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

[Anshuman Khandual]

On 9/21/22 07:21, Barry Song wrote:
> 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;
> }

This is a good starting point.