Re: [PATCH v7 1/2] mm/tlbbatch: Introduce arch_tlbbatch_should_defer()

[Yicong Yang <yangyicong@huawei.com>]

On 2022/11/30 7:23, Andrew Morton wrote:
> On Thu, 17 Nov 2022 16:26:47 +0800 Yicong Yang <yangyicong@huawei.com> wrote:
> 
>> From: Anshuman Khandual <khandual@linux.vnet.ibm.com>
>>
>> The entire scheme of deferred TLB flush in reclaim path rests on the
>> fact that the cost to refill TLB entries is less than flushing out
>> individual entries by sending IPI to remote CPUs. But architecture
>> can have different ways to evaluate that. Hence apart from checking
>> TTU_BATCH_FLUSH in the TTU flags, rest of the decision should be
>> architecture specific.
>>
>> ...
>>
>> --- a/arch/x86/include/asm/tlbflush.h
>> +++ b/arch/x86/include/asm/tlbflush.h
>> @@ -240,6 +240,18 @@ static inline void flush_tlb_page(struct vm_area_struct *vma, unsigned long a)
>>  	flush_tlb_mm_range(vma->vm_mm, a, a + PAGE_SIZE, PAGE_SHIFT, false);
>>  }
>>  
>> +static inline bool arch_tlbbatch_should_defer(struct mm_struct *mm)
>> +{
>> +	bool should_defer = false;
>> +
>> +	/* If remote CPUs need to be flushed then defer batch the flush */
>> +	if (cpumask_any_but(mm_cpumask(mm), get_cpu()) < nr_cpu_ids)
>> +		should_defer = true;
>> +	put_cpu();
>> +
>> +	return should_defer;
>> +}
>> +
>>  static inline u64 inc_mm_tlb_gen(struct mm_struct *mm)
>>  {
>>  	/*
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index 2ec925e5fa6a..a9ab10bc0144 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -685,17 +685,10 @@ static void set_tlb_ubc_flush_pending(struct mm_struct *mm, bool writable)
>>   */
>>  static bool should_defer_flush(struct mm_struct *mm, enum ttu_flags flags)
>>  {
>> -	bool should_defer = false;
>> -
>>  	if (!(flags & TTU_BATCH_FLUSH))
>>  		return false;
>>  
>> -	/* If remote CPUs need to be flushed then defer batch the flush */
>> -	if (cpumask_any_but(mm_cpumask(mm), get_cpu()) < nr_cpu_ids)
>> -		should_defer = true;
>> -	put_cpu();
>> -
>> -	return should_defer;
>> +	return arch_tlbbatch_should_defer(mm);
>>  }
> 
> I think this conversion could have been done better.
> 
> should_defer_flush() is compiled if
> CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH.  So the patch implicitly
> assumes that only x86 implements
> CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH.  Presently true, but what
> happens if sparc (for example) wants to set
> CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH?  Now sparc needs its private
> version of arch_tlbbatch_should_defer(), even if that is identical to
> x86's.
> 

The current logic is if architecture want to enable batched TLB flush, they
need to implement their own version of arch_tlbbatch_should_defer() (for the
hint to defer the TLB flush) and arch_tlbbatch_add_mm() (for pending TLB flush)
and select ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH. That's what we do in Patch 2/2 for
enabling this on arm64.

Since it is architecture specific, we must rely on the architecture to implement
these two functions. Only select the ARCH_HAS_ARCH_TLBBATCH_SHOULD_DEFER is not
enough.

> Wouldn't it be better to make should_defer_flush() a __weak
> function in rmap.c, or a static inline inside #ifndef
> ARCH_HAS_ARCH_TLBBATCH_SHOULD_DEFER, or whatever technique best fits?
> 

When ARCH_HAS_ARCH_TLBBATCH_SHOULD_DEFER is not selected, should_defer_flush()
is implemented to only return false. I think this match what you want already.

Thanks.