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>

arch/powerpc/include/asm/mmu_context.h

@@ -72,6 +72,7 @@ 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 */
+	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 */