Re: [RFC 0/8] Additional kmsg devices

[Marcin Niesluchowski]

On 07/03/2015 01:21 PM, Richard Weinberger wrote:
> On Fri, Jul 3, 2015 at 12:49 PM, Marcin Niesluchowski
> <m.niesluchow@xxxxxxxxxxx> wrote:
> > Dear All,
> >
> > This series of patches extends kmsg interface with ability to dynamicaly
> > create (and destroy) kmsg-like devices which can be used by user space
> > for logging. Logging to kernel has number of benefits, including but not
> > limited to - always available, requiring no userspace, automatically
> > rotating and low overhead.
> >
> > User-space logging to kernel cyclic buffers was already successfully used
> > in android logger concept but it had certain flaws that this commits try
> > to address:
> > * drops hardcoded number of devices and static paths in favor for dynamic
> >    configuration by ioctl interface in userspace
> > * extends existing driver instead of creating completely new one
> So, now we start moving syslogd into kernel land because userspace is
> too broken to provide
> decent logging?
>
> I can understand the systemd is using kmsg if no other logging service
> is available
> but I really don't think we should encourage other programs to do so.
>
> Why can't you just make sure that your target has a working
> syslogd/rsyslogd/journald/whatever?
> All can be done perfectly fine in userspace.
* Message credibility: Lets imagine simple service which collects logs 
via unix sockets. There is no reliable way of identifying logging 
process. getsockopt() with SO_PEERCRED option would give pid form cred 
structure, but according to manual it may not be of actual logging process:
  "The returned credentials are those that were in effect at the time 
of the call to connect(2) or socketpair(2)."
      - select(7)

* Early userspace tool: Helpful especially for embeded systems.

* Reliability: Userspace service may be killed due to out of memory 
(OOM). This is kernel cyclic buffer, which size can be specified 
differently according to situation.

* Possibility of using it with pstore: This code could be extended to 
log additional buffers to persistent storage same way main (kmsg) log 
buffer is.

* Use case of attaching file descriptor to stdout/stderr: Especially in 
early userspace.

* Performance: Those services mentioned by You are weeker solutions in 
that case. Especially systemd-journald is much too heavy soulution.

--

Best Regards,
Marcin Niesluchowski

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