Date: Mon, 26 Jun 2023 23:34:34 -0700 From: bob prohaska <fbsd@www.zefox.net> To: Mark Millard <marklmi@yahoo.com> Cc: Jamie Landeg-Jones <jamie@catflap.org>, freebsd-net@freebsd.org, freebsd-arm@freebsd.org Subject: Re: -current dropping ssh connections Message-ID: <ZJqC%2BgEcAD5uwilE@www.zefox.net> In-Reply-To: <601E35C4-CDE1-47A3-B2D2-29501AB5FB93@yahoo.com> References: <ZJMyPquk32fXhT%2BI@www.zefox.net> <E84D47F1-4E75-4DA1-8EE2-3E2F36DF0EF1@yahoo.com> <ZJM7VXyWotSGwAtH@www.zefox.net> <202306212305.35LN5ITH069587@donotpassgo.dyslexicfish.net> <ZJR5FzEEFofoh8f1@www.zefox.net> <202306222238.35MMcoQm017939@donotpassgo.dyslexicfish.net> <ZJpCqCVzcMGIruMU@www.zefox.net> <601E35C4-CDE1-47A3-B2D2-29501AB5FB93@yahoo.com>
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On Mon, Jun 26, 2023 at 07:38:22PM -0700, Mark Millard wrote: > On Jun 26, 2023, at 19:00, bob prohaska <fbsd@www.zefox.net> wrote: > > > On Thu, Jun 22, 2023 at 11:38:50PM +0100, Jamie Landeg-Jones wrote: > >> bob prohaska <fbsd@www.zefox.net> wrote: > >> > >>> That seems worth a try. > >>> The notion of an ssh escape (~. in this case) finding its way into the data stream is new to me. > >> > >> Thinking again, that looks like corruption coming down the ssh connection. > >> For the ssh escape char to affect anything (note it needs to be preceeded > >> by a new line) it would have to be sent up the line. > >> > >> For an example, assuming ssh has the default escape char, look at the difference between: > >> > >> printf 'pwd;\n~.;echo sleeping.; sleep 5' | ssh -tt user@host > >> > >> and with the same printf, but no escape char: > >> > >> printf 'pwd;\n~.;echo sleeping.; sleep 5' | ssh -tt -e none user@host > >> > >> You can see how the former closes the connection due to the \n~. > >> > >> (The -tt forces a terminal/interactive session to be set up - normally, as we are > >> piping input to ssh in this case, the terminal isnt set up, and the escape character > >> isn't used - it's only recongnised in interactive sesions by default) > >> > >> I personally have "EscapeChar none" in my ssh_config, but I suspect this > >> is probably not the issue here, still, can't hurt to try it! > > > > Indeed, connection was dropped with escape character set to none. > > There does seem to be a some dependence on system load. Connections > > survive at light or no load and drop when the sshd side is busy. > > So how busy is the disk media (really the USB bus) from > somewhat before the failures happen to the failure point? > The question applies separately to both systems involved > in the ssh session. > I've no quantitative measure. Is there any command I could add to the swap activity logger that can report USB load? > I'll remind that on a RPi2B v1.1, RPi2B v1.2, or RPi3B* > the Ethernet and the 4 USB ports all share the one USB2 > path at an internal stage for the USB subsystem, if I > remember right. > > Saturating that USB2 path with data transfer bytes > to/from disk media might be able to block other activity > long enough to cause the Ethernet communications to have > problems for all I know. (Amount of data queued for > transfer would be relevant.) > > This might be a reason to avoid settings for the > combination vm.pfault_oom_attempts and vm.pfault_oom_wait > to values that could contribute to a sustained period of > saturating the USB channel. > At this point the vm.* settings are default, with no warnings on the serial console apparent.. > (I have no detailed knowledge of how the tradeoffs work > for the competing uses of the USB channel. So I can not > validly claim to know that the above is realistic.) > It is tempting to think USB saturation would account for the trouble. A tool similar to gstat, but for USB as a whole, would be helpful if it exists. It's perhaps notable that the dropped connections are seen in connection with usb-serial adapters more than interactive connections running shells. Thanks for reading! bob prohaska
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