linux-kernel - Re: [PATCH v1] ringbuffer: Don't choose the process with adj equal OOM_SCORE_ADJ

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Message-ID: <20180410090128.GY21835@dhcp22.suse.cz>
Date:   Tue, 10 Apr 2018 11:01:28 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Zhaoyang Huang <huangzhaoyang@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] ringbuffer: Don't choose the process with adj equal
 OOM_SCORE_ADJ_MIN

On Tue 10-04-18 16:38:32, Zhaoyang Huang wrote:
> On Tue, Apr 10, 2018 at 4:12 PM, Michal Hocko <mhocko@...nel.org> wrote:
> > On Tue 10-04-18 16:04:40, Zhaoyang Huang wrote:
> >> On Tue, Apr 10, 2018 at 3:49 PM, Michal Hocko <mhocko@...nel.org> wrote:
> >> > On Tue 10-04-18 14:39:35, Zhaoyang Huang wrote:
> >> >> On Tue, Apr 10, 2018 at 2:14 PM, Michal Hocko <mhocko@...nel.org> wrote:
> > [...]
> >> >> > OOM_SCORE_ADJ_MIN means "hide the process from the OOM killer completely".
> >> >> > So what exactly do you want to achieve here? Because from the above it
> >> >> > sounds like opposite things. /me confused...
> >> >> >
> >> >> Steve's patch intend to have the process be OOM's victim when it
> >> >> over-allocating pages for ring buffer. I amend a patch over to protect
> >> >> process with OOM_SCORE_ADJ_MIN from doing so. Because it will make
> >> >> such process to be selected by current OOM's way of
> >> >> selecting.(consider OOM_FLAG_ORIGIN first before the adj)
> >> >
> >> > I just wouldn't really care unless there is an existing and reasonable
> >> > usecase for an application which updates the ring buffer size _and_ it
> >> > is OOM disabled at the same time.
> >> There is indeed such kind of test case on my android system, which is
> >> known as CTS and Monkey etc.
> >
> > Does the test simulate a real workload? I mean we have two things here
> >
> > oom disabled task and an updater of the ftrace ring buffer to a
> > potentially large size. The second can be completely isolated to a
> > different context, no? So why do they run in the single user process
> > context?
> ok. I think there are some misunderstandings here. Let me try to
> explain more by my poor English. There is just one thing here. The
> updater is originally a oom disabled task with adj=OOM_SCORE_ADJ_MIN.
> With Steven's patch, it will periodically become a oom killable task
> by calling set_current_oom_origin() for user process which is
> enlarging the ring buffer. What I am doing here is limit the user
> process to the ones that adj > -1000.

I've understood that part. And I am arguing whether this is really such
an important case to play further tricks. Wouldn't it be much simpler to
put the updater out to a separate process? OOM disabled processes
shouldn't really do unexpectedly large allocations. Full stop. Otherwise
you risk a large system disruptions.
-- 
Michal Hocko
SUSE Labs