Messages in this thread | | | From | Andy Lutomirski <> | Date | Thu, 19 Jul 2018 09:45:47 -0700 | Subject | Re: [PATCH 4/7] x86,tlb: make lazy TLB mode lazier |
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[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@surriel.com> 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 shouldn’t be hot, so I assume you’re 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.
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