linux-kernel - Re: [PATCH 2/3] drm/scheduler: Don't call wait_event

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Message-ID: <87a7trwbh0.fsf@xmission.com>
Date:   Wed, 25 Apr 2018 10:29:47 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Andrey Grodzovsky <Andrey.Grodzovsky@....com>
Cc:     David.Panariti@....com,
        Michel Dänzer <michel@...nzer.net>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        oleg@...hat.com, amd-gfx@...ts.freedesktop.org,
        Alexander.Deucher@....com, akpm@...ux-foundation.org,
        Christian.Koenig@....com
Subject: Re: [PATCH 2/3] drm/scheduler: Don't call wait_event_killable for signaled process.

Andrey Grodzovsky <Andrey.Grodzovsky@....com> writes:

> On 04/25/2018 03:14 AM, Daniel Vetter wrote:
>> On Tue, Apr 24, 2018 at 05:37:08PM -0400, Andrey Grodzovsky wrote:
>>>
>>> On 04/24/2018 05:21 PM, Eric W. Biederman wrote:
>>>> Andrey Grodzovsky <Andrey.Grodzovsky@....com> writes:
>>>>
>>>>> On 04/24/2018 03:44 PM, Daniel Vetter wrote:
>>>>>> On Tue, Apr 24, 2018 at 05:46:52PM +0200, Michel Dänzer wrote:
>>>>>>> Adding the dri-devel list, since this is driver independent code.
>>>>>>>
>>>>>>>
>>>>>>> On 2018-04-24 05:30 PM, Andrey Grodzovsky wrote:
>>>>>>>> Avoid calling wait_event_killable when you are possibly being called
>>>>>>>> from get_signal routine since in that case you end up in a deadlock
>>>>>>>> where you are alreay blocked in singla processing any trying to wait
>>>>>>> Multiple typos here, "[...] already blocked in signal processing and [...]"?
>>>>>>>
>>>>>>>
>>>>>>>> on a new signal.
>>>>>>>>
>>>>>>>> Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@....com>
>>>>>>>> ---
>>>>>>>>     drivers/gpu/drm/scheduler/gpu_scheduler.c | 5 +++--
>>>>>>>>     1 file changed, 3 insertions(+), 2 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/drivers/gpu/drm/scheduler/gpu_scheduler.c b/drivers/gpu/drm/scheduler/gpu_scheduler.c
>>>>>>>> index 088ff2b..09fd258 100644
>>>>>>>> --- a/drivers/gpu/drm/scheduler/gpu_scheduler.c
>>>>>>>> +++ b/drivers/gpu/drm/scheduler/gpu_scheduler.c
>>>>>>>> @@ -227,9 +227,10 @@ void drm_sched_entity_do_release(struct drm_gpu_scheduler *sched,
>>>>>>>>     		return;
>>>>>>>>     	/**
>>>>>>>>     	 * The client will not queue more IBs during this fini, consume existing
>>>>>>>> -	 * queued IBs or discard them on SIGKILL
>>>>>>>> +	 * queued IBs or discard them when in death signal state since
>>>>>>>> +	 * wait_event_killable can't receive signals in that state.
>>>>>>>>     	*/
>>>>>>>> -	if ((current->flags & PF_SIGNALED) && current->exit_code == SIGKILL)
>>>>>>>> +	if (current->flags & PF_SIGNALED)
>>>>>> You want fatal_signal_pending() here, instead of inventing your own broken
>>>>>> version.
>>>>> I rely on current->flags & PF_SIGNALED because this being set from
>>>>> within get_signal,
>>>> It doesn't mean that.  Unless you are called by do_coredump (you
>>>> aren't).
>>> Looking in latest code here
>>> https://elixir.bootlin.com/linux/v4.17-rc2/source/kernel/signal.c#L2449
>>> i see that current->flags |= PF_SIGNALED; is out side of
>>> if (sig_kernel_coredump(signr)) {...} scope
>> Ok I read some more about this, and I guess you go through process exit
>> and then eventually close. But I'm not sure.
>>
>> The code in drm_sched_entity_fini also looks strange: You unpark the
>> scheduler thread before you remove all the IBs. At least from the comment
>> that doesn't sound like what you want to do.
>
> I think it should be safe for the dying scheduler entity since before that (in
> drm_sched_entity_do_release) we set it's runqueue to NULL
> so no new jobs will be dequeued form it by the scheduler thread.
>
>>
>> But in general, PF_SIGNALED is really something deeply internal to the
>> core (used for some book-keeping and accounting). The drm scheduler is the
>> only thing looking at it, so smells like a layering violation. I suspect
>> (but without knowing what you're actually trying to achive here can't be
>> sure) you want to look at something else.
>>
>> E.g. PF_EXITING seems to be used in a lot more places to cancel stuff
>> that's no longer relevant when a task exits, not PF_SIGNALED. There's the
>> TIF_MEMDIE flag if you're hacking around issues with the oom-killer.
>>
>> This here on the other hand looks really fragile, and probably only does
>> what you want to do by accident.
>> -Daniel
>
> Yes , that what Eric also said and in the V2 patches i will try  to change
> PF_EXITING
>
> Another issue is changing wait_event_killable to wait_event_timeout where I need
> to understand
> what TO value is acceptable for all the drivers using the scheduler, or maybe it
> should come as a property
> of drm_sched_entity.

It would not surprise me if you could pick a large value like 1 second
and issue a warning if that time outever triggers.  It sounds like the
condition where we wait indefinitely today is because something went
wrong in the driver.

Eric