linux-kernel - Re: [PATCH 2/3] drm/scheduler: Don't call wait_event_killable for signaled process.

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Date:   Mon, 30 Apr 2018 17:25:35 +0200
From:   Christian König <ckoenig.leichtzumerken@...il.com>
To:     Andrey Grodzovsky <Andrey.Grodzovsky@....com>,
        christian.koenig@....com,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     David.Panariti@....com, Oleg Nesterov <oleg@...hat.com>,
        amd-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Alexander.Deucher@....com, akpm@...ux-foundation.org
Subject: Re: [PATCH 2/3] drm/scheduler: Don't call wait_event_killable for
 signaled process.

Am 30.04.2018 um 16:32 schrieb Andrey Grodzovsky:
>
>
> On 04/30/2018 08:08 AM, Christian König wrote:
>> Hi Eric,
>>
>> sorry for the late response, was on vacation last week.
>>
>> Am 26.04.2018 um 02:01 schrieb Eric W. Biederman:
>>> Andrey Grodzovsky <Andrey.Grodzovsky@....com> writes:
>>>
>>>> On 04/25/2018 01:17 PM, Oleg Nesterov wrote:
>>>>> On 04/25, Andrey Grodzovsky wrote:
>>>>>> here (drm_sched_entity_fini) is also a bad idea, but we still 
>>>>>> want to be
>>>>>> able to exit immediately
>>>>>> and not wait for GPU jobs completion when the reason for reaching 
>>>>>> this code
>>>>>> is because of KILL
>>>>>> signal to the user process who opened the device file.
>>>>> Can you hook f_op->flush method?
>>
>> THANKS! That sounds like a really good idea to me and we haven't 
>> investigated into that direction yet.
>>
>>>> But this one is called for each task releasing a reference to the 
>>>> the file, so
>>>> not sure I see how this solves the problem.
>>> The big question is why do you need to wait during the final closing a
>>> file?
>>
>> As always it's because of historical reasons. Initially user space 
>> pushed commands directly to a hardware queue and when a processes 
>> finished we didn't need to wait for anything.
>>
>> Then the GPU scheduler was introduced which delayed pushing the jobs 
>> to the hardware queue to a later point in time.
>>
>> This wait was then added to maintain backward compability and not 
>> break userspace (but see below).
>>
>>> The wait can be terminated so the wait does not appear to be simply a
>>> matter of correctness.
>>
>> Well when the process is killed we don't care about correctness any 
>> more, we just want to get rid of it as quickly as possible (OOM 
>> situation etc...).
>>
>> But it is perfectly possible that a process submits some render 
>> commands and then calls exit() or terminates because of a SIGTERM, 
>> SIGINT etc.. In this case we need to wait here to make sure that all 
>> rendering is pushed to the hardware because the scheduler might need 
>> resources/settings from the file descriptor.
>>
>> For example if you just remove that wait you could close firefox and 
>> get garbage on the screen for a millisecond because the remaining 
>> rendering commands where not executed.
>>
>> So what we essentially need is to distinct between a SIGKILL (which 
>> means stop processing as soon as possible) and any other reason 
>> because then we don't want to annoy the user with garbage on the 
>> screen (even if it's just for a few milliseconds).
>>
>> Constructive ideas how to handle this would be very welcome, cause I 
>> completely agree that what we have at the moment by checking 
>> PF_SIGNAL is just a very very hacky workaround.
>
> What about changing PF_SIGNALED to  PF_EXITING in 
> drm_sched_entity_do_release
>
> -       if ((current->flags & PF_SIGNALED) && current->exit_code == 
> SIGKILL)
> +      if ((current->flags & PF_EXITING) && current->exit_code == 
> SIGKILL)
>
> From looking into do_exit and it's callers , current->exit_code will 
> get assign the signal which was delivered to the task. If SIGINT was 
> sent then it's SIGINT, if SIGKILL then SIGKILL.

That's at least a band aid to stop us from abusing PF_SIGNALED.

But additional to that change, can you investigate when f_ops->flush() 
is called when the process exists normally, because of SIGKILL or 
because of some other signal?

Could be that this is more closely to what we are searching for,
Christian.

>
> Andrey
>
>
>>
>> Thanks,
>> Christian.
>
>
>>
>>>
>>> Eric
>>> _______________________________________________
>>> amd-gfx mailing list
>>> amd-gfx@...ts.freedesktop.org
>>> https://lists.freedesktop.org/mailman/listinfo/amd-gfx
>>
>
> _______________________________________________
> amd-gfx mailing list
> amd-gfx@...ts.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/amd-gfx

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