Re: [PATCH] x86: only use ERMS for user copies for larger sizes

[Ingo Molnar <mingo@kernel.org>]

[ Cc:-ed a few other gents and lkml. ]

* Jens Axboe <axboe@kernel.dk> 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.yu@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 <axboe@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)

So I'm inclined to do something like yours, because clearly the changelog 
of 954e482bde20 was at least partly false: Intel can say whatever they 
want, it's a fact that ERMS has high setup costs for low buffer sizes - 
ERMS is optimized for large size, cache-aligned copies mainly.

But the result is counter-intuitive in terms of kernel text footprint, 
plus the '128' is pretty arbitrary - we should at least try to come up 
with a break-even point where manual copy is about as fast as ERMS - on 
at least a single CPU ...

Thanks,

	Ingo