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powerpc: Create context switch helpers save_sprs() and restore_sprs()

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Move all our context switch SPR save and restore code into two
helpers. We do a few optimisations:

- Group all mfsprs and all mtsprs. In many cases an mtspr sets a
scoreboarding bit that an mfspr waits on, so the current practise of
mfspr A; mtspr A; mfpsr B; mtspr B is the worst scheduling we can
do.

- SPR writes are slow, so check that the value is changing before
writing it.

A context switch microbenchmark using yield():

http://ozlabs.org/~anton/junkcode/context_switch2.c

./context_switch2 --test=yield 0 0

shows an improvement of almost 10% on POWER8.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
This commit is contained in:
Anton Blanchard
2015-10-29 11:43:55 +11:00
committed by Michael Ellerman
parent af72ab646a
commit 152d523e63
4 changed files with 82 additions and 82 deletions
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11
arch/powerpc/include/asm/switch_to.h
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@@ -15,17 +15,6 @@ extern struct task_struct *__switch_to(struct task_struct *,
struct thread_struct;
extern struct task_struct *_switch(struct thread_struct *prev,
struct thread_struct *next);
#ifdef CONFIG_PPC_BOOK3S_64
static inline void save_early_sprs(struct thread_struct *prev)
{
if (cpu_has_feature(CPU_FTR_ARCH_207S))
prev->tar = mfspr(SPRN_TAR);
if (cpu_has_feature(CPU_FTR_DSCR))
prev->dscr = mfspr(SPRN_DSCR);
}
#else
static inline void save_early_sprs(struct thread_struct *prev) {}
#endif
extern void enable_kernel_fp(void);
extern void enable_kernel_altivec(void);