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

[Andrew Morton <akpm@linux-foundation.org>]

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.

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