Re: [Intel-gfx] [PATCH 2/3] mm, notifier: Catch sleeping/blocking for !blockable

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 23/11/2018 13:12, Daniel Vetter wrote:
> On Fri, Nov 23, 2018 at 1:46 PM Michal Hocko <mhocko@kernel.org> wrote:
>>
>> On Fri 23-11-18 13:38:38, Daniel Vetter wrote:
>>> On Fri, Nov 23, 2018 at 12:12:37PM +0100, Michal Hocko wrote:
>>>> On Thu 22-11-18 17:51:05, Daniel Vetter wrote:
>>>>> We need to make sure implementations don't cheat and don't have a
>>>>> possible schedule/blocking point deeply burried where review can't
>>>>> catch it.
>>>>>
>>>>> I'm not sure whether this is the best way to make sure all the
>>>>> might_sleep() callsites trigger, and it's a bit ugly in the code flow.
>>>>> But it gets the job done.
>>>>
>>>> Yeah, it is quite ugly. Especially because it makes DEBUG config
>>>> bahavior much different. So is this really worth it? Has this already
>>>> discovered any existing bug?
>>>
>>> Given that we need an oom trigger to hit this we're not hitting this in CI
>>> (oom is just way to unpredictable to even try). I'd kinda like to also add
>>> some debug interface so I can provoke an oom kill of a specially prepared
>>> process, to make sure we can reliably exercise this path without killing
>>> the kernel accidentally. We do similar tricks for our shrinker already.
>>
>> Create a task with oom_score_adj = 1000 and trigger the oom killer via
>> sysrq and you should get a predictable oom invocation and execution.
> 
> Ah right. We kinda do that already in an attempt to get the tests
> killed without the runner, for accidental oom. Just didn't think about
> this in the context of intentionally firing the oom. I'll try whether
> I can bake up some new subtest in our userptr/mmu-notifier testcases.

Very handy trick - I think I will think of applying it in the shrinker 
area as well.

Regards,

Tvrtko