Re: [PATCH 2/4] mm: rmap: Allow platforms without mm_cpumask to defer TLB flush

From: Barry Song <21cnbao@gmail.com>
To: Nadav Amit <namit@vmware.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Jonathan Corbet <corbet@lwn.net>,
	"darren@os.amperecomputing.com" <darren@os.amperecomputing.com>,
	"guojian@oppo.com" <guojian@oppo.com>,
	"huzhanyuan@oppo.com" <huzhanyuan@oppo.com>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	"linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-mm@kvack.org" <linux-mm@kvack.org>,
	"lipeifeng@oppo.com" <lipeifeng@oppo.com>,
	"mgorman@suse.de" <mgorman@suse.de>,
	"realmz6@gmail.com" <realmz6@gmail.com>,
	"v-songbaohua@oppo.com" <v-songbaohua@oppo.com>,
	"will@kernel.org" <will@kernel.org>,
	"x86@kernel.org" <x86@kernel.org>,
	"yangyicong@hisilicon.com" <yangyicong@hisilicon.com>,
	"zhangshiming@oppo.com" <zhangshiming@oppo.com>
Subject: Re: [PATCH 2/4] mm: rmap: Allow platforms without mm_cpumask to defer TLB flush
Date: Fri, 8 Jul 2022 20:17:12 +1200	[thread overview]
Message-ID: <CAGsJ_4wp03dV0JqRk5NiSSEwPxvMH7z3SE0FhX8Dc8DjDPuVyw@mail.gmail.com> (raw)
In-Reply-To: <EE0C38ED-A213-4F5C-9F87-FD90E5035C21@vmware.com>

On Fri, Jul 8, 2022 at 8:08 PM Nadav Amit <namit@vmware.com> wrote:
>
> On Jul 7, 2022, at 11:59 PM, Barry Song <21cnbao@gmail.com> wrote:
>
> >> 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?
> >
> > Yep. good suggestion, Nadav. So the code will be as below, right?
>
> Hmmm… Although it is likely to work (because only x86 and arm would use this
> batch flushing), I think that for consistency ARCH_HAS_MM_CPUMASK should be
> correct for all architectures.
>
> Is it really only x86 that has mm_cpumask()?

i am quite sure there are some other platforms having mm_cpumask().
for example, arm(not arm64).
but i am not exactly sure of the situation of each individual arch. thus,
i don't risk changing their source code.
but arm64 is the second platform looking for tlbbatch, and
ARCH_HAS_MM_CPUMASK only affects tlbbatch. so i would
expect those platforms to fill their ARCH_HAS_MM_CPUMASK
while they start to bringup their tlbbatch? for this moment,
we only need to make certain we don't break x86?
does it make sense?

Thanks
Barry

>