apply sane memory limits

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Date:   Wed, 10 Jul 2019 18:23:33 +0000
From:   "Prout, Andrew - LLSC - MITLL" <aprout@...mit.edu>
To:     Christoph Paasch <christoph.paasch@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>
CC:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jonathan Looney <jtl@...flix.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Tyler Hicks <tyhicks@...onical.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Bruce Curtis <brucec@...flix.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>,
        "Dustin Marquess" <dmarquess@...le.com>
Subject: RE: [PATCH net 2/4] tcp: tcp_fragment() should apply sane memory
 limits

On 6/17/19 8:19 PM, Christoph Paasch wrote:
> 
> Yes, this does the trick for my packetdrill-test.
> 
> I wonder, is there a way we could end up in a situation where we can't
> retransmit anymore?
> For example, sk_wmem_queued has grown so much that the new test fails.
> Then, if we legitimately need to fragment in __tcp_retransmit_skb() we
> won't be able to do so. So we will never retransmit. And if no ACK
> comes back in to make some room we are stuck, no?

We seem to be having exactly this problem. We’re running on the 4.14 branch. After recently updating our kernel, we’ve been having a problem with TCP connections stalling / dying off without disconnecting. They're stuck and never recover.

I bisected the problem to 4.14.127 commit 9daf226ff92679d09aeca1b5c1240e3607153336 (commit f070ef2ac66716357066b683fb0baf55f8191a2e upstream): tcp: tcp_fragment() should apply sane memory limits. That lead me to this thread.

Our environment is a supercomputing center: lots of servers interconnected with a non-blocking 10Gbit ethernet network. We’ve zeroed in on the problem in two situations: remote users on VPN accessing large files via samba and compute jobs using Intel MPI over TCP/IP/ethernet. It certainly affects other situations, many of our workloads have been unstable since this patch went into production, but those are the two we clearly identified as they fail reliably every time. We had to take the system down for unscheduled maintenance to roll back to an older kernel.

The TCPWqueueTooBig count is incrementing when the problem occurs.

Using ftrace/trace-cmd on an affected process, it appears the call stack is:
run_timer_softirq
expire_timers
call_timer_fn
tcp_write_timer
tcp_write_timer_handler
tcp_retransmit_timer
tcp_retransmit_skb
__tcp_retransmit_skb
tcp_fragment

Andrew Prout
MIT Lincoln Laboratory Supercomputing Center