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From: Michael Neuling on 8 Jun 2010 01:00
The POWER7 core has dynamic SMT mode switching which is controlled by
the hypervisor. There are 3 SMT modes:
SMT1 uses thread 0
SMT2 uses threads 0 & 1
SMT4 uses threads 0, 1, 2 & 3
When in any particular SMT mode, all threads have the same performance
as each other (ie. at any moment in time, all threads perform the same).

The SMT mode switching works such that when linux has threads 2 & 3 idle
and 0 & 1 active, it will cede (H_CEDE hypercall) threads 2 and 3 in the
idle loop and the hypervisor will automatically switch to SMT2 for that
core (independent of other cores). The opposite is not true, so if
threads 0 & 1 are idle and 2 & 3 are active, we will stay in SMT4 mode.

Similarly if thread 0 is active and threads 1, 2 & 3 are idle, we'll go
into SMT1 mode.

If we can get the core into a lower SMT mode (SMT1 is best), the threads
will perform better (since they share less core resources). Hence when
we have idle threads, we want them to be the higher ones.

This adds a feature bit for asymmetric packing to powerpc and then
enables it on POWER7.

Signed-off-by: Michael Neuling <mikey(a)neuling.org>

---

arch/powerpc/include/asm/cputable.h | 3 ++-
arch/powerpc/kernel/process.c | 9 +++++++++
2 files changed, 11 insertions(+), 1 deletion(-)

Index: linux-2.6-ozlabs/arch/powerpc/include/asm/cputable.h
===================================================================
--- linux-2.6-ozlabs.orig/arch/powerpc/include/asm/cputable.h
+++ linux-2.6-ozlabs/arch/powerpc/include/asm/cputable.h
@@ -195,6 +195,7 @@ extern const char *powerpc_base_platform
#define CPU_FTR_SAO LONG_ASM_CONST(0x0020000000000000)
#define CPU_FTR_CP_USE_DCBTZ LONG_ASM_CONST(0x0040000000000000)
#define CPU_FTR_UNALIGNED_LD_STD LONG_ASM_CONST(0x0080000000000000)
+#define CPU_FTR_ASYM_SMT LONG_ASM_CONST(0x0100000000000000)

#ifndef __ASSEMBLY__

@@ -409,7 +410,7 @@ extern const char *powerpc_base_platform
CPU_FTR_MMCRA | CPU_FTR_SMT | \
CPU_FTR_COHERENT_ICACHE | CPU_FTR_LOCKLESS_TLBIE | \
CPU_FTR_PURR | CPU_FTR_SPURR | CPU_FTR_REAL_LE | \
- CPU_FTR_DSCR | CPU_FTR_SAO)
+ CPU_FTR_DSCR | CPU_FTR_SAO | CPU_FTR_ASYM_SMT)
#define CPU_FTRS_CELL (CPU_FTR_USE_TB | CPU_FTR_LWSYNC | \
CPU_FTR_PPCAS_ARCH_V2 | CPU_FTR_CTRL | \
CPU_FTR_ALTIVEC_COMP | CPU_FTR_MMCRA | CPU_FTR_SMT | \
Index: linux-2.6-ozlabs/arch/powerpc/kernel/process.c
===================================================================
--- linux-2.6-ozlabs.orig/arch/powerpc/kernel/process.c
+++ linux-2.6-ozlabs/arch/powerpc/kernel/process.c
@@ -1265,3 +1265,12 @@ unsigned long randomize_et_dyn(unsigned

return ret;
}
+
+int arch_sd_sibiling_asym_packing(void)
+{
+ if (cpu_has_feature(CPU_FTR_ASYM_SMT)){
+ printk_once(KERN_INFO "Enabling Asymmetric SMT scheduling\n");
+ return SD_ASYM_PACKING;
+ }
+ return 0;
+}
--
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