Re: [RFC 0/8] Additional kmsg devices

[Petr Mladek]

On Fri 2015-07-03 17:09:03, Marcin Niesluchowski wrote:
> 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.

But then many services will fight for the space in the kernel ring
buffer. We will need a mechanism to guarantee a space for each
service. We will need priorities to throttle various services
various ways. It will be easier to lost messages. It might be
harder to get the important messages on the console when
the system is going down. It will be harder to handle continuous
lines. I am not sure that we want to go this way.

Best Regards,
Petr
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