linux-kernel - Re: [RFC 0/8] Additional kmsg devices

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Message-id: <5596A58F.7090208@samsung.com>
Date:	Fri, 03 Jul 2015 17:09:03 +0200
From:	Marcin Niesluchowski <m.niesluchow@...sung.com>
To:	Richard Weinberger <richard.weinberger@...il.com>
Cc:	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"open list:ABI/API" <linux-api@...r.kernel.org>,
	Jonathan Corbet <corbet@....net>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Petr Mladek <pmladek@...e.cz>, Tejun Heo <tj@...nel.org>,
	Kay Sievers <kay@...y.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Joe Perches <joe@...ches.com>,
	Karol Lewandowski <k.lewandowsk@...sung.com>,
	Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>
Subject: Re: [RFC 0/8] Additional kmsg devices

On 07/03/2015 01:21 PM, Richard Weinberger wrote:
> On Fri, Jul 3, 2015 at 12:49 PM, Marcin Niesluchowski
> <m.niesluchow@...sung.com> 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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