Re: [PATCH v2 11/11] mm,sched: conditionally skip lazy TLB mm refcounting

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Jul 30, 2018 at 10:30:11AM -0400, Rik van Riel wrote:

> > What happened to the rework I did there? That not only avoided
> > fiddling
> > with active_mm, but also avoids grab/drop cycles for the other
> > architectures when doing task->kthread->kthread->task things.
> 
> I don't think I saw that. I only saw your email from
> July 20th with this fragment of code, which does not
> appear to avoid the grab/drop cycles, and still fiddles
> with active_mm:

Yeah, that's it. Note how it doesn't do a grab+drop for kernel->kernel,
where the current could would have.

And also note that it only fiddles with active_mm if it does the
grab+drop thing (the below should have s/ifdef/ifndef/ to make more
sense maybe).

So for ARCH_NO_ACTIVE_MM we never touch ->active_mm and therefore
->active_mm == ->mm.

> +       /*
> +        * kernel -> kernel   lazy + transfer active
> +        *   user -> kernel   lazy + mmgrab() active
> +        *
> +        * kernel ->   user   switch + mmdrop() active
> +        *   user ->   user   switch
> +        */
> +       if (!next->mm) {                                // to kernel
> +               enter_lazy_tlb(prev->active_mm, next);
> +
#ifndef ARCH_NO_ACTIVE_MM
> +               next->active_mm = prev->active_mm;
> +               if (prev->mm)                           // from user
> +                       mmgrab(prev->active_mm);
		else
			prev->active_mm = NULL;
> +#endif
> +       } else {                                        // to user
> +               switch_mm_irqs_off(prev->active_mm, next->mm, next);
> +
#ifndef ARCH_NO_ACTIVE_MM
> +               if (!prev->mm) {                        // from kernel
> +                       /* will mmdrop() in finish_task_switch(). */
> +                       rq->prev_mm = prev->active_mm;
> +                       prev->active_mm = NULL;
> +               }
> +#endif