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SubjectRe: [PATCH v3 09/21] x86/asm: add clear_pages_movnt()
On Mon, Jun 6, 2022 at 11:39 PM Ankur Arora <ankur.a.arora@oracle.com> wrote:
>
> Add clear_pages_movnt(), which uses MOVNTI as the underlying primitive.
> With this, page-clearing can skip the memory hierarchy, thus providing
> a non cache-polluting implementation of clear_pages().
>
> MOVNTI, from the Intel SDM, Volume 2B, 4-101:
> "The non-temporal hint is implemented by using a write combining (WC)
> memory type protocol when writing the data to memory. Using this
> protocol, the processor does not write the data into the cache
> hierarchy, nor does it fetch the corresponding cache line from memory
> into the cache hierarchy."
>
> The AMD Arch Manual has something similar to say as well.
>
> One use-case is to zero large extents without bringing in never-to-be-
> accessed cachelines. Also, often clear_pages_movnt() based clearing is
> faster once extent sizes are O(LLC-size).
>
> As the excerpt notes, MOVNTI is weakly ordered with respect to other
> instructions operating on the memory hierarchy. This needs to be
> handled by the caller by executing an SFENCE when done.
>
> The implementation is straight-forward: unroll the inner loop to keep
> the code similar to memset_movnti(), so that we can gauge
> clear_pages_movnt() performance via perf bench mem memset.
>
> # Intel Icelakex
> # Performance comparison of 'perf bench mem memset -l 1' for x86-64-stosb
> # (X86_FEATURE_ERMS) and x86-64-movnt:
>
> System: Oracle X9-2 (2 nodes * 32 cores * 2 threads)
> Processor: Intel Xeon(R) Platinum 8358 CPU @ 2.60GHz (Icelakex, 6:106:6)
> Memory: 512 GB evenly split between nodes
> LLC-size: 48MB for each node (32-cores * 2-threads)
> no_turbo: 1, Microcode: 0xd0001e0, scaling-governor: performance
>
> x86-64-stosb (5 runs) x86-64-movnt (5 runs) Delta(%)
> ---------------------- --------------------- --------
> size BW ( stdev) BW ( stdev)
>
> 2MB 14.37 GB/s ( +- 1.55) 12.59 GB/s ( +- 1.20) -12.38%
> 16MB 16.93 GB/s ( +- 2.61) 15.91 GB/s ( +- 2.74) -6.02%
> 128MB 12.12 GB/s ( +- 1.06) 22.33 GB/s ( +- 1.84) +84.24%
> 1024MB 12.12 GB/s ( +- 0.02) 23.92 GB/s ( +- 0.14) +97.35%
> 4096MB 12.08 GB/s ( +- 0.02) 23.98 GB/s ( +- 0.18) +98.50%

For these sizes it may be worth it to save/rstor an xmm register to do
the memset:

Just on my Tigerlake laptop:
model name : 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz

movntdq xmm (5 runs) movnti GPR (5 runs)
Delta(%)
----------------------- -----------------------
size BW GB/s ( +- stdev) BW GB/s ( +-
stdev) %
2 MB 35.71 GB/s ( +- 1.02) 34.62 GB/s ( +-
0.77) -3.15%
16 MB 36.43 GB/s ( +- 0.35) 31.3 GB/s ( +-
0.1) -16.39%
128 MB 35.6 GB/s ( +- 0.83) 30.82 GB/s ( +-
0.08) -15.5%
1024 MB 36.85 GB/s ( +- 0.26) 30.71 GB/s ( +-
0.2) -20.0%
>
> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
> ---
> arch/x86/include/asm/page_64.h | 1 +
> arch/x86/lib/clear_page_64.S | 21 +++++++++++++++++++++
> 2 files changed, 22 insertions(+)
>
> diff --git a/arch/x86/include/asm/page_64.h b/arch/x86/include/asm/page_64.h
> index a88a3508888a..3affc4ecb8da 100644
> --- a/arch/x86/include/asm/page_64.h
> +++ b/arch/x86/include/asm/page_64.h
> @@ -55,6 +55,7 @@ extern unsigned long __phys_addr_symbol(unsigned long);
> void clear_pages_orig(void *page, unsigned long npages);
> void clear_pages_rep(void *page, unsigned long npages);
> void clear_pages_erms(void *page, unsigned long npages);
> +void clear_pages_movnt(void *page, unsigned long npages);
>
> #define __HAVE_ARCH_CLEAR_USER_PAGES
> static inline void clear_pages(void *page, unsigned int npages)
> diff --git a/arch/x86/lib/clear_page_64.S b/arch/x86/lib/clear_page_64.S
> index 2cc3b681734a..83d14f1c9f57 100644
> --- a/arch/x86/lib/clear_page_64.S
> +++ b/arch/x86/lib/clear_page_64.S
> @@ -58,3 +58,24 @@ SYM_FUNC_START(clear_pages_erms)
> RET
> SYM_FUNC_END(clear_pages_erms)
> EXPORT_SYMBOL_GPL(clear_pages_erms)
> +
> +SYM_FUNC_START(clear_pages_movnt)
> + xorl %eax,%eax
> + movq %rsi,%rcx
> + shlq $PAGE_SHIFT, %rcx
> +
> + .p2align 4
> +.Lstart:
> + movnti %rax, 0x00(%rdi)
> + movnti %rax, 0x08(%rdi)
> + movnti %rax, 0x10(%rdi)
> + movnti %rax, 0x18(%rdi)
> + movnti %rax, 0x20(%rdi)
> + movnti %rax, 0x28(%rdi)
> + movnti %rax, 0x30(%rdi)
> + movnti %rax, 0x38(%rdi)
> + addq $0x40, %rdi
> + subl $0x40, %ecx
> + ja .Lstart
> + RET
> +SYM_FUNC_END(clear_pages_movnt)
> --
> 2.31.1
>

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