[RFC PATCH 6/7] powerpc/64s/radix: reset mm_cpumask for single thread process when possible

[Balbir Singh]

On Tue, Oct 31, 2017 at 6:18 PM, Nicholas Piggin <npiggin at gmail.com> wrote:
> When a single-threaded process has a non-local mm_cpumask and requires
> a full PID tlbie invalidation, use that as an opportunity to reset the
> cpumask back to the current CPU we're running on.
>
> There is a lot of tuning we can do with this, and more sophisticated
> management of PIDs and stale translations across CPUs, but this is
> something simple that can be done to significantly help single threaded
> processes without changing behaviour too much.
>
> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
> ---
>  arch/powerpc/include/asm/mmu_context.h | 19 +++++++++++++
>  arch/powerpc/mm/tlb-radix.c            | 52 +++++++++++++++++++++++++++-------
>  2 files changed, 60 insertions(+), 11 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/mmu_context.h b/arch/powerpc/include/asm/mmu_context.h
> index 20eae6f76247..05516027fd82 100644
> --- a/arch/powerpc/include/asm/mmu_context.h
> +++ b/arch/powerpc/include/asm/mmu_context.h
> @@ -5,6 +5,7 @@
>  #include <linux/kernel.h>
>  #include <linux/mm.h>
>  #include <linux/sched.h>
> +#include <linux/sched/mm.h>
>  #include <linux/spinlock.h>
>  #include <asm/mmu.h>
>  #include <asm/cputable.h>
> @@ -153,6 +154,24 @@ static inline void activate_mm(struct mm_struct *prev, struct mm_struct *next)
>  static inline void enter_lazy_tlb(struct mm_struct *mm,
>                                   struct task_struct *tsk)
>  {
> +#ifdef CONFIG_PPC_BOOK3S_64
> +       /*
> +        * Under radix, we do not want to keep lazy PIDs around because
> +        * even if the CPU does not access userspace, it can still bring
> +        * in translations through speculation and prefetching.
> +        *
> +        * Switching away here allows us to trim back the mm_cpumask in
> +        * cases where we know the process is not running on some CPUs
> +        * (see mm/tlb-radix.c).
> +        */
> +       if (radix_enabled() && mm != &init_mm) {
> +               mmgrab(&init_mm);
> +               tsk->active_mm = &init_mm;
> +               switch_mm_irqs_off(mm, tsk->active_mm, tsk);
> +               mmdrop(mm);
> +       }
> +#endif
> +

I thought we did not use enter_lazy_tlb for anything. Even unuse_mm() is not
called from common paths, do we care?

>         /* 64-bit Book3E keeps track of current PGD in the PACA */
>  #ifdef CONFIG_PPC_BOOK3E_64
>         get_paca()->pgd = NULL;
> diff --git a/arch/powerpc/mm/tlb-radix.c b/arch/powerpc/mm/tlb-radix.c
> index 49cc581a31cd..db7e696e4faf 100644
> --- a/arch/powerpc/mm/tlb-radix.c
> +++ b/arch/powerpc/mm/tlb-radix.c
> @@ -255,10 +255,18 @@ void radix__flush_tlb_mm(struct mm_struct *mm)
>                 return;
>
>         preempt_disable();
> -       if (!mm_is_thread_local(mm))
> -               _tlbie_pid(pid, RIC_FLUSH_TLB);
> -       else
> +       if (!mm_is_thread_local(mm)) {
> +               if (atomic_read(&mm->mm_users) == 1 && current->mm == mm) {
> +                       _tlbie_pid(pid, RIC_FLUSH_ALL);
> +                       atomic_set(&mm->context.active_cpus, 1);
> +                       cpumask_clear(mm_cpumask(mm));

I wonder if we can delegate this back to switch_mm_irqs_off() to lazily clear
the previous cpumask and check for changes to the current mask

Balbir