Re: [PATCH] drm/i915: Optimise VMA lookup slightly

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

On 13/12/2016 13:41, Chris Wilson wrote:
> On Tue, Dec 13, 2016 at 01:30:19PM +0000, Tvrtko Ursulin wrote:
>>
>> On 13/12/2016 12:41, Chris Wilson wrote:
>>> On Tue, Dec 13, 2016 at 12:22:18PM +0000, Tvrtko Ursulin wrote:
>>>> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>>>
>>>> A few details to hopefully make a very hot function a tiny bit
>>>> more efficient:
>>>>
>>>> 1. Cast VM pointers before substraction to save the compiler
>>>>    doing a smart one which includes multiplication.
>>>
>>> Indeed. Not pretty though.
>>>
>>> static always_inline __kernel_ptrdiff_t ptrdiff(const void *a, const void *b)
>>> {
>>> 	return a - b;
>>> }
>>>
>>> cmp = ptrdiff(vma->vm, vm);
>>> if (cmp)
>>> 	return cmp;
>>
>> Pffifle as you would say. :)
>>
>>>> 2. Use smaller type for comparison since we only care about
>>>>    the sign.
>>>
>>> Should be a no-op since the compiler also should only care about the
>>> sign and not be moving the registers about, just the cc and we should be
>>> inlining... Is gcc not smart enough? :(
>>
>> I am pretty sure there was a significant difference in the size of
>> generated code so apparently it isn't.
>
> If you have two available, can you attach the asm?

http://paste.debian.net/901995/ if you can figure it out.

Dropped it from v2 though.

>>>> 3. Prefer the ppgtt lookup branch and inline it, allowing the
>>>>    compiler to optimise out the second part of i915_vma_compare
>>>>    and save one call indirection.
>>>
>>> This runs counter to a better optimisation that completely avoids
>>> calling obj_to_vma for ppgtt lookups (i.e. in execbuffer we go straight
>> >from handle to vma, skipping the handle to obj intermediate lookup).
>>>
>>> Primary caller for this function should be ggtt users, with single
>>> negative lookups before creating the ppgtt vma.
>>
>> Empirically ppgtt users call this much much more on any benchmarks
>> I've tried since with the change all I see is lookup_and_create in
>> the profiles and not obj_to_vma.
>
> Exactly, hence why we should tackle the root of the problem... Replace a
> slow idr lookup and rbtree walk with a direct lookup (hashtable) from
> the fpriv. Since it doesn't just affect full-ppgtt but the introduction
> of the second lookup regressed everyone,
> https://bugs.freedesktop.org/show_bug.cgi?id=87131 and the myriad of
> igt/benchmark/gem_exec_reloc benchmarks.

Dropped this from v2 as well, only kept ptrdiff.

Will have a look around for the things you mentioned here.

Regards,

Tvrtko
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