Re: [PATCH] drm/i915: Remove temporary allocation of dma addresses when rotating

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 14/11/2017 18:14, Chris Wilson wrote:
> Quoting Tvrtko Ursulin (2017-02-27 14:31:17)
>>
>> On 27/02/2017 10:21, Chris Wilson wrote:
>>> On Mon, Feb 27, 2017 at 10:14:12AM +0000, Tvrtko Ursulin wrote:
>>>>
>>>> On 27/02/2017 10:06, Chris Wilson wrote:
>>>>> On Mon, Feb 27, 2017 at 09:55:10AM +0000, Tvrtko Ursulin wrote:
>>>>>>
>>>>>> On 22/02/2017 08:44, Chris Wilson wrote:
>>>>>>> I also think that's an argument for improving the general cache rather
>>>>>>> than arguing against using it.
>>>>>>
>>>>>> Well I wasn't concerned about the cache per se, but about whether it
>>>>>> is completely appropriate (best choice) to use it in this particular
>>>>>> case.
>>>>>>
>>>>>> Because as I said before, for 1920x1080x32 we are talking about a
>>>>>> 16KiB extremely short lived temporary allocation, vs the similar
>>>>>> size for the sg radix cache. But radix cache sticks around the the
>>>>>> lifetime of obj->mm.pages and it wouldn't otherwise be there since
>>>>>> AFAICS in practice no one really touches frame buffers in a way to
>>>>>> trigger its creation.
>>>>>>
>>>>>> Those amounts of memory are not a concern, but again, is the
>>>>>> simplification of the code worth the conceptual downsides mentioned
>>>>>> above? Even if we considered 4K frame buffers, when both allocations
>>>>>> go to ~64KiB, would that change anything? I am not sure, probably
>>>>>> not for me.
>>>>>>
>>>>>> So I am still unsure that we should go with this change.
>>>>>
>>>>> Again, the complaint you have here are general concerns about caching
>>>>> the mapping. Avoiding using the cache instead of improving the cache
>>>>> seems the wrong approach.
>>>>
>>>> Depends what kind of improvments to the cache you have in mind. If
>>>> you are thinking about size then I disagree, I think it is efficient
>>>> enough already. But if you are thinking about the lifetime
>>>> management then it is obvious from all that I have written so far
>>>> that I would agree with that. Since the core of my "complaint" is
>>>> the lifetime mismatch, and not the size.
>>>>
>>>> For lifetime I am not sure what you could do. Exposing the size of
>>>> it, with maybe some other bits attached the the object, to the
>>>> shrinker I think doesn't make much sense since the sizes are so
>>>> small compared to the backing store sizes.
>>>>
>>>> Perhaps you could add an explicit reset of the cache after the
>>>> rotation is done with it, but then the only remaining benefit will
>>>> be avoiding greater than zero order allocations. I say the only
>>>> one.. it would still be a good one. Just have no idea if this level
>>>> of cache usage would satisfy you!
>>>>
>>>> Perhaps you could say what kind of optimisation you have in mind to
>>>> save me guessing? :)
>>>
>>> I was thinking you would like an inactivity timer. Or we could have a
>>> separate shrinker, as that's the principal cache management system.
>>
>> I thought about the shrinker myself. Even wrote some code to more
>> accurately size the objects as part of the existing passes. But as I
>> said the contribution of anything object and not backing store is so
>> small that, even though it would conceptually be more correct and
>> perhaps avoid some marginal over-shrinking, I am not sure it is worth
>> doing it. Assuming of course that I got the sizing of the radix tree
>> correct! I just hacked something up based on some debug dumping code
>> from radix-tree.c.
>>
>> So the complication is there is no API to get the size of the radix tree
>> (or the scatter list table) and we would have to add something, either
>> internally to i915, or try and upstream it.
>>
>> Or we avoid that with your timer idea and just purge all caches which
>> haven't been used in a while. Maybe from idle work or something.
> 
> Tempting. I like hooking into mark_idle/park more than adding a new
> timer, and we already have the precedent of using that to initiate a
> cache flush.
> 
> What's the impact of us keeping pages pinned when idle -- (a lot) more
> work in the shrinker. Let's see where the cost-beneift lies.
>   
>> But for this immediate patch, would you be happy with adding and
>> exporting i915_gem_object_reset_page_iter and calling it after rotation
>> is done with accessing the pages? Benefit would be avoidance of
>> drm_malloc_gfp if that bothers you most.
> 
> Honestly I think the page_iter cache is useful and likely to already
> exist or be used shortly after a portion of the object is rotated.

How come? I thought CPU access to framebuffers is atypical nowadays.

Regards,

Tvrtko
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