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[PATCH 0/2] *** SUBJECT HERE ***

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  [PATCH 0/2] *** SUBJECT HERE *** Andrew Pinski <apinski@cavium.com> - 2016-03-13 21:00 +0100
    [PATCH 1/2] ARM64:VDSO: Improve gettimeofday, don't use udiv Andrew Pinski <apinski@cavium.com> - 2016-03-13 21:00 +0100
      Re: [PATCH 1/2] ARM64:VDSO: Improve gettimeofday, don't use udiv Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2016-03-14 08:00 +0100
        Re: [PATCH 1/2] ARM64:VDSO: Improve gettimeofday, don't use udiv Mark Rutland <mark.rutland@arm.com> - 2016-03-14 09:00 +0100

#1356733 — [PATCH 0/2] *** SUBJECT HERE ***

*** BLURB HERE ***

Andrew Pinski (2):
  ARM64:VDSO: Improve gettimeofday, don't use udiv
  ARM64:VDSO: Improve __do_get_tspec, don't use udiv

 arch/arm64/kernel/vdso/gettimeofday.S |   47 ++++++++++++++++++++++++--------
 1 files changed, 35 insertions(+), 12 deletions(-)

-- 
1.7.2.5

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#1356735 — [PATCH 1/2] ARM64:VDSO: Improve gettimeofday, don't use udiv

On many cores, udiv with a large value is slow, expand instead
the division out to be what GCC would have generated for the
divide by 1000.

On ThunderX, the speeds up gettimeofday by 5%.

Signed-off-by: Andrew Pinski <apinski@cavium.com>
---
 arch/arm64/kernel/vdso/gettimeofday.S |   20 ++++++++++++++++----
 1 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/kernel/vdso/gettimeofday.S b/arch/arm64/kernel/vdso/gettimeofday.S
index efa79e8..e5caef9 100644
--- a/arch/arm64/kernel/vdso/gettimeofday.S
+++ b/arch/arm64/kernel/vdso/gettimeofday.S
@@ -64,10 +64,22 @@ ENTRY(__kernel_gettimeofday)
 	bl	__do_get_tspec
 	seqcnt_check w9, 1b
 
-	/* Convert ns to us. */
-	mov	x13, #1000
-	lsl	x13, x13, x12
-	udiv	x11, x11, x13
+	/* Undo the shift. */
+        lsr     x11, x11, x12
+
+	/* Convert ns to us (division by 1000 by using multiply high).
+	 * This is how GCC converts the division by 1000 into.
+	 * This is faster than divide on most cores.
+	 */
+        mov     x13, 63439
+        movk    x13, 0xe353, lsl 16
+        lsr     x11, x11, 3
+        movk    x13, 0x9ba5, lsl 32
+        movk    x13, 0x20c4, lsl 48
+	/* x13 = 0x20c49ba5e353f7cf */
+        umulh   x11, x11, x13
+        lsr     x11, x11, 4
+
 	stp	x10, x11, [x0, #TVAL_TV_SEC]
 2:
 	/* If tz is NULL, return 0. */
-- 
1.7.2.5

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#1356977 — Re: [PATCH 1/2] ARM64:VDSO: Improve gettimeofday, don't use udiv

On 13 March 2016 at 20:50, Andrew Pinski <apinski@cavium.com> wrote:
> On many cores, udiv with a large value is slow, expand instead
> the division out to be what GCC would have generated for the
> divide by 1000.
>
> On ThunderX, the speeds up gettimeofday by 5%.
>
> Signed-off-by: Andrew Pinski <apinski@cavium.com>
> ---
>  arch/arm64/kernel/vdso/gettimeofday.S |   20 ++++++++++++++++----
>  1 files changed, 16 insertions(+), 4 deletions(-)
>
> diff --git a/arch/arm64/kernel/vdso/gettimeofday.S b/arch/arm64/kernel/vdso/gettimeofday.S
> index efa79e8..e5caef9 100644
> --- a/arch/arm64/kernel/vdso/gettimeofday.S
> +++ b/arch/arm64/kernel/vdso/gettimeofday.S
> @@ -64,10 +64,22 @@ ENTRY(__kernel_gettimeofday)
>         bl      __do_get_tspec
>         seqcnt_check w9, 1b
>
> -       /* Convert ns to us. */
> -       mov     x13, #1000
> -       lsl     x13, x13, x12
> -       udiv    x11, x11, x13
> +       /* Undo the shift. */
> +        lsr     x11, x11, x12
> +
> +       /* Convert ns to us (division by 1000 by using multiply high).
> +        * This is how GCC converts the division by 1000 into.
> +        * This is faster than divide on most cores.
> +        */
> +        mov     x13, 63439

Please don't mix hex and decimal constants

> +        movk    x13, 0xe353, lsl 16
> +        lsr     x11, x11, 3
> +        movk    x13, 0x9ba5, lsl 32
> +        movk    x13, 0x20c4, lsl 48
> +       /* x13 = 0x20c49ba5e353f7cf */

Could we clean this up a bit? Something along the lines of

.set m, 0x20c49ba5e353f7cf
movz x13,#:abs_g3:m
movk x13, #:abs:g2_nc:m
movk x13, #:abs_g1_nc:m
movk x13, #:abs_g0_nc:m

Actually, the movz/movk sequence should probably be implemented as a
macro in asm/assembler.h, with parameters for the register and the
symbol name. I think Mark proposed such a patch at some point

> +        umulh   x11, x11, x13
> +        lsr     x11, x11, 4
> +
>         stp     x10, x11, [x0, #TVAL_TV_SEC]
>  2:
>         /* If tz is NULL, return 0. */
> --
> 1.7.2.5
>
>
> _______________________________________________
> linux-arm-kernel mailing list
> linux-arm-kernel@lists.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel

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#1357022 — Re: [PATCH 1/2] ARM64:VDSO: Improve gettimeofday, don't use udiv

On Mon, Mar 14, 2016 at 07:55:38AM +0100, Ard Biesheuvel wrote:
> On 13 March 2016 at 20:50, Andrew Pinski <apinski@cavium.com> wrote:
> > +        movk    x13, 0xe353, lsl 16
> > +        lsr     x11, x11, 3
> > +        movk    x13, 0x9ba5, lsl 32
> > +        movk    x13, 0x20c4, lsl 48
> > +       /* x13 = 0x20c49ba5e353f7cf */
> 
> Could we clean this up a bit? Something along the lines of
> 
> .set m, 0x20c49ba5e353f7cf
> movz x13,#:abs_g3:m
> movk x13, #:abs:g2_nc:m
> movk x13, #:abs_g1_nc:m
> movk x13, #:abs_g0_nc:m
> 
> Actually, the movz/movk sequence should probably be implemented as a
> macro in asm/assembler.h, with parameters for the register and the
> symbol name.

Agreed.

> I think Mark proposed such a patch at some point

That would be [1], which needs the relocations fixed up [2,3] to match the
above.

I didn't respin that as it turned out to be unnecessary at the time, but I'm
more than happy for someone to pick it up.

Mark.

[1] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-January/397563.html
[2] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-January/397572.html
[3] http://lists.infradead.org/pipermail/linux-arm-kernel/2016-January/397573.html

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