linux-kernel - Re: [PATCH 2/4] mm: rmap: Allow platforms without mm

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Date:   Fri, 8 Jul 2022 06:36:47 +0000
From:   Nadav Amit <namit@...are.com>
To:     Barry Song <21cnbao@...il.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>, X86 ML <x86@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Arnd Bergmann <arnd@...db.de>,
        LKML <linux-kernel@...r.kernel.org>,
        "darren@...amperecomputing.com" <darren@...amperecomputing.com>,
        "yangyicong@...ilicon.com" <yangyicong@...ilicon.com>,
        "huzhanyuan@...o.com" <huzhanyuan@...o.com>,
        "lipeifeng@...o.com" <lipeifeng@...o.com>,
        "zhangshiming@...o.com" <zhangshiming@...o.com>,
        "guojian@...o.com" <guojian@...o.com>,
        "realmz6@...il.com" <realmz6@...il.com>,
        Barry Song <v-songbaohua@...o.com>,
        Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH 2/4] mm: rmap: Allow platforms without mm_cpumask to defer
 TLB flush

On Jul 7, 2022, at 5:52 AM, Barry Song <21cnbao@...il.com> wrote:

> From: Barry Song <v-songbaohua@...o.com>
> 
> Platforms like ARM64 have hareware TLB shootdown broadcast. They
> don't maintain mm_cpumask and they just send tlbi and related
> sync instructions for TLB flush.
> So if mm_cpumask is empty, we also allow deferred TLB flush
> 
> Cc: Nadav Amit <namit@...are.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Signed-off-by: Barry Song <v-songbaohua@...o.com>>
> ---
> mm/rmap.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 5bcb334cd6f2..d320c29a4ad8 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -692,8 +692,13 @@ static bool should_defer_flush(struct mm_struct *mm, enum ttu_flags flags)
> 	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)
> +	/*
> +	 * If remote CPUs need to be flushed then defer batch the flush;
> +	 * If ARCHs like ARM64 have hardware TLB flush broadcast, thus
> +	 * they don't maintain mm_cpumask() at all, defer batch as well.
> +	 */
> +	if (cpumask_any_but(mm_cpumask(mm), get_cpu()) < nr_cpu_ids ||
> +	    cpumask_empty(mm_cpumask(mm)))

The cpumask_empty() is indeed just another memory access, which is most
likely ok. But wouldn’t adding something like CONFIG_ARCH_HAS_MM_CPUMASK
make the code simpler and (slightly, certainly slightly) more performant?