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SubjectRe: arm: Only load TLS values when needed
Hi André,

On 15/07/13 18:14, André Hentschel wrote:
> From: André Hentschel <nerv@dawncrow.de>
>
> This patch intents to reduce loading instructions when the resulting value is not used.
> It's a follow up on a4780adeefd042482f624f5e0d577bf9cdcbb760
>

Have you done any benchmarking to see that this has any real impact? Or
tested on a !Vv6k system? It looks possible that the only case where
this will perform better is where we're using switch_tls_none or
switch_tls_software (both rare cases, I think) and there's some change
it will make things worse in other cases?

One of the reasons for Russell's suggestion of placing the ldrd (which
became the two ldr instructions you've removed from __switch_to, in
order to maintain building for older cores) where it is was in order to
reduce the chance of pipeline stalls.

As I've pointed out below, there is some risk that changing that has
implications for the v6 only case below (and the v6k case is now more
prone to stalls with !CONFIG_CPU_USE_DOMAINS, but newer cores should
have more advanced scheduling to avoid such issues anyway...)

> Signed-off-by: André Hentschel <nerv@dawncrow.de>
>
> ---
> This patch is against Linux 3.11-rc1 (ad81f0545ef01ea651886dddac4bef6cec930092)
>
> Thanks to everyone who helped me with a4780adeefd042482f624f5e0d577bf9cdcbb760
>
> diff --git a/arch/arm/include/asm/tls.h b/arch/arm/include/asm/tls.h
> index 83259b8..3742722 100644
> --- a/arch/arm/include/asm/tls.h
> +++ b/arch/arm/include/asm/tls.h
> @@ -3,29 +3,32 @@
>
> #ifdef __ASSEMBLY__
> #include <asm/asm-offsets.h>
> - .macro switch_tls_none, base, tp, tpuser, tmp1, tmp2
> + .macro switch_tls_none, prev, next, tp, tpuser, tmp1, tmp2
> .endm
>
> - .macro switch_tls_v6k, base, tp, tpuser, tmp1, tmp2
> + .macro switch_tls_v6k, prev, next, tp, tpuser, tmp1, tmp2
> + ldrd \tp, \tpuser, [\next, #TI_TP_VALUE] @ get the next TLS and user r/w register
> mrc p15, 0, \tmp2, c13, c0, 2 @ get the user r/w register
> mcr p15, 0, \tp, c13, c0, 3 @ set TLS register
> mcr p15, 0, \tpuser, c13, c0, 2 @ and the user r/w register
> - str \tmp2, [\base, #TI_TP_VALUE + 4] @ save it
> + str \tmp2, [\prev, #TI_TP_VALUE + 4] @ save it
> .endm
>
> - .macro switch_tls_v6, base, tp, tpuser, tmp1, tmp2
> + .macro switch_tls_v6, prev, next, tp, tpuser, tmp1, tmp2
> ldr \tmp1, =elf_hwcap
> ldr \tmp1, [\tmp1, #0]
> mov \tmp2, #0xffff0fff
> + ldr \tp, [\next, #TI_TP_VALUE] @ get the next TLS register
> tst \tmp1, #HWCAP_TLS @ hardware TLS available?
> streq \tp, [\tmp2, #-15] @ set TLS value at 0xffff0ff0
> - mrcne p15, 0, \tmp2, c13, c0, 2 @ get the user r/w register
> + mrcne p15, 0, \tmp2, c13, c0, 2 @ get the previous user r/w register
> + ldrne \tpuser, [\next, #TI_TP_VALUE + 4] @ get the next user r/w register
> mcrne p15, 0, \tp, c13, c0, 3 @ yes, set TLS register
> mcrne p15, 0, \tpuser, c13, c0, 2 @ set user r/w register

Now we've only got one instruction between the store and the load and
risk stalling the pipeline...

Dave M cautiously says "The ancient advice was that one instruction was
enough" but this is very core dependent... I wonder if anyone has a good
idea about whether this is an issue here...?

> - strne \tmp2, [\base, #TI_TP_VALUE + 4] @ save it
> + strne \tmp2, [\prev, #TI_TP_VALUE + 4] @ save it
> .endm
>
> - .macro switch_tls_software, base, tp, tpuser, tmp1, tmp2
> + .macro switch_tls_software, prev, next, tp, tpuser, tmp1, tmp2
> mov \tmp1, #0xffff0fff
> str \tp, [\tmp1, #-15] @ set TLS value at 0xffff0ff0
> .endm
> diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
> index a39cfc2a1..1484b59 100644
> --- a/arch/arm/kernel/entry-armv.S
> +++ b/arch/arm/kernel/entry-armv.S
> @@ -689,12 +689,10 @@ ENTRY(__switch_to)
> THUMB( stmia ip!, {r4 - sl, fp} ) @ Store most regs on stack
> THUMB( str sp, [ip], #4 )
> THUMB( str lr, [ip], #4 )
> - ldr r4, [r2, #TI_TP_VALUE]
> - ldr r5, [r2, #TI_TP_VALUE + 4]
> #ifdef CONFIG_CPU_USE_DOMAINS
> ldr r6, [r2, #TI_CPU_DOMAIN]
> #endif
> - switch_tls r1, r4, r5, r3, r7
> + switch_tls r1, r2, r4, r5, r3, r7
> #if defined(CONFIG_CC_STACKPROTECTOR) && !defined(CONFIG_SMP)
> ldr r7, [r2, #TI_TASK]
> ldr r8, =__stack_chk_guard
>

Jonny

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