On Mon, 2017-06-05 at 15:36 -0700, Andy Lutomirski wrote:

> +++ b/arch/x86/include/asm/mmu_context.h
> @@ -122,8 +122,10 @@ static inline void switch_ldt(struct mm_struct
> *prev, struct mm_struct *next)
>  
>  static inline void enter_lazy_tlb(struct mm_struct *mm, struct
> task_struct *tsk)
>  {
> -	if (this_cpu_read(cpu_tlbstate.state) == TLBSTATE_OK)
> -		this_cpu_write(cpu_tlbstate.state, TLBSTATE_LAZY);
> +	int cpu = smp_processor_id();
> +
> +	if (cpumask_test_cpu(cpu, mm_cpumask(mm)))
> +		cpumask_clear_cpu(cpu, mm_cpumask(mm));
>  }

This is an atomic write to a shared cacheline,
every time a CPU goes idle.

I am not sure you really want to do this, since
there are some workloads out there that have a
crazy number of threads, which go idle hundreds,
or even thousands of times a second, on dozens
of CPUs at a time. *cough*Java*cough*

Keeping track of the state in a CPU-local variable,
written with a non-atomic write, would be much more
CPU cache friendly here.

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