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powerpc: During context switch, check before setting mm_cpumask 

During context switch, switch_mm() sets our current CPU in mm_cpumask.
We can avoid this atomic sequence in most cases by checking before
setting the bit.

Testing on a POWER8 using our context switch microbenchmark:

tools/testing/selftests/powerpc/benchmarks/context_switch \
	--process --no-fp --no-altivec --no-vector

Performance improves 2%.

Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
This commit is contained in:
Anton Blanchard 2016-10-03 17:40:29 +11:00 committed by Michael Ellerman
parent 91ac730b8b
commit bb85fb5803
1 changed files with 2 additions and 1 deletions
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3
arch/powerpc/include/asm/mmu_context.h
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@ -72,7 +72,8 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
struct task_struct *tsk) struct task_struct *tsk)
{ {
/* Mark this context has been used on the new CPU */ /* Mark this context has been used on the new CPU */
cpumask_set_cpu(smp_processor_id(), mm_cpumask(next)); if (!cpumask_test_cpu(smp_processor_id(), mm_cpumask(next)))
cpumask_set_cpu(smp_processor_id(), mm_cpumask(next));
/* 32-bit keeps track of the current PGDIR in the thread struct */ /* 32-bit keeps track of the current PGDIR in the thread struct */
#ifdef CONFIG_PPC32 #ifdef CONFIG_PPC32