Forgot to CC the mailing list...

On 11/20/18 1:18 PM, Jens Axboe wrote:
> Hi,
> 
> So this is a fun one... While I was doing the aio polled work, I noticed
> that the submitting process spent a substantial amount of time copying
> data to/from userspace. For aio, that's iocb and io_event, which are 64
> and 32 bytes respectively. Looking closer at this, and it seems that
> ERMS rep movsb is SLOWER for smaller copies, due to a higher startup
> cost.
> 
> I came up with this hack to test it out, and low and behold, we now cut
> the time spent in copying in half. 50% less.
> 
> Since these kinds of patches tend to lend themselves to bike shedding, I
> also ran a string of kernel compilations out of RAM. Results are as
> follows:
> 
> Patched       : 62.86s avg, stddev 0.65s
> Stock : 63.73s avg, stddev 0.67s
> 
> which would also seem to indicate that we're faster punting smaller
> (< 128 byte) copies.
> 
> CPU: Intel(R) Xeon(R) CPU E5-2650 v4 @ 2.20GHz
> 
> Interestingly, text size is smaller with the patch as well?!
> 
> I'm sure there are smarter ways to do this, but results look fairly
> conclusive. FWIW, the behaviorial change was introduced by:
> 
> commit 954e482bde20b0e208fd4d34ef26e10afd194600
> Author: Fenghua Yu <fenghua...@intel.com>
> Date:   Thu May 24 18:19:45 2012 -0700
> 
>     x86/copy_user_generic: Optimize copy_user_generic with CPU erms feature
> 
> which contains nothing in terms of benchmarking or results, just claims
> that the new hotness is better.
> 
> Signed-off-by: Jens Axboe <ax...@kernel.dk>
> ---
> 
> diff --git a/arch/x86/include/asm/uaccess_64.h 
> b/arch/x86/include/asm/uaccess_64.h
> index a9d637bc301d..7dbb78827e64 100644
> --- a/arch/x86/include/asm/uaccess_64.h
> +++ b/arch/x86/include/asm/uaccess_64.h
> @@ -29,16 +29,27 @@ copy_user_generic(void *to, const void *from, unsigned 
> len)
>  {
>       unsigned ret;
>  
> +     /*
> +      * For smaller copies, don't use ERMS as it's slower.
> +      */
> +     if (len < 128) {
> +             alternative_call(copy_user_generic_unrolled,
> +                              copy_user_generic_string, X86_FEATURE_REP_GOOD,
> +                              ASM_OUTPUT2("=a" (ret), "=D" (to), "=S" (from),
> +                                          "=d" (len)),
> +                              "1" (to), "2" (from), "3" (len)
> +                              : "memory", "rcx", "r8", "r9", "r10", "r11");
> +             return ret;
> +     }
> +
>       /*
>        * If CPU has ERMS feature, use copy_user_enhanced_fast_string.
>        * Otherwise, if CPU has rep_good feature, use copy_user_generic_string.
>        * Otherwise, use copy_user_generic_unrolled.
>        */
>       alternative_call_2(copy_user_generic_unrolled,
> -                      copy_user_generic_string,
> -                      X86_FEATURE_REP_GOOD,
> -                      copy_user_enhanced_fast_string,
> -                      X86_FEATURE_ERMS,
> +                      copy_user_generic_string, X86_FEATURE_REP_GOOD,
> +                      copy_user_enhanced_fast_string, X86_FEATURE_ERMS,
>                        ASM_OUTPUT2("=a" (ret), "=D" (to), "=S" (from),
>                                    "=d" (len)),
>                        "1" (to), "2" (from), "3" (len)
> 


-- 
Jens Axboe

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