Re: [PATCH 4/7] x86,tlb: make lazy TLB mode lazier

[Andy Lutomirski]

[I added PeterZ and Vitaly -- can you see any way in which this would
break something obscure?  I don't.]

On Thu, Jul 19, 2018 at 7:14 AM, Rik van Riel <riel@xxxxxxxxxxx> wrote:
> I guess we can skip both switch_ldt and load_mm_cr4 if real_prev equals
> next?

Yes, AFAICS.

>
> On to the lazy TLB mm_struct refcounting stuff :)
>
>>
>> Which refcount?  mm_users shouldnât be hot, so I assume youâre talking about
>> mm_count. My suggestion is to get rid of mm_count instead of trying to
>> optimize it.
>
>
> Do you have any suggestions on how? :)
>
> The TLB shootdown sent at __exit_mm time does not get rid of the
> kernelthread->active_mm
> pointer pointing at the mm that is exiting.
>

Ah, but that's conceptually very easy to fix.  Add a #define like
ARCH_NO_TASK_ACTIVE_MM.  Then just get rid of active_mm if that
#define is set.  After some grepping, there are very few users.  The
only nontrivial ones are the ones in kernel/ and mm/mmu_context.c that
are involved in the rather complicated dance of refcounting active_mm.
If that field goes away, it doesn't need to be refcounted.  Instead, I
think the refcounting can get replaced with something like:

/*
 * Release any arch-internal references to mm.  Only called when
mm_users is zero
 * and all tasks using mm have either been switch_mm()'d away or have had
 * enter_lazy_tlb() called.
 */
extern void arch_shoot_down_dead_mm(struct mm_struct *mm);

which the kernel calls in __mmput() after tearing down all the page
tables.  The body can be something like:

if (WARN_ON(cpumask_any_but(mm_cpumask(...), ...)) {
  /* send an IPI.  Maybe just call tlb_flush_remove_tables() */
}

(You'll also have to fix up the highly questionable users in
arch/x86/platform/efi/efi_64.c, but that's easy.)

Does all that make sense?  Basically, as I understand it, the
expensive atomic ops you're seeing are all pointless because they're
enabling an optimization that hasn't actually worked for a long time,
if ever.