Re: [PATCH] scatterlist: Speed up for_each_sg() loop macro

Sultan Alsawaf
2019-10-28T16:18:53Z

On Mon, Oct 28, 2019 at 11:17:34AM -0300, Jason Gunthorpe wrote:
> This is a big change in the algorithm, why are you sure it is OK?

I'm sure it's OK because the test module I provided in the commit message
encapsulates all the possible edge cases of sg chaining:
-An sglist with >=1 && <=(SG_MAX_SINGLE_ALLOC-1) nents (no chaining, the last
element in the array is unused)
-An sglist with SG_MAX_SINGLE_ALLOC nents (no chaining, the last element in the
array isn't an sg chain link)
-An sglist with >SG_MAX_SINGLE_ALLOC && <=2*(SG_MAX_SINGLE_ALLOC-1) nents (there
is one chain to another array, and the other array's last element is unused)
-An sglist with (2*SG_MAX_SINGLE_ALLOC)-1 nents (there is one chain to another
array, and the other array's last element isn't an sg chain link)
-An sglist with 2*SG_MAX_SINGLE_ALLOC nents (there are two chains to other
arrays, and the 3rd array contains 2 sgs & its last element is unused)
-An sglist with >2*SG_MAX_SINGLE_ALLOC && <(3*SG_MAX_SINGLE_ALLOC)-1 nents
(there are two chains to other arrays, and the 3rd array's last element isn't
an sg chain)

I just made my module test nents >=1 && <=3*SG_MAX_SINGLE_ALLOC for simplicity.
My proposed for_each_sg() also handles nents==0 the same as before by doing
nothing.

> Did you compare with just inlining sg_net?

Yes. Forcefully inlining sg_next() had no impact on performance.

Sultan