Re: [PATCH v5 2/2] arm64: support batched/deferred tlb shootdown during page reclamation

[Yicong Yang]

On 2022/11/14 22:19, Anshuman Khandual wrote:
> 
> 
> On 11/14/22 14:16, Yicong Yang wrote:
>> On 2022/11/14 11:29, Anshuman Khandual wrote:
>>>
>>> On 10/28/22 13:42, Yicong Yang wrote:
>>>> +static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm)
>>>> +{
>>>> +	/*
>>>> +	 * TLB batched flush is proved to be beneficial for systems with large
>>>> +	 * number of CPUs, especially system with more than 8 CPUs. TLB shutdown
>>>> +	 * is cheap on small systems which may not need this feature. So use
>>>> +	 * a threshold for enabling this to avoid potential side effects on
>>>> +	 * these platforms.
>>>> +	 */
>>>> +	if (num_online_cpus() <= CONFIG_ARM64_NR_CPUS_FOR_BATCHED_TLB)
>>>> +		return false;
>>>> +
>>>> +#ifdef CONFIG_ARM64_WORKAROUND_REPEAT_TLBI
>>>> +	if (unlikely(this_cpu_has_cap(ARM64_WORKAROUND_REPEAT_TLBI)))
>>>> +		return false;
>>>> +#endif
>>> should_defer_flush() is immediately followed by set_tlb_ubc_flush_pending() which calls
>>> arch_tlbbatch_add_mm(), triggering the actual TLBI flush via __flush_tlb_page_nosync().
>>> It should be okay to check capability with this_cpu_has_cap() as the entire call chain
>>> here is executed on the same cpu. But just wondering if cpus_have_const_cap() would be
>>> simpler, consistent, and also cost effective ?
>>>
>> ok. Checked cpus_have_const_cap() I think it matches your words.
>>
>>> Regardless, a comment is needed before the #ifdef block explaining why it does not make
>>> sense to defer/batch when __tlbi()/__tlbi_user() implementation will execute 'dsb(ish)'
>>> between two TLBI instructions to workaround the errata.
>>>
>> The workaround for the errata mentioned the affected platforms need the tlbi+dsb to be done
>> twice, so I'm not sure if we defer the final dsb will cause any problem so I think the judgement
>> here is used for safety. I have no such platform to test if it's ok to defer the last dsb.
> 
> We should not defer TLB flush on such systems, as ensured by the above test and 'false'
> return afterwards. The only question is whether this decision should be taken at a CPU
> level (which is affected by the errata) or the whole system level.
> 
> What is required now
> 
> - Replace this_cpu_has_cap() with cpus_have_const_cap ?
> - Add the following comment before the #ifdef check
> 

Have respin the series according to the above comments:
https://lore.kernel.org/lkml/20221115031425.44640-3-yangyicong@xxxxxxxxxx/

Thanks.

> /*
>  * TLB flush deferral is not required on systems, which are affected with
>  * ARM64_WORKAROUND_REPEAT_TLBI, as __tlbi()/__tlbi_user() implementation
>  * will have two consecutive TLBI instructions with a dsb(ish) in between
>  * defeating the purpose (i.e save overall 'dsb ish' cost).
>  */
> 
> .
>