linux-kernel - Re: [PATCH] scatterlist: Speed up for_each

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Date:   Mon, 28 Oct 2019 09:18:48 -0700
From:   Sultan Alsawaf <sultan@...neltoast.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     "Martin K. Petersen" <martin.petersen@...cle.com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        Ming Lei <ming.lei@...hat.com>,
        Gal Pressman <galpress@...zon.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] scatterlist: Speed up for_each_sg() loop macro

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