linux-kernel - Re: [PATCH v3 2/4] mm/tlbbatch: Introduce arch_tlbbatch_should

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Date:   Fri, 9 Sep 2022 09:46:04 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Yicong Yang <yangyicong@...wei.com>, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-arm-kernel@...ts.infradead.org,
        x86@...nel.org, catalin.marinas@....com, will@...nel.org,
        linux-doc@...r.kernel.org
Cc:     corbet@....net, peterz@...radead.org, arnd@...db.de,
        linux-kernel@...r.kernel.org, darren@...amperecomputing.com,
        yangyicong@...ilicon.com, huzhanyuan@...o.com, lipeifeng@...o.com,
        zhangshiming@...o.com, guojian@...o.com, realmz6@...il.com,
        linux-mips@...r.kernel.org, openrisc@...ts.librecores.org,
        linuxppc-dev@...ts.ozlabs.org, linux-riscv@...ts.infradead.org,
        linux-s390@...r.kernel.org, Barry Song <21cnbao@...il.com>,
        wangkefeng.wang@...wei.com, xhao@...ux.alibaba.com,
        prime.zeng@...ilicon.com,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Subject: Re: [PATCH v3 2/4] mm/tlbbatch: Introduce
 arch_tlbbatch_should_defer()

On 8/22/22 13:51, Yicong Yang wrote:
> From: Anshuman Khandual <khandual@...ux.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.
> 
> Signed-off-by: Anshuman Khandual <khandual@...ux.vnet.ibm.com>
> [https://lore.kernel.org/linuxppc-dev/20171101101735.2318-2-khandual@linux.vnet.ibm.com/]
> Signed-off-by: Yicong Yang <yangyicong@...ilicon.com>
> [Rebase and fix incorrect return value type]

>From semantics perspective, this patch still makes sense, even on its own.

Reviewed-by: Anshuman Khandual <anshuman.khandual@....com>

> ---
>  arch/x86/include/asm/tlbflush.h | 12 ++++++++++++
>  mm/rmap.c                       |  9 +--------
>  2 files changed, 13 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/include/asm/tlbflush.h b/arch/x86/include/asm/tlbflush.h
> index cda3118f3b27..8a497d902c16 100644
> --- 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 edc06c52bc82..a17a004550c6 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -687,17 +687,10 @@ 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);
>  }
>  
>  /*