Re: [RFC 0/8] Additional kmsg devices

On Tue 2015-07-07 19:10:56, Karol Lewandowski wrote:

On 2015-07-07 15:11, Petr Mladek wrote:

quoted

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

quoted

On 07/03/2015 01:21 PM, Richard Weinberger wrote:

quoted

On Fri, Jul 3, 2015 at 12:49 PM, Marcin Niesluchowski
[off-list ref] wrote:

quoted

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.

Yes.   Please note however that problems you describe are also valid for
/dev/kmsg today.

Mea culpa, I should not write when I do not have enough time to read
the patches. I somehow missed the patch that added more buffers, so
many opinions were misleading.

quoted

It will be harder to handle continuous lines.

I don't see how it would be different from what we have today.

There is currently only one buffer for continuous lines. It is flushed
when you write from another CPU even before the current message is completed.
It needs to be flushed also when you write from another devkmsg.

I thought that there would be much bigger chance to mix parts of
messages in a single buffer but it is not true after all.


Best Regards,
Petr