[RFC] Exclusive gpu access for SteamVR usecases
Christian König
deathsimple at vodafone.de
Wed May 31 06:53:40 UTC 2017
>
>> 2. How are the priorities from processes supposed to interact with
>> the per context priority?
>
> Do you mean process niceness?
>
> There isn't any relationship between niceness and gpu priority.
>
> Let me know if you meant something different here.
I meant something different.
The application controls the per context priority when creating the
context and the compositor should control the per process priority.
Those two needs to be handled separately, otherwise we would could
override the context priority from the compositor and so confuse the
application.
I suggest to cleanly separate the two.
> (ii) Job dependencies between two processes.
>
> This case is mentioned separately as it is probably the most common
> use case we will encounter for this feature. Most graphics
> applications enter producer/consumer relationship with the compositor
> process (window swapchain).
>
> In this case the compositor should already have all the information
> required to avoid a deadlock.
That is nonsense. The kernel is the supervisor of resource management,
so only the kernel knows how to avoid a deadlock.
Let's imagine the following example:
Process A is a low priority task (for example updating the clock bitmap)
which needs a resource for it's command submission.
Process B is a high priority task (rendering of the VR) which needs a
bunch of memory for it's command submission.
Now the kernel memory management decides that it needs to evict process
A from VRAM to make room for the command submission of process B. To do
this all command submissions of process A need to finish.
In this moment the compositor hands over exclusive access to process B
and never gives process A a chance to run.
Now B depends on A, but A can never run because B has exclusive access
-> deadlock.
We somehow need to handle this inside the kernel or this whole approach
won't work.
Regards,
Christian.
Am 30.05.2017 um 23:38 schrieb Andres Rodriguez:
>
>
> On 2017-05-30 11:19 AM, Christian König wrote:
>> Looks like a good start, but a few notes in general:
>>
>> 1. Split the patches into two sets.
>>
>> One for implementing changing the priorities and one for limiting the
>> priorities.
>>
>
> No problem.
>
>> 2. How are the priorities from processes supposed to interact with
>> the per context priority?
>
> Do you mean process niceness?
>
> There isn't any relationship between niceness and gpu priority.
>
> Let me know if you meant something different here.
>
>>
>> 3. Thinking more about it we can't limit the minimum priority in the
>> scheduler.
>>
>> For example a low priority job might block resources the high
>> priority job needs to run. E.g. VRAM memory.
>>
>
> We avoid deadlocks by making sure that all dependencies of an
> exclusive task are also elevated to the same priority as said task.
> Usermode (the DRM_MASTER) is responsible to maintain this guarantee.
> The kernel does provide an ioctl that makes this task simple,
> amdgpu_sched_process_priority_set().
>
> Lets take a look at this issue through three different scenarios.
>
> (i) Job dependencies are all process internal, i.e. multiple contexts
> in one process.
>
> This is the trivial case. A call to
> amdgpu_sched_process_priority_set() will change the priority of all
> contexts belonging to a process in lockstep.
>
> Once amdgpu_sched_process_priority_set() returns, it is safe to raise
> the minimum priority using amdgpu_sched_min_priority_get(). At this
> point we have a guarantee that all contexts belonging to the process
> will be in a runnable state, or all the contexts will be in a
> not-runnable state. There won't be a mix of runnable and non-runnable
> processes.
>
> Getting into that mixed state is what could cause a deadlock, a
> runnable context depends on a non-runnable one.
>
> Note: the current patchset needs a fix to provide this guarantee in
> multi-gpu systems.
>
> (ii) Job dependencies between two processes.
>
> This case is mentioned separately as it is probably the most common
> use case we will encounter for this feature. Most graphics
> applications enter producer/consumer relationship with the compositor
> process (window swapchain).
>
> In this case the compositor should already have all the information
> required to avoid a deadlock. It knows:
> - Itself (as a process)
> - The application process
> - The dependencies between both processes
>
> At this stage it is simple for the compositor to understand that if it
> wishes to perform an exclusive mode transition, all dependencies
> (which are known) should also be part of the exclusive group.
>
> We should be able to implement this feature without modifying a
> game/application.
>
> (iii) Job dependencies between multiple (3+) processes.
>
> This scenario is very uncommon for games. For example, if a game or
> application is split into multiple processes. Process A interacts with
> the compositor. Process B does some physics/compute calculations and
> send the results to Process A.
>
> To support this use case, we would require an interface for the
> application to communicate to the compositor its dependencies. I.e.
> Process A would say, "Also keep Process B's priority in sync with
> mine". This should be a simple bit of plumbing to allow Process A to
> share an fd from Process B with the compositor.
>
> B --[pipe_send(fdB)]--> A
> --[compositor_ext_priority_group_add(fdB)]--> Compositor
>
> Once the compositor is aware of all of A's dependencies, this can be
> handled in the same fashion as (ii).
>
> A special extension would be required for compositor protocols to
> communicate the dependencies fd. Applications would also need to be
> updated to use this extension.
>
> I think this case would be very uncommon. But it is something that we
> would be able to handle if the need would arise.
>
> > We need something like blocking the submitter instead (bad) or
> detection
> > of dependencies in the scheduler (good, but tricky to implement).
> >
>
> I definitely agree that detecting dependencies is tricky. Which is why
> I prefer an approach where usermode defines the dependencies. It is
> simple for both the kernel and usermode to implement.
>
> > Otherwise we can easily run into a deadlock situation with that
> approach.
> >
>
> The current API does allow you to deadlock yourself pretty easily if
> misused. But so do many other APIs, like having a thread trying to
> grab the same lock twice :)
>
> Thanks for the comments,
> Andres
>
>> Regards,
>> Christian.
>>
>> Am 25.05.2017 um 02:00 schrieb Andres Rodriguez:
>>> When multiple environments are running simultaneously on a system, e.g.
>>> an X desktop + a SteamVR game session, it may be useful to sacrifice
>>> performance in one environment in order to boost it on the other.
>>>
>>> This series provides a mechanism for a DRM_MASTER to provide exclusive
>>> gpu access to a group of processes.
>>>
>>> Note: This series is built on the assumption that the drm lease
>>> patch series
>>> will extend DRM_MASTER status to lesees.
>>>
>>> The libdrm we intend to provide is as follows:
>>>
>>> /**
>>> * Set the priority of all contexts in a process
>>> *
>>> * This function will change the priority of all contexts owned by
>>> * the process identified by fd.
>>> *
>>> * \param dev - \c [in] device handle
>>> * \param fd - \c [in] fd from target process
>>> * \param priority - \c [in] target priority
>>> AMDGPU_CTX_PRIORITY_*
>>> *
>>> * \return 0 on success\n
>>> * <0 - Negative POSIX error code
>>> *
>>> * \notes @fd can be *any* file descriptor from the target process.
>>> * \notes this function requires DRM_MASTER
>>> */
>>> int amdgpu_sched_process_priority_set(amdgpu_device_handle dev,
>>> int fd, int32_t priority);
>>>
>>> /**
>>> * Request to raise the minimum required priority to schedule a gpu
>>> job
>>> *
>>> * Submit a request to increase the minimum required priority to
>>> schedule
>>> * a gpu job. Once this function returns, the gpu scheduler will no
>>> longer
>>> * consider jobs from contexts with priority lower than @priority.
>>> *
>>> * The minimum priority considered by the scheduler will be the
>>> highest from
>>> * all currently active requests.
>>> *
>>> * Requests are refcounted, and must be balanced using
>>> * amdgpu_sched_min_priority_put()
>>> *
>>> * \param dev - \c [in] device handle
>>> * \param priority - \c [in] target priority
>>> AMDGPU_CTX_PRIORITY_*
>>> *
>>> * \return 0 on success\n
>>> * <0 - Negative POSIX error code
>>> *
>>> * \notes this function requires DRM_MASTER
>>> */
>>> int amdgpu_sched_min_priority_get(amdgpu_device_handle dev,
>>> int32_t priority);
>>>
>>> /**
>>> * Drop a request to raise the minimum required scheduler priority
>>> *
>>> * This call balances amdgpu_sched_min_priority_get()
>>> *
>>> * If no other active requests exists for @priority, the minimum
>>> required
>>> * priority will decay to a lower level until one is reached with
>>> an active
>>> * request or the lowest priority is reached.
>>> *
>>> * \param dev - \c [in] device handle
>>> * \param priority - \c [in] target priority
>>> AMDGPU_CTX_PRIORITY_*
>>> *
>>> * \return 0 on success\n
>>> * <0 - Negative POSIX error code
>>> *
>>> * \notes this function requires DRM_MASTER
>>> */
>>> int amdgpu_sched_min_priority_put(amdgpu_device_handle dev,
>>> int32_t priority);
>>>
>>> Using this app, VRComposer can raise the priority of the VRapp and
>>> itself. Then
>>> it can restrict the minimum scheduler priority in order to become
>>> exclusive gpu
>>> clients.
>>>
>>> One of the areas I'd like feedback is the following scenario. If a
>>> VRapp opens
>>> a new fd and creates a new context after a call to set_priority,
>>> this specific
>>> context will be lower priority than the rest. If the minimum
>>> required priority
>>> is then raised, it is possible that this new context will be starved
>>> and
>>> deadlock the VRapp.
>>>
>>> One solution I had in mind to address this situation, is to make
>>> set_priority
>>> also raise the priority of future contexts created by the VRapp.
>>> However, that
>>> would require keeping track of the requested priority on a
>>> per-process data
>>> structure. The current design appears to steer clean of keeping any
>>> process
>>> specific data, and everything instead of stored on a per-file basis.
>>> Which is
>>> why I did not pursue this approach. But if this is something you'd
>>> like me to
>>> implement let me know.
>>>
>>> One could also argue that preventing an application deadlock should
>>> be handled
>>> between the VRComposer and the VRApp. It is not the kernel's
>>> responsibility to
>>> babysit userspace applications and prevent themselves from shooting
>>> themselves
>>> in the foot. The same could be achieved by improper usage of shared
>>> fences
>>> between processes.
>>>
>>> Thoughts/feedback/comments on this issue, or others, are appreciated.
>>>
>>> Regards,
>>> Andres
>>>
>>
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