Fwd: [PATCH 14/20] drm/radeon: multiple ring allocator v2

[Jerome Glisse]

> On 07.05.2012 17:23, Jerome Glisse wrote:
>>
>> On Mon, May 7, 2012 at 7:42 AM, Christian König<deathsimple at vodafone.de>
>>  wrote:
>>>
>>> A startover with a new idea for a multiple ring allocator.
>>> Should perform as well as a normal ring allocator as long
>>> as only one ring does somthing, but falls back to a more
>>> complex algorithm if more complex things start to happen.
>>>
>>> We store the last allocated bo in last, we always try to allocate
>>> after the last allocated bo. Principle is that in a linear GPU ring
>>> progression was is after last is the oldest bo we allocated and thus
>>> the first one that should no longer be in use by the GPU.
>>>
>>> If it's not the case we skip over the bo after last to the closest
>>> done bo if such one exist. If none exist and we are not asked to
>>> block we report failure to allocate.
>>>
>>> If we are asked to block we wait on all the oldest fence of all
>>> rings. We just wait for any of those fence to complete.
>>>
>>> v2: We need to be able to let hole point to the list_head, otherwise
>>>    try free will never free the first allocation of the list. Also
>>>    stop calling radeon_fence_signalled more than necessary.
>>>
>>> Signed-off-by: Christian König<deathsimple at vodafone.de>
>>> Signed-off-by: Jerome Glisse<jglisse at redhat.com>
>>
>> This one is NAK please use my patch. Yes in my patch we never try to
>> free anything if there is only on sa_bo in the list if you really care
>> about this it's a one line change:
>>
>> http://people.freedesktop.org/~glisse/reset5/0001-drm-radeon-multiple-ring-allocator-v2.patch
>
> Nope that won't work correctly, "last" is pointing to the last allocation
> and that's the most unlikely to be freed at this time. Also in this version
> (like in the one before) radeon_sa_bo_next_hole lets hole point to the
> "prev" of the found sa_bo without checking if this isn't the lists head.
> That might cause a crash if an to be freed allocation is the first one in
> the buffer.
>
> What radeon_sa_bo_try_free would need to do to get your approach working is
> to loop over the end of the buffer and also try to free at the beginning,
> but saying that keeping the last allocation results in a whole bunch of
> extra cases and "if"s, while just keeping a pointer to the "hole" (e.g.
> where the next allocation is most likely to succeed) simplifies the code
> quite a bit (but I agree that on the down side it makes it harder to
> understand).
>
>> Your patch here can enter in infinite loop and never return holding
>> the lock. See below.
>>
>> [SNIP]
>>
>>> +               } while (radeon_sa_bo_next_hole(sa_manager, fences));
>>
>> Here you can infinite loop, in the case there is a bunch of hole in
>> the allocator but none of them allow to full fill the allocation.
>> radeon_sa_bo_next_hole will keep returning true looping over and over
>> on all the all. That's why i only restrict my patch to 2 hole skeeping
>> and then fails the allocation or try to wait. I believe sadly we need
>> an heuristic and 2 hole skeeping at most sounded like a good one.
>
> Nope, that can't be an infinite loop, cause radeon_sa_bo_next_hole in
> conjunction with radeon_sa_bo_try_free are eating up the opportunities for
> holes.
>
> Look again, it probably will never loop more than RADEON_NUM_RINGS + 1, with
> the exception for allocating in a complete scattered buffer, and even then
> it will never loop more often than halve the number of current allocations
> (and that is really really unlikely).
>
> Cheers,
> Christian.

I looked again and yes it can loop infinitly, think of hole you can
never free ie radeon_sa_bo_try_free can't free anything. This
situation can happen if you have several thread allocating sa bo at
the same time while none of them are yet done with there sa_bo (ie
none have call sa_bo_free yet). I updated a v3 that track oldest and
fix all things you were pointing out above.

http://people.freedesktop.org/~glisse/reset5/0001-drm-radeon-multiple-ring-allocator-v3.patch

Cheers,
Jerome