[PATCH] drm/radeon: add an exclusive lock for GPU reset

[Jerome Glisse]

On Tue, Jul 3, 2012 at 10:52 AM, Christian König
<deathsimple at vodafone.de> wrote:
> On 03.07.2012 16:09, Jerome Glisse wrote:
>>
>> On Tue, Jul 3, 2012 at 5:26 AM, Christian König <deathsimple at vodafone.de>
>> wrote:
>>>
>>> [SNIP]
>>>
>>> Yeah, but I thought that fixing those oops as the second step. I see the
>>> fact that suspend/resume is unpinning all the ttm memory and then pinning
>>> it
>>> again as a bug that needs to be fixed. Or as an alternative we could
>>> split
>>> the suspend/resume calls into suspend/disable and resume/enable calls,
>>> where
>>> we only call disable/enable in the gpu_reset path rather than a complete
>>> suspend/resume (not really sure about that).
>>
>> Fixing oops are not second step, they are first step. I am not saying
>> that the suspend/resume as it happens right now is a good thing, i am
>> saying it's what it's and we have to deal with it until we do
>> something better. There is no excuse to not fix oops with a simple
>> patch 16 lines patch.
>
> Completely agree.
>
>
>>> Also a GPU reset isn't such a rare event, currently it just occurs when
>>> userspace is doing something bad, for example submitting an invalid
>>> shader
>>> or stuff like that. But with VM and IO protection coming into the driver
>>> we
>>> are going to need a GPU reset when there is an protection fault, and with
>>> that it is really desirable to just reset the hardware in a way where we
>>> can
>>> say: This IB was faulty skip over it and resume with whatever is after it
>>> on
>>> the ring.
>>
>> There is mecanism to handle that properly from irq handler that AMD
>> need to release, the pagefault thing could be transparent and should
>> only need few lines in the irq handler (i think i did a patch for that
>> and sent it to AMD for review but i am wondering if i wasn't lacking
>> some doc).
>
> I also searched the AMD internal docs for a good description of how the
> lightweight recovery is supposed to work, but haven't found anything clear
> so far. My expectation is that there should be something like a "abort
> current IB" command you can issue by writing an register, but that doesn't
> seems to be the case.

Doc are scarce on that, my understanding is that you can't skip IB
that trigger the segfault. The handling was designed with page fault
in mind, ie you page in the missing page and you resume the ib. That
being said i think one can abuse that by moving the rptr of the ring
and resuming.

>>> And todo that we need to keep the auxiliary data like sub allocator
>>> memory,
>>> blitting shader bo, and especially vm page tables at the same place in
>>> hardware memory.
>>
>> I agree that we need a lightweight reset but we need to keep the heavy
>> reset as it is right now, if you want to do a light weight reset do it
>> as a new function. I always intended to have two reset path, hint gpu
>> soft reset name vs what is call hard reset but not released, i even
>> made patch for that long time ago but never got them cleared from AMD
>> review.
>
> My idea was to pass in some extra informations, so asic_reset more clearly
> knows what todo. An explicit distinction between a soft and a hard reset
> also seems like a possible solution, but sounds like a bit of code
> duplication.

I think people should stop on the mantra of don't duplicate code, i do
agree that it's better to not duplicate code but in some case there is
no way around it and it's worse to try sharing the code, i think the
worst example in radeon kernel is the modesetting section where we try
to share same code for all different variation of DCE but in the end
it's a mess of if/else/switch/case and what not. Code duplication is
sometime the cleanest and best choice, might we fix a bug in one and
not the other yes it might happen but it's life ;)

>
>>>> I stress it we need at very least a mutex to protect gpu reset and i
>>>> will stand on that position because there is no other way around.
>>>> Using rw lock have a bonus of allowing proper handling of gpu reset
>>>> failure and that what the patch i sent to linus fix tree is about, so
>>>> to make drm next merge properly while preserving proper behavior in
>>>> gpu reset failure the rw semaphore is the best option.
>>>
>>> Oh well, I'm not arguing that we don't need a look here. I'm just
>>> questioning to put it at the ioctl level (e.g. the driver entry from
>>> userspace), that wasn't such a good idea with the cs_mutex and doesn't
>>> seems
>>> like a good idea now. Instead we should place the looking between the
>>> ioctl/ttm and the actual hardware submission and that brings it pretty
>>> close
>>> (if not identically) to the ring mutex.
>>>
>>> Cheers,
>>> Christian.
>>
>> No, locking at the ioctl level make sense please show me figure that
>> it hurt performance, i did a quick sysprof and i couldn't see them
>> impacting anything. No matter how much you hate this, this is the best
>> solution, it avoids each ioctl to do useless things in case of gpu
>> lockup and it touch a lot less code than moving a lock down the call
>> path would. So again this is the best solution for the heavy reset,
>> and i am not saying that a soft reset would need to take this lock or
>> that we can't improve the way it's done. All i am saying is that ring
>> lock is the wrong solution for heavy reset, it should be ok for light
>> weight reset.
>>
> I'm not into any performance concerns, it just doesn't seems to be the right
> place to me. So are you really sure that the ttm_bo_delayed_workqueue,
>  pageflips or callbacks to radeon_bo_move can't hit us here? IIRC that
> always was the big concern with the cs_mutex not being held in all cases.
>
> Anyway, if you think it will work and fix the crash problem at hand then I'm
> ok with commit it.

On GPU lockup the assumption have always been that irq handler won't
be call. Thus no pageflip. For ttm the reset function suspend ttm
delayed workqueue. Suspending bo creation/deletion of domain change
like my patch does avoid the other possibilities of any ttm work
starting. I don't think i have miss anything, i am quite confident i
did not, ... but you never know.

Cheers,
Jerome