linux-kernel - Re: [PATCH v1] x86/lib: Optimize 8x loop and memory clobbers in csum

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Date:   Fri, 26 Nov 2021 13:50:32 -0600
From:   Noah Goldstein <goldstein.w.n@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     tglx@...utronix.de, mingo@...hat.com,
        Borislav Petkov <bp@...en8.de>, dave.hansen@...ux.intel.com,
        X86 ML <x86@...nel.org>, hpa@...or.com, peterz@...radead.org,
        alexanderduyck@...com, open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] x86/lib: Optimize 8x loop and memory clobbers in csum_partial.c

On Fri, Nov 26, 2021 at 1:21 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Fri, Nov 26, 2021 at 11:14 AM Noah Goldstein <goldstein.w.n@...il.com> wrote:
> >
> > On Fri, Nov 26, 2021 at 12:50 PM Noah Goldstein <goldstein.w.n@...il.com> wrote:
> > >
> > > On Fri, Nov 26, 2021 at 12:27 PM Eric Dumazet <edumazet@...gle.com> wrote:
> > > >
> > > > On Fri, Nov 26, 2021 at 10:17 AM Noah Goldstein <goldstein.w.n@...il.com> wrote:
> > > > >
> > > >
> > > > >
> > > > > Makes sense. Although if you inline I think you definitely will want a more
> > > > > conservative clobber than just "memory". Also I think with 40 you also will
> > > > > get some value from two counters.
> > > > >
> > > > > Did you see the number/question I posted about two accumulators for 32
> > > > > byte case?
> > > > > Its a judgement call about latency vs throughput that I don't really have an
> > > > > answer for.
> > > > >
> > > >
> > > > The thing I do not know is if using more units would slow down the
> > > > hyper thread ?
> >
> > Did some quick tests with the latency/throughput benchmarks running
> > in parallel on two hyperthreads on the same processors. The 32 byte case
> > latency advantage goes with 2 accum and there is still a slight regression
> > in throughput. The larger cases that hit the loop still still have improvements
> > both in tput and latency with 2 accum.
>
> Great. I also played with rorx instruction, because it removes one
> "adcl $0,..." step

Bright :) but it will need a BMI support check.

>
> __wsum ipv6_csum_partial(const void *buff, int len, __wsum sum)
> {
> u64 temp64;
> u64 tmp;
> u32 res32;
>
> if (unlikely(len != 40))
> return csum_partial(buff, len, sum);
>
> temp64 = (__force u64)sum;
> asm("addq 0*8(%[src]),%[temp64]\n\t"
>     "adcq 1*8(%[src]),%[temp64]\n\t"
>     "adcq 2*8(%[src]),%[temp64]\n\t"
>     "adcq 3*8(%[src]),%[temp64]\n\t"
>     "adcq 4*8(%[src]),%[temp64]\n\t"
>     "mov  %k[temp64],%[res32]\n\t"
>     "rorx $32,%[temp64],%[temp64]\n\t"
>     "adcl %k[temp64],%[res32]\n\t"
>     "adcl $0,%[res32]"
>     : [temp64] "+r" (temp64), [res32] "=r" (res32)
>     : [src] "r" (buff)
>     : "memory");
> return (__force __wsum)res32;
> }

I actually get better performance in hyperthread benchmarks with 2 accum:

Used:

        u64 res;
        temp64 = (__force uint64_t)sum;
        asm("movq 0*8(%[src]),%[res]\n\t"
            "addq 1*8(%[src]),%[res]\n\t"
            "adcq 2*8(%[src]),%[res]\n\t"
            "adcq   $0, %[res]\n"
            "addq 3*8(%[src]),%[temp64]\n\t"
            "adcq 4*8(%[src]),%[temp64]\n\t"
            "adcq   %[res], %[temp64]\n\t"
            "mov  %k[temp64],%k[res]\n\t"
            "rorx $32,%[temp64],%[temp64]\n\t"
            "adcl %k[temp64],%k[res]\n\t"
            "adcl $0,%k[res]"
            : [temp64] "+r"(temp64), [res] "=&r"(res)
            : [src] "r"(buff)
            : "memory");
        return (__force __wsum)res;

w/ hyperthread:
size,    2acc lat,    1acc lat,   2acc tput,   1acc tput
  40,       6.511,       7.863,       6.177,       6.157

w/o hyperthread:
size,    2acc lat,    1acc lat,   2acc tput,   1acc tput
  40,       5.577,       6.764,       3.150,       3.210