Re: arm: Only load TLS values when needed

From: Jonathan Austin
Date: Thu Aug 15 2013 - 13:29:57 EST

Hi AndrÃ

(I've put RMK and LAKML back on Cc: they got dropped somewhere along the way)

On 14/08/13 22:21, Andrà Hentschel wrote:
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...?

We could use a ldrd at the top, that'd be nearly what we have right
now, don't we?

Yea, that'd be good - as far as I can see from an 1136 TRM, the ldrd *may* be two cycles (depending on alignment of the words) but the ldr and ldrne will always be two cycles. Ahhh, the joys of modifying the fast path ;)

Was expecting to see something that reflected this discussion,

Ah ok, i misunderstood that, sry.
Something like that?

From: Andrà Hentschel <nerv@xxxxxxxxxxx>

This patch intents to reduce loading instructions when the resulting value is not used.
It's a follow up on a4780adeefd042482f624f5e0d577bf9cdcbb760

You caught me just before running away for a week of holiday, but I've managed to squeeze in a bit of testing - hope this helps - I'll try to be clear about what I have/haven't tested so you can judge for yourself how much weight to give this.

What I've tested:

* Apply your code to 3.11-rc5
* Boot on Integrator CP (1176 r1p0) and run a full Debian Wheezy init

--> It works!

I also applied the following diff and rebuilt to check that we were really testing the V6 NOT V6K path here:

diff --git a/arch/arm/include/asm/tls.h b/arch/arm/include/asm/tls.h
index 31743f7..71dfe82 100644
--- a/arch/arm/include/asm/tls.h
+++ b/arch/arm/include/asm/tls.h
@@ -43,6 +43,7 @@
#define switch_tls switch_tls_v6
#elif defined(CONFIG_CPU_32v6K)
#define tls_emu 0
#define has_tls_reg 1
#define switch_tls switch_tls_v6k

What I've *not* tested:
* As this CPU does not have the tls register, I haven't run any tests that try to read/write it, but we wouldn't expect that to work, right?
* Haven't tried a V7 platform, as I understand you've already done that.
* Haven't verified it still builds for v4 (which was the problem for ldrd in the past and now we've put it back, so that's important)
* Performance impact

I think it'd be good if you can verify the bottom two points - especially the ldrd one: perhaps Russell can tell you which platform it broke on before?

So, that's a
Tested-by: Jonathan Austin <jonathan.austin@xxxxxxx>

If you want it :)
Hope that helps,


Signed-off-by: Andrà Hentschel <nerv@xxxxxxxxxxx>

This patch is against 28fbc8b6a29c849a3f03a6b05010d4b584055665

diff --git a/arch/arm/include/asm/tls.h b/arch/arm/include/asm/tls.h
index 83259b8..31743f7 100644
--- a/arch/arm/include/asm/tls.h
+++ b/arch/arm/include/asm/tls.h
@@ -3,29 +3,31 @@

#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

- .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

- .macro switch_tls_v6, base, tp, tpuser, tmp1, tmp2
+ .macro switch_tls_v6, prev, next, tp, tpuser, tmp1, tmp2
+ ldrd \tp, \tpuser, [\next, #TI_TP_VALUE]
ldr \tmp1, =elf_hwcap
ldr \tmp1, [\tmp1, #0]
mov \tmp2, #0xffff0fff
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
mcrne p15, 0, \tp, c13, c0, 3 @ yes, set TLS register
mcrne p15, 0, \tpuser, c13, c0, 2 @ set user r/w register
- strne \tmp2, [\base, #TI_TP_VALUE + 4] @ save it
+ strne \tmp2, [\prev, #TI_TP_VALUE + 4] @ save it

- .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
diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
index d40d0ef..11112de 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]
ldr r6, [r2, #TI_CPU_DOMAIN]
- switch_tls r1, r4, r5, r3, r7
+ switch_tls r1, r2, r4, r5, r3, r7
ldr r7, [r2, #TI_TASK]
ldr r8, =__stack_chk_guard

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