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

[Anshuman Khandual]

On 9/20/22 09:09, Barry Song wrote:
> On Tue, Sep 20, 2022 at 3:00 PM Anshuman Khandual
> <anshuman.khandual at arm.com> 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 !