powerpc: During context switch, check before setting mm_cpumask

From: Anton Blanchard <anton@samba.org>

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

On 03/10/16 17:40, Anton Blanchard wrote:
> From: Anton Blanchard <anton@samba.org>
> 
> 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>
> ---
>  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));
>  

I think this makes sense, in fact I think in the longer term we can
even use __set_bit() reorder-able version since we have a sync
coming out of schedule(). The read side for TLB flush can use a RMB

Acked-by: Balbir Singh <bsingharora@gmail.com>

Balbir Singh.

On Tue, 2016-10-04 at 10:25 +1100, Balbir Singh wrote:
> I think this makes sense, in fact I think in the longer term we can
> even use __set_bit() reorder-able version since we have a sync
> coming out of schedule(). The read side for TLB flush can use a RMB

No, that wouldn't be atomic vs. other threads accessing the same
bitmap.

Ben.

On 04/10/16 10:58, Benjamin Herrenschmidt wrote:
> On Tue, 2016-10-04 at 10:25 +1100, Balbir Singh wrote:
>> I think this makes sense, in fact I think in the longer term we can
>> even use __set_bit() reorder-able version since we have a sync
>> coming out of schedule(). The read side for TLB flush can use a RMB
> 
> No, that wouldn't be atomic vs. other threads accessing the same
> bitmap.

Somethings distorted my thought process. Thanks!

Balbir Singh.

On Mon, 2016-03-10 at 06:40:29 UTC, Anton Blanchard wrote:
> From: Anton Blanchard <anton@samba.org>
> 
> 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>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/bb85fb5803270c52863b983596c2a0

cheers