Re: [RFC][PATCH] sched: Use lightweight hazard pointers to grab lazy mms

[Peter Zijlstra]

On Thu, Jun 17, 2021 at 11:08:03AM +0200, Peter Zijlstra wrote:
> On Wed, Jun 16, 2021 at 10:32:15PM -0700, Andy Lutomirski wrote:

> --- a/arch/x86/include/asm/mmu.h
> +++ b/arch/x86/include/asm/mmu.h
> @@ -66,4 +66,9 @@ typedef struct {
>  void leave_mm(int cpu);
>  #define leave_mm leave_mm
>  
> +/* On x86, mm_cpumask(mm) contains all CPUs that might be lazily using mm */
> +#define for_each_possible_lazymm_cpu(cpu, mm) \
> +	for_each_cpu((cpu), mm_cpumask((mm)))
> +
> +
>  #endif /* _ASM_X86_MMU_H */

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 8ac693d542f6..e102ec53c2f6 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -19,6 +19,7 @@
>  

> +
> +#ifndef for_each_possible_lazymm_cpu
> +#define for_each_possible_lazymm_cpu(cpu, mm) for_each_online_cpu((cpu))
> +#endif
> +

Why can't the x86 implementation be the default? IIRC the problem with
mm_cpumask() is that (some) architectures don't clear bits, but IIRC
they all should be setting bits, or were there archs that didn't even do
that?