Re: [PATCH RFT 3/5] tcp: Force TCP_WINDOW_CLAMP before resetting STALLED flag

[David Gibson <david@gibson.dropbear.id.au>]

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On Mon, Sep 25, 2023 at 02:09:41PM +1000, David Gibson wrote:
> On Sat, Sep 23, 2023 at 12:06:08AM +0200, Stefano Brivio wrote:
> > It looks like we need it as workaround for this situation, readily
> > reproducible at least with a 6.5 Linux kernel, with default rmem_max
> > and wmem_max values:
> > 
> > - an iperf3 client on the host sends about 160 KiB, typically
> >   segmented into five frames by passt. We read this data using
> >   MSG_PEEK
> > 
> > - the iperf3 server on the guest starts receiving
> > 
> > - meanwhile, the host kernel advertised a zero-sized window to the
> >   receiver, as expected
> > 
> > - eventually, the guest acknowledges all the data sent so far, and
> >   we drop it from the buffer, courtesy of tcp_sock_consume(), using
> >   recv() with MSG_TRUNC
> > 
> > - the client, however, doesn't get an updated window value, and
> >   even keepalive packets are answered with zero-window segments,
> >   until the connection is closed
> > 
> > It looks like dropping data from a socket using MSG_TRUNC doesn't
> > cause a recalculation of the window, which would be expected as a
> > result of any receiving operation that invalidates data on a buffer
> > (that is, not with MSG_PEEK).
> > 
> > Strangely enough, setting TCP_WINDOW_CLAMP via setsockopt(), even to
> > the previous value we clamped to, forces a recalculation of the
> > window which is advertised to the guest.
> > 
> > I couldn't quite confirm this issue by following all the possible
> > code paths in the kernel, yet. If confirmed, this should be fixed in
> > the kernel, but meanwhile this workaround looks robust to me (and it
> > will be needed for backward compatibility anyway).
> 
> So, I tested this, and things got a bit complicated.
> 
> First, I reproduced the "read side" problem by setting
> net.core.rmem_max to 256kiB while setting net.core.wmem_max to 16MiB.
> The "160kiB" stall happened almost every time.  Applying this patch
> appears to fix it completely, getting GiB/s throughput consistently.
> So, yah.
> 
> Then I tried reproducing it differently: by setting both
> net.core.rmem_max and net.core.wmem_max to 16MiB, but setting
> SO_RCVBUF to 128kiB explicitly in tcp_sock_set_bufsize() (which
> actually results in a 256kiB buffer, because of the kernel's weird
> interpretation).
> 
> With the SO_RCVBUF clamp and without this patch, I don't get the
> 160kiB stall consistently any more.  What I *do* get is nearly every
> time - but not *every* time - is slow transfers, ~40Mbps vs. ~12Gbps.
> Sometimes it stalls after several seconds. The stall is slightly
> different from the 160kiB stall though: the 160kiB stall seems 0 bytes
> transferred on both sides.  With the RCVBUF stall I get a trickle of
> bytes (620 bytes/s) on the receiver/guest side, with mostly 0 bytes
> per interval on the sender but occasionally an interval with several
> hundred KB.
> 
> That is it seems like there's a buffer somewhere that's very slowly
> draining into the receiver, then getting topped up in an instant once
> it gets low enough.
> 
> When I have both this patch and the RCVBUF clamp, I don't seem to be
> able to reproduce the trickle-stall anymore, but I still get the slow
> transfer speeds most, but not every time.  Sometimes, but only rarely,
> I do seem to still get a complete stall (0 bytes on both sides).

I noted another oddity.  With this patch, _no_ RCVBUF clamp and 16MiB
wmem_max fixed, things seem to behave much better with a small
rmem_max than large.  With rmem_max=256KiB I get pretty consistent
37Gbps throughput and iperf3 -c reports 0 retransmits.

With rmem_max=16MiB, the throughput fluctuates from second to second
between ~3Gbps and ~30Gbps.  The client reports retransmits in some
intervals, which is pretty weird over lo.

Urgh... so many variables.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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