Messages in this thread | | | Date | Mon, 28 Oct 2019 09:29:55 -0700 | From | Sultan Alsawaf <> | Subject | Re: [PATCH] scatterlist: Speed up for_each_sg() loop macro |
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On Mon, Oct 28, 2019 at 01:23:20PM -0300, Jason Gunthorpe wrote: > On Mon, Oct 28, 2019 at 09:18:48AM -0700, Sultan Alsawaf wrote: > > 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) > > This testing is making assumptions about how 'nr' is used and the > construction of the sgl though > > If any chains are partially populated, or for some reason the driver > starts at a different sgl, it will break. You'll need to somehow > show none of those possibilities are happening.
I haven't personally witnessed any for_each_sg() use that starts at a different sgl, but this is something I considered as well. The optimal solution would be to alter for_each_sg() to replace the `sg` and `nr` arguments with a single sg_table pointer argument instead. Users that genuinely need to start at a different sgl can just craft their own for-loop with sg_next.
This would require changes to a lot of files though, and I wanted to see how the mailing list felt about this change before embarking on that.
Sultan
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