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From:   Zhaoyang Huang <huangzhaoyang@gmail.com>
Date:   Tue, 10 Apr 2018 17:32:44 +0800
Message-ID: <CAGWkznEf0siz1-RFSJwuKCjUtbKM=P1djo2Nc7sU1GS0AgWv2w@mail.gmail.com>
Subject: Re: [PATCH v1] ringbuffer: Don't choose the process with adj equal OOM_SCORE_ADJ_MIN
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Tue, Apr 10, 2018 at 5:01 PM, Michal Hocko <mhocko@kernel.org> wrote:
> On Tue 10-04-18 16:38:32, Zhaoyang Huang wrote:
>> On Tue, Apr 10, 2018 at 4:12 PM, Michal Hocko <mhocko@kernel.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@kernel.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@kernel.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.
> --
It is a real problem(my android system just hung there while running
the test case for the innocent key process killed by OOM), however,
the problem is we can not define the userspace's behavior as you
suggested. What Steven's patch doing here is to keep the system to be
stable by having the updater to take the responsbility itself. My
patch is to let the OOM disabled processes remain the unkillable
status.

> Michal Hocko
> SUSE Labs