Re: [PATCH] powerpc: During context switch, check before setting mm_cpumask

From: Anton Blanchard <redacted>

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 <redacted>
---
 arch/powerpc/include/asm/mmu_context.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/powerpc/include/asm/mmu_context.h b/arch/powerpc/include/asm/mmu_context.h
index 475d1be..5c45114 100644
--- a/arch/powerpc/include/asm/mmu_context.h
+++ b/arch/powerpc/include/asm/mmu_context.h

@@ -72,7 +72,8 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 			     struct task_struct *tsk)
 {
 	/* 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));