Re: [PATCH v3 4/4] arm64: support batched/deferred tlb shootdown during page reclamation

[Barry Song]

On Wed, Sep 21, 2022 at 1:50 PM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> On Tue, Sep 20, 2022 at 8:45 PM Anshuman Khandual
> <anshuman.khandual@xxxxxxx> wrote:
> >
> >
> >
> > On 9/20/22 09:09, Barry Song wrote:
> > > On Tue, Sep 20, 2022 at 3:00 PM Anshuman Khandual
> > > <anshuman.khandual@xxxxxxx> wrote:
> > >>
> > >>
> > >> On 8/22/22 13:51, Yicong Yang wrote:
> > >>> +static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm)
> > >>> +{
> > >>> +     return true;
> > >>> +}
> > >>
> > >> This needs to be conditional on systems, where there will be performance
> > >> improvements, and should not just be enabled all the time on all systems.
> > >> num_online_cpus() > X, which does not hold any cpu hotplug lock would be
> > >> a good metric ?
> > >
> > > for a small system, i don't see how this patch will help, e.g. cpus <= 4;
> > > so we can actually disable tlb-batch on small systems.
> >
> > Do not subscribe ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH based on NR_CPUS ?
> > That might not help much as the default value is 256 for NR_CPUS.
> >
> > OR
> >
> > arch_tlbbatch_should_defer() checks on
> >
> > 1. online cpus                  (dont enable batched TLB if <= X)
> > 2. ARM64_WORKAROUND_REPEAT_TLBI (dont enable batched TLB)
> >
> > > just need to check if we will have any race condition since hotplug will
> > > make the condition true and false dynamically.
> >
> > If should_defer_flush() evaluate to be false, then ptep_clear_flush()
> > clears and flushes the entry right away. This should not race with other
> > queued up TLBI requests, which will be flushed separately. Wondering how
> > there can be a race here !
>
> Right. How about we make something as below?
>
> static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm)
> {
>     /* for a small system very small number of CPUs, TLB shootdown is cheap */
>     if (num_online_cpus() <= 4 ||
> unlikely(this_cpu_has_cap(ARM64_WORKAROUND_REPEAT_TLBI)))
>           return false;
>
> #ifdef CONFIG_ARM64_WORKAROUND_REPEAT_TLBI
>     if (unlikely(this_cpu_has_cap(ARM64_WORKAROUND_REPEAT_TLBI)))
>          return false;
> #endif
>
>     return true;
> }

sorry, i mean

static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm)
{
    /* for a small system very small number of CPUs, TLB shootdown is cheap */
    if (num_online_cpus() <= 4)
            return false;

#ifdef CONFIG_ARM64_WORKAROUND_REPEAT_TLBI
    if (unlikely(this_cpu_has_cap(ARM64_WORKAROUND_REPEAT_TLBI)))
            return false;
#endif

    return true;
}

>
> Thanks
> Barry