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SubjectRe: [PATCH 11/11] mm,sched: conditionally skip lazy TLB mm refcounting
On Wed, Aug 01, 2018 at 06:02:55AM -0400, Rik van Riel wrote:
> Conditionally skip lazy TLB mm refcounting. When an architecture has
> CONFIG_ARCH_NO_ACTIVE_MM_REFCOUNTING enabled, an mm that is used in
> lazy TLB mode anywhere will get shot down from exit_mmap, and there
> in no need to incur the cache line bouncing overhead of refcounting
> a lazy TLB mm.
>
> Implement this by moving the refcounting of a lazy TLB mm to helper
> functions, which skip the refcounting when it is not necessary.
>
> Deal with use_mm and unuse_mm by fully splitting out the refcounting
> of the lazy TLB mm a kernel thread may have when entering use_mm from
> the refcounting of the mm that use_mm is about to start using.


> @@ -2803,16 +2803,29 @@ context_switch(struct rq *rq, struct task_struct *prev,
> * membarrier after storing to rq->curr, before returning to
> * user-space.
> */
> + /*
> + * kernel -> kernel lazy + transfer active
> + * user -> kernel lazy + grab_lazy_mm active
> + *
> + * kernel -> user switch + drop_lazy_mm active
> + * user -> user switch
> + */
> + if (!mm) { // to kernel
> next->active_mm = oldmm;
> enter_lazy_tlb(oldmm, next);
> +
> + if (prev->mm) // from user
> + grab_lazy_mm(oldmm);
> + else
> + prev->active_mm = NULL;
> + } else { // to user
> switch_mm_irqs_off(oldmm, mm, next);
>
> + if (!prev->mm) { // from kernel
> + /* will drop_lazy_mm() in finish_task_switch(). */
> + rq->prev_mm = oldmm;
> + prev->active_mm = NULL;
> + }
> }

So this still confuses the heck out of me; and the Changelog doesn't
seem to even mention it. You still track and swizzle ->active_mm but no
longer refcount it.

Why can't we skip the ->active_mm swizzle and keep ->active_mm == ->mm.

Doing the swizzle but not the refcount just makes me itch.

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