[PATCH 2/5] drm/nouveau: add reservation to nouveau_gem_ioctl_cpu_prep

[Thomas Hellstrom]

On 01/22/2014 11:58 AM, Maarten Lankhorst wrote:
> op 22-01-14 11:27, Thomas Hellstrom schreef:
>> On 01/22/2014 10:55 AM, Maarten Lankhorst wrote:
>>> op 22-01-14 10:40, Thomas Hellstrom schreef:
>>>> On 01/22/2014 09:19 AM, Maarten Lankhorst wrote:
>>>>> op 21-01-14 18:44, Thomas Hellstrom schreef:
>>>>>> On 01/21/2014 04:29 PM, Maarten Lankhorst wrote:
>>>>>>> Hey,
>>>>>>>
>>>>>>> op 21-01-14 16:17, Thomas Hellstrom schreef:
>>>>>>>> Maarten, for this and the other patches in this series,
>>>>>>>>
>>>>>>>> I seem to recall we have this discussion before?
>>>>>>>> IIRC I stated that reservation was a too heavy-weight lock to
>>>>>>>> hold to
>>>>>>>> determine whether a buffer was idle? It's a pretty nasty thing to
>>>>>>>> build in.
>>>>>>>>
>>>>>>> I've sent this patch after determining that this already didn't
>>>>>>> end up
>>>>>>> being heavyweight.
>>>>>>> Most places were already using the fence_lock and reservation, I
>>>>>>> just
>>>>>>> fixed up the few
>>>>>>> places that didn't hold a reservation while waiting. Converting the
>>>>>>> few places that didn't
>>>>>>> ended up being trivial, so I thought I'd submit it.
>>>>>> Actually the only *valid* reason for holding a reservation when
>>>>>> waiting
>>>>>> for idle is
>>>>>> 1) You want to block further command submission on the buffer.
>>>>>> 2) You want to switch GPU engine and don't have access to gpu
>>>>>> semaphores
>>>>>> / barriers.
>>>>>>
>>>>>> Reservation has the nasty side effect that it blocks command
>>>>>> submission
>>>>>> and pins the buffer (in addition now makes the evict list traversals
>>>>>> skip the buffer) which in general is *not* necessary for most wait
>>>>>> cases, so we should instead actually convert the wait cases that
>>>>>> don't
>>>>>> fulfill 1) and 2) above in the other direction if we have
>>>>>> performance
>>>>>> and latency-reduction in mind. I can't see how a spinlock
>>>>>> protecting a
>>>>>> fence pointer or fence list is stopping you from using RW fences as
>>>>>> long
>>>>>> as the spinlock is held while manipulating the fence list?
>>>>>>
>>>>> You wish. Fine I'll enumerate all cases of ttm_bo_wait (with the
>>>>> patchset, though) and enumerate if they can be changed to work
>>>>> without
>>>>> reservation or not.
>>>>>
>>>>> ttm/ttm_bo.c
>>>>> ttm_bo_cleanup_refs_or_queue: needs reservation and ttm_bo_wait to
>>>>> finish for the direct destroy fastpath, if either fails it needs
>>>>> to be
>>>>> queued. Cannot work without reservation.
>>>> Doesn't block and no significant reservation contention expected.
>>>>
>>>>> ttm_bo_cleanup_refs_and_unlock: already drops reservation to wait,
>>>>> doesn't need to re-acquire. Simply reordering ttm_bo_wait until after
>>>>> re-reserve is enough.
>>>> Currently follows the above rules.
>>>>
>>>>> ttm_bo_evict: already has the reservation, cannot be dropped since
>>>>> only trylock is allowed. Dropping reservation would cause badness,
>>>>> cannot be converted.
>>>> Follows rule 2 above. We're about to move the buffer and if that can't
>>>> be pipelined using the GPU (which TTM currently doesn't allow), we
>>>> need
>>>> to wait. Although eviction should be low priority compared to new
>>>> command submission, so I can't really see why we couldn't wait before
>>>> trying to reserve here?
>>>>
>>>>> ttm_bo_move_buffer: called from ttm_bo_validate, cannot drop
>>>>> reservation for same reason as ttm_bo_evict. It might be part of a
>>>>> ticketed reservation so really don't drop lock here.
>>>> Part of command submission and as such follows rule 2 above. If we can
>>>> pipeline the move with the GPU, no need to wait (but needs to be
>>>> implemented, of course).
>>>>
>>>>> ttm_bo_synccpu_write_grab: the wait could be converted to be done
>>>>> afterwards, without  fence_lock. But in this case reservation could
>>>>> take the role of fence_lock too,
>>>>>
>>>>> so no separate fence_lock would be needed.
>>>> With the exception that reservation is more likely to be contended.
>>> True but rule 1.
>>>>> ttm_bo_swapout: see ttm_bo_evict.
>>>>>
>>>>> ttm/ttm_bo_util.c:
>>>>> ttm_bo_move_accel_cleanup: calls ttm_bo_wait, cannot drop lock, see
>>>>> ttm_bo_move_buffer, can be called from that function.
>>>> Rule 2.
>>>>
>>>>> ttm/ttm_bo_vm.c
>>>>> ttm_bo_vm_fault_idle: I guess you COULD drop the reservation here,
>>>>> but
>>>>> you already had the reservation, so a similar optimization to
>>>>> ttm_bo_synccpu_write_grab could be done without requiring fence_lock.
>>>>> If you would write it like that, you would end up with a patch
>>>>> similar
>>>>> to drm/nouveau: add reservation to nouveau_gem_ioctl_cpu_prep. I
>>>>> think
>>>>> we should do this, an
>>>>>
>>>>> Ok, so the core does NOT need fence_lock because we can never drop
>>>>> reservations except in synccpu_write_grab and maybe
>>>>> ttm_bo_vm_fault_idle, but even in those cases reservation is done. So
>>>>> that could be used instead of fence_lock.
>>>>>
>>>>> nouveau_gem_ioctl_cpu_prep:
>>>>> Either block on a global spinlock or a local reservation lock.
>>>>> Doesn't
>>>>> matter much which, I don't need the need to keep a global lock for
>>>>> this function...
>>>>> 2 cases can happen in the trylock reservation failure case: buffer is
>>>>> not reserved, so it's not in the process of being evicted. buffer is
>>>>> reserved, which means it's being used in command submission right
>>>>> now,
>>>>> or in one of the functions described above (eg not idle).
>>>>>
>>>>> nouveau_gem_pushbuf_reloc_apply:
>>>>> has to call ttm_bo_wait with reservation, cannot be dropped.
>>>>>
>>>>> So for core ttm and nouveau the fence_lock is never needed, radeon
>>>>> has
>>>>> only 1 function that calls ttm_bo_wait which uses a reservation too.
>>>>> It doesn't need the fence_lock either.
>>>> And vmwgfx now also has a syccpu IOCTL (see drm-next).
>>>>
>>>> So assuming that we converted the functions that can be converted to
>>>> wait outside of reservation, the same way you have done with Nouveau,
>>>> leaving the ones that fall under 1) and 2) above, I would still argue
>>>> that a spinlock should be used because taking a reservation may
>>>> implicitly mean wait for gpu, and could give bad performance- and
>>>> latency charateristics. You shouldn't need to wait for gpu to check
>>>> for
>>>> buffer idle.
>>> Except that without reservation you can't tell if the buffer is really
>>> idle, or is currently being
>>> used as part of some command submission/eviction before the fence
>>> pointer is set.
>>>
>> Yes, but when that matters, you're either in case 1 or case 2 again.
>> Otherwise, when you release the reservation, you still don't know.
>> A typical example of this is the vmwgfx synccpu ioctl, where you can
>> either choose to block command submission (not used currently)
>> or not (user-space inter-process synchronization). The former is a case
>> 1 wait and holds reservation while waiting for idle and then ups
>> cpu_writers. The latter waits without reservation for previously
>> submitted rendering to finish.
> Yeah you could, but what exactly are you waiting on then? If it's some
> specific existing rendering,
> I would argue that you should create an android userspace fence during
> command submission,
> or provide your own api to block on a specfic fence in userspace.
>
> If you don't then I think taking a reservation is not unreasonable. In
> the most common case the buffer is
> idle and not reserved, so it isn't contested. The actual waiting
> itself can be done without reservation held,
> by taking a reference on the fence.
Yeah, here is where we disagree. I'm afraid people will start getting
sloppy with reservations and use them to protect more stuff, and after a
while they start wondering why the GPU command queue drains...

Perhaps we could agree on a solution (building on one of your original
ideas) where we require reservation to modify the fence pointers, and
the buffer object moving flag, but the structure holding the fence
pointer(s) is RCU safe, so that the pointers can be safely read under an
rcu lock.

People who don't care about potential reservation contention could just
use the reservation lock, and clear pointers of fences that have
signaled. People who do care could do a reservation trylock, and if it
fails read the fence pointers under RCU. This of course means those
fence objects need to be freed after an RCU grace period.

/Thomas

>
> ~Maarten