[systemd-devel] /dev/log tends to block on socket based activation ...

[Umut Tezduyar Lindskog]

Hi,

On Wed, Aug 6, 2014 at 8:36 AM, Hoyer, Marko (ADITG/SW2)
<mhoyer at de.adit-jv.com> wrote:
> Good morning everyone,
>
>
>
> I’m playing around a bit with systemd’s socket based activation of
> systemd-journald. My intention is to shift back in time the actual startup
> of systemd-journald.service to save resources (CPU) for early applications
> during startup. The respective socket is activated as usual to not lose any
> early (sys)logs. The actual startup of the service is delayed by adding some
> dependencies to targets (basic.target for instance).
>
>
>
> In principal, the idea is working as expected but sometimes the logging via
> syslog(“..”) blocks applications until the daemon is actually started.
> Depending on how the startup of such application is integrated into the
> startup configuration, this might lead to deadlock situations.
>
>
>
> Has anyone here any idea how one can prevent the blocking situation.
>
>
>
> Some observations:
>
> - The blocking situation is not happening on each syslog call, sometimes
> this happens after one call, sometimes after a few calls. I wasn’t able by
> now isolating the concrete case that leads to a blocking socket
>
> - I doubt that the underlying socket buffer is full
>
> - The call is blocked by the kernel syscall send() that is invoked by the
> syslog() call
>
>
>
> Thx in advance for any hints …
>
>

syslog() is a sync call and on the other side of the syslog() socket
is systemd-journald. If you deprioritize (or delay) systemd-journald,
eventually socket holding syslog() (it is /dev/log ->
/run/systemd/journal/dev-log) will get filled up and calls to syslog()
will wait until systemd-journald empties the socket buffer.

The default limit for /dev/log is 10 messages (implementation is 10 +
1, so it actually stores 11 messages) and can be changed by a similar
trick as I did on
http://lists.freedesktop.org/archives/systemd-devel/2014-July/021536.html.
In your case the socket you are interested in is
systemd-journald-dev-log.socket.

Let me know how it goes.
Umut

>
>
>
> Best regards
>
>
>
> Marko Hoyer
>
> Advanced Driver Information Technology GmbH
> Software Group II (ADITG/SW2)
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> 31139 Hildesheim
> Germany
>
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> Fax +49 5121 49 6999
> mhoyer at de.adit-jv.com
>
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