linux-kernel - Re: [PATCH 00/10] OOM Debug print selection and additional information

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Message-ID: <20190828111248.GE28313@dhcp22.suse.cz>
Date:   Wed, 28 Aug 2019 13:12:48 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Edward Chron <echron@...sta.com>, Qian Cai <cai@....pw>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Johannes Weiner <hannes@...xchg.org>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Ivan Delalande <colona@...sta.com>
Subject: Re: [PATCH 00/10] OOM Debug print selection and additional
 information

On Wed 28-08-19 19:56:58, Tetsuo Handa wrote:
> On 2019/08/28 19:32, Michal Hocko wrote:
> >> Speak of my cases, those who take care of their systems are not developers.
> >> And they afraid changing code that runs in kernel mode. They unlikely give
> >> permission to install SystemTap/eBPF scripts. As a result, in many cases,
> >> the root cause cannot be identified.
> > 
> > Which is something I would call a process problem more than a kernel
> > one. Really if you need to debug a problem you really have to trust
> > those who can debug that for you. We are not going to take tons of code
> > to the kernel just because somebody is afraid to run a diagnostic.
> > 
> 
> This is a problem of kernel development process.

I disagree. Expecting that any larger project can be filled with the
(close to) _full_ and ready to use introspection built in is just
insane. We are trying to help with a generally useful information but
you simply cannot cover most existing failure paths.

> >> Moreover, we are talking about OOM situations, where we can't expect userspace
> >> processes to work properly. We need to dump information we want, without
> >> counting on userspace processes, before sending SIGKILL.
> > 
> > Yes, this is an inherent assumption I was making and that means that
> > whatever dynamic hooks would have to be registered in advance.
> > 
> 
> No. I'm saying that neither static hooks nor dynamic hooks can work as
> expected if they count on userspace processes. Registering in advance is
> irrelevant. Whether it can work without userspace processes is relevant.

I am not saying otherwise. I do not expect any userspace process to dump
any information or read it from elswhere than from the kernel log.

> Also, out-of-tree codes tend to become defunctional. We are trying to debug
> problems caused by in-tree code. Breaking out-of-tree debugging code just
> because in-tree code developers don't want to pay the burden of maintaining
> code for debugging problems caused by in-tree code is a very bad idea.

This is a simple math of cost/benefit. The maintenance cost is not free
and paying it for odd cases most people do not care about is simply not
sustainable, we simply do not have that much of a man power.
-- 
Michal Hocko
SUSE Labs