[PATCH 04/10] drm/ttm: change fence_lock to inner lock, v3

Tue Nov 27 04:35:09 PST 2012

Op 22-11-12 21:29, Thomas Hellstrom schreef:
> On 11/22/2012 04:51 PM, Maarten Lankhorst wrote:
>> Op 21-11-12 14:27, Thomas Hellstrom schreef:
>>> On 11/21/2012 02:12 PM, Maarten Lankhorst wrote:
>>>> Op 21-11-12 13:42, Thomas Hellstrom schreef:
>>>>> On 11/21/2012 12:38 PM, Maarten Lankhorst wrote:
>>>>>> Hey,
>>>>>>
>>>>>> Op 20-11-12 16:08, Thomas Hellstrom schreef:
>>>>>>> On 11/20/2012 02:13 PM, Maarten Lankhorst wrote:
>>>>>>>> Op 20-11-12 13:03, Thomas Hellstrom schreef:
>>>>>>>>> On 11/20/2012 12:33 PM, Maarten Lankhorst wrote:
>>>>>>>>>> Op 20-11-12 08:48, Thomas Hellstrom schreef:
>>>>>>>>>>> On 11/19/2012 04:33 PM, Maarten Lankhorst wrote:
>>>>>>>>>>>> Op 19-11-12 16:04, Thomas Hellstrom schreef:
>>>>>>>>>>>>> On 11/19/2012 03:17 PM, Thomas Hellstrom wrote:
>>>>>>>>>>>>>> Hi,
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> This patch looks mostly good, although I think ttm_bo_cleanup_refs becomes overly complicated:
>>>>>>>>>>>>>> Could this do, or am I missing something?
>>>>>>>>>>>>>>
>>>>>>>>>>>>> Actually, my version is bad, because ttm_bo_wait() is called with the lru lock held.
>>>>>>>>>>>>>
>>>>>>>>>>>>> /Thomas
>>>>>>>>>>>> Oh digging through it made me remember why I had to release the reservation early and
>>>>>>>>>>>> had to allow move_notify to be called without reservation.
>>>>>>>>>>>>
>>>>>>>>>>>> Fortunately move_notify has a NULL parameter, which is the only time that happens,
>>>>>>>>>>>> so you can still check do BUG_ON(mem != NULL && !ttm_bo_reserved(bo)); in your
>>>>>>>>>>>> move_notify handler.
>>>>>>>>>>>>
>>>>>>>>>>>> 05/10 removed the loop and assumed no new fence could be attached after the driver has
>>>>>>>>>>>> declared the bo dead.
>>>>>>>>>>>>
>>>>>>>>>>>> However, at that point it may no longer hold a reservation to confirm this, that's why
>>>>>>>>>>>> I moved the cleanup to be done in the release_list handler. It could still be done in
>>>>>>>>>>>> ttm_bo_release, but we no longer have a reservation after we waited. Getting
>>>>>>>>>>>> a reservation can fail if the bo is imported for example.
>>>>>>>>>>>>
>>>>>>>>>>>> While it would be true that in that case a new fence may be attached as well, that
>>>>>>>>>>>> would be less harmful since that operation wouldn't involve this device, so the
>>>>>>>>>>>> ttm bo can still be removed in that case. When that time comes I should probably
>>>>>>>>>>>> fix up that WARN_ON(ret) in ttm_bo_cleanup_refs. :-)
>>>>>>>>>>>>
>>>>>>>>>>>> I did add a WARN_ON(!atomic_read(&bo->kref.refcount)); to
>>>>>>>>>>>> ttm_bo_reserve and ttm_eu_reserve_buffers to be sure nothing is done on the device
>>>>>>>>>>>> itself. If that is too paranoid, those WARN_ON's could be dropped. I prefer to leave them
>>>>>>>>>>>> in for a kernel release or 2. But according to the rules that would be the only time you
>>>>>>>>>>>> could attach a new fence and trigger the WARN_ON for now..
>>>>>>>>>>> Hmm, I'd appreciate if you could group patches with functional changes that depend on eachother togeteher,
>>>>>>>>>>> and "this is done because ...", which makes it much easier to review, (and to follow the commit history in case
>>>>>>>>>>> something goes terribly wrong and we need to revert).
>>>>>>>>>>>
>>>>>>>>>>> Meanwhile I'll take a look at the final ttm_bo.c and see if I can spot any culprits.
>>>>>>>>>>>
>>>>>>>>>>> In general, as long as a bo is on a LRU list, we must be able to attach fences because of accelerated eviction.
>>>>>>>>>> I thought it was deliberately designed in such a way that it was kept on the lru list,
>>>>>>>>>> but since it's also on the ddestroy list it won't start accelerated eviction,
>>>>>>>>>> since it branches into cleanup_refs early, and lru_lock still protects all the list entries.
>>>>>>>>> I used bad wording. I meant that unbinding might be accelerated, but  currently (quite inefficiently)
>>>>>>>>> do synchronized unbinding, assuming that only the CPU can do that. When we start to support
>>>>>>>>> unsynchronized moves, we need to be able to attach fences at least at the last move_notify(bo, NULL);
>>>>>>>> Would you need to wait in that case on fence_wait being completed before calling move_notify?
>>>>>>>>
>>>>>>>> If not, you would still only need to perform one wait, but you'd have to make sure move_notify only gets
>>>>>>>> called by 1 thread before checking the fence pointer and performing a wait. At that point you still hold the
>>>>>>>> lru_lock though, so it shouldn't be too hard to make something safe.
>>>>>>> I think typically a driver that wants to implement asynchronous moves don't want to wait before calling
>>>>>>> move_notify, but may wait in move_notify or move. Typically (upcoming vmwgfx) it would invalidate the buffer in move_notify(bo, NULL), attach a fence and then use the normal delayed destroy to wait on that fence before destroying the buffer.
>>>>>>>
>>>>>>> Otherwise, since binds / unbinds are handled in the GPU command stream there's never any need to wait for moves except when there's a CPU
>>>>>>> access.
>>>>>> Well, nouveau actually needs fence_wait to finish first, since vm changes are out of band.
>>>>>> But I guess it should be possible to attach it as work to the fence when it's signaled, and I
>>>>>> may want to do something like that already for performance reasons in a different place,
>>>>>> so I guess it doesn't matter.
>>>>> Actions to be performed on fence signaling tend to be very cpu consuming, I think due to the context switches involved.
>>>>> We had to replace that in the old psb driver and batch things like TTM does instead.
>>>>>
>>>>> Also remember that TTM fences are not required to signal in finite time unless fence_flush is called.
>>>>>
>>>>> I think nouveau doesn't use fence irqs to signal its fences.
>>>>>
>>>>>> Is calling move_notify(bo, NULL) legal and a noop the second time?
>>>>> I see no fundamental reason why it shouldn't be OK, although we might need to patch drivers to cope with it.
>>>>>
>>>>>>     That would save a flag in the bo to check if it's called already,
>>>>>> although I suppose we could always define a TTM_BO_PRIV_FLAG_* for it otherwise.
>>>>>>
>>>>>> move_notify might end up being called with the lru_lock held, but that shouldn't be a problem.
>>>>> I don't think that's a good idea. Drivers sleeping in move_notify will need to release the spinlock, and that means it's
>>>>> better to release it before move_notify is called.
>>>> Is the only sleeping being done on fences? In that case we might wish to split it up in 2 pieces for destruction,
>>>> the piece that runs immediately, and a piece to run after the new fence has signaled (current behavior).
>>>>
>>>> Nouveau needs the final move_notify unmap to be called after object is idle, like it is now. It doesn't need
>>>> to attach a new fence.
>>> In that case it might be best to worry about asynchronous stuff later?
>>> We will eventually implement it on the new vmwgfx hardware revision, but it's not ready yet.
>>>
>>> /Thomas
>> Ok sounds good.
>>
>> In that case what do you want me to change from the first 4 patches apart from more verbose commit messages?
>> - 03/10 I got that I need to re-add the list_empty check after -EBUSY was returned in evict_mem_first.
>>
>> Also PATCH 05/10 cleans up the spinning in ttm_bo_cleanup_refs, so I hope it's ok that it's a big
>> ugly in 04/10, as long as it doesn't result in any new bugs being introduced.
>>
>> ~Maarten
>>
>> PS: I did a plain rebase of my git tree to deal with the conflicts in drm-next.
>>
>
> Maarten, it seems to me the purpose of the patches are the following (not necessarily the correct order).
>
> 1) Change fence lock locking order w r t LRU lock - should be a trivial and very small change.
Hm yeah, this seems to be small in itself if I only do that.

> 2) Change reservations from lists to always be trylock, skipping already reserved bos.
Yeah, but unfortunately this was easier to be done after some of the cleanups.

> 3) Remove the lru lock around reservations.
This is a separate patch, but unfortunately dependent on all previous optimizations/cleanups.
> 4) Various optimizations / cleanups.
This was a bit harder, some of the changes are a lot easier with the cleanups/optimizations done first.

I need the cleanup_refs changes before the reservation trylock change, since they cause cleanup_refs to be
called with reservation and lru lock held, this will prevent ever blocking inside that function on something
other than wait.

The real reason those patches are in this order is because some patches can only be done after some
previous changes have been made first. However I can decrease the amount of changes slightly,
I was thinking of this:

0. change cleanup_refs_or_queue order of reservation and wait check
1. fence_lock <-> lru_lock nesting change, needed for patch 3
- small patch if I only focus on the inversion itself, I fear there is no race free to do this
without squashing this with patch 0. There will be a race otherwise where we waited on the
previous fence and a new fence was attached between unlocking fence lock and locking lru lock.
Both patches only touch ttm_bo_cleanup_refs_or_queue (and 2 lines in ttm_eu_fence_buffer_objects),
so it won't affect reviewability much if it's done in 1 patch only.

2. fix radeon move_notify to be callable without reservation, needed for patch 3
- small patch

3. call ttm_bo_cleanup_refs with reservation and lru lock held, drop the looping in ttm_bo_cleanup_refs, makes patch 4 and 5 easier
- instead of touching this function multiple times, just touch it first, to the final cleaner form to be done with it.

4. loop the trylocking in swapout
5. loop the trylocking in ttm_mem_evict_first
6. drop now unused no_wait_reserve argument from ttm_mem_evict_first
- separate patch since it's just touching a lot of functions without any functional change

Bonus series, those are probably independent patches, but might depend on above first:
7. cleanup ttm_bo_force_list_clean's taking lru lock twice for every bo
- depends loosely on no_wait_reserve argument being dropped to apply cleanly, no real depends otherwise
8. lru lock is now no longer needed to protect reservations, cleanup
- depends on the whole series
9. replace calls to ttm_bo_wait_unreserved with ttm_bo_reserve_slowpath, maps a lot better to mutexes
- Probably best if I split this one up in 4 patches, first introduce new function, make nouveau/ttm_eu use it, then drop old function.
> If you could reorganize and make 4 patch series like this, it would be much easier to follow what happens and why, and would make it much easier for me to review. It seems to me if patch series 1-3 focus on the intended changes and the intended changes only, they would be quite small?
Would the above patches 1 to 6 in 1 series be ok too?

If so, a completely untested version is up at my git tree http://cgit.freedesktop.org/~mlankhorst/linux/log/

"drm/ttm: change fence_lock to inner lock" up to "drm/ttm: remove no_wait_reserve, v2"

At this point those patches are not even boot-tested, so I'll do some testing first before
resubmitting those.

The last 3 changes can be reviewed independently, but last 2 changes
are probably best committed in that order, else I need to fixup reserve_slowpath too.

~Maarten