powerpc: Copy only required pieces of the mm_context_t to the paca

Currently we copy the whole mm_context_t to the paca but only access a
few bits of it.  This is wasteful of space paca and also takes quite
some time in the hot path of context switching.

This patch pulls in only the required bits from the mm_context_t to
the paca and on context switch, copies only those.

Benchmarking this (On top of Anton's recent MSR context switching
changes [1]) using processes and yield shows an improvement of almost
3% on POWER8:

  http://ozlabs.org/~anton/junkcode/context_switch2.c
  ./context_switch2 --test=yield --process 0 0

1. https://lists.ozlabs.org/pipermail/linuxppc-dev/2015-October/135700.html

Signed-off-by: Michael Neuling <mikey@neuling.org>
[mpe: Rename paca fields to be mm_ctx_foo rather than context_foo]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 16 insertions, 2 deletions

diff --git a/arch/powerpc/include/asm/paca.h b/arch/powerpc/include/asm/paca.h
index 1cc6e0828907..ef78c288c712 100644
--- a/arch/powerpc/include/asm/paca.h
+++ b/arch/powerpc/include/asm/paca.h
@@ -16,6 +16,7 @@
 
 #ifdef CONFIG_PPC64
 
+#include <linux/string.h>
 #include <asm/types.h>
 #include <asm/lppaca.h>
 #include <asm/mmu.h>
@@ -132,7 +133,13 @@ struct paca_struct {
 #endif /* CONFIG_PPC_BOOK3E */
 
 #ifdef CONFIG_PPC_BOOK3S
-	mm_context_t context;
+	mm_context_id_t mm_ctx_id;
+#ifdef CONFIG_PPC_MM_SLICES
+	u64 mm_ctx_low_slices_psize;
+	unsigned char mm_ctx_high_slices_psize[SLICE_ARRAY_SIZE];
+#else
+	u16 mm_ctx_sllp;
+#endif
 #endif
 
 	/*
@@ -199,7 +206,14 @@ struct paca_struct {
 #ifdef CONFIG_PPC_BOOK3S
 static inline void copy_mm_to_paca(mm_context_t *context)
 {
-	get_paca()->context = *context;
+	get_paca()->mm_ctx_id = context->id;
+#ifdef CONFIG_PPC_MM_SLICES
+	get_paca()->mm_ctx_low_slices_psize = context->low_slices_psize;
+	memcpy(&get_paca()->mm_ctx_high_slices_psize,
+	       &context->high_slices_psize, SLICE_ARRAY_SIZE);
+#else
+	get_paca()->mm_ctx_sllp = context->sllp;
+#endif
 }
 #else
 static inline void copy_mm_to_paca(mm_context_t *context){}