Re: [PATCH v2 0/5] mm: Enable CONFIG_NODES_SPAN_OTHER_NODES by default for NUMA

From: Michal Hocko <mhocko@kernel.org>
To: Will Deacon <will@kernel.org>
Cc: Hoan Tran OS <hoan@os.amperecomputing.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	"open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>,
	Paul Mackerras <paulus@samba.org>,
	"H . Peter Anvin" <hpa@zytor.com>,
	"sparclinux@vger.kernel.org" <sparclinux@vger.kernel.org>,
	Alexander Duyck <alexander.h.duyck@linux.intel.com>,
	"linux-s390@vger.kernel.org" <linux-s390@vger.kernel.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	"x86@kernel.org" <x86@kernel.org>,
	Mike Rapoport <rppt@linux.ibm.com>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Ingo Molnar <mingo@redhat.com>, Vlastimil Babka <vbabka@suse.cz>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Open Source Submission <patches@amperecomputing.com>,
	Pavel Tatashin <pavel.tatashin@microsoft.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Will Deacon <will.deacon@arm.com>, Borislav Petkov <bp@alien8.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	"linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
	Oscar Salvador <osalvador@suse.de>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
	"David S . Miller" <davem@davemloft.net>,
	willy@infradead.org
Subject: Re: [PATCH v2 0/5] mm: Enable CONFIG_NODES_SPAN_OTHER_NODES by default for NUMA
Date: Fri, 12 Jul 2019 17:00:07 +0200	[thread overview]
Message-ID: <20190712150007.GU29483@dhcp22.suse.cz> (raw)
In-Reply-To: <20190712143730.au3662g4ua2tjudu@willie-the-truck>

On Fri 12-07-19 15:37:30, Will Deacon wrote:
> Hi all,
> 
> On Fri, Jul 12, 2019 at 02:12:23PM +0200, Michal Hocko wrote:
> > On Fri 12-07-19 10:56:47, Hoan Tran OS wrote:
> > [...]
> > > It would be good if we can enable it by-default. Otherwise, let arch 
> > > enables it by them-self. Do you have any suggestions?
> > 
> > I can hardly make any suggestions when it is not really clear _why_ you
> > want to remove this config option in the first place. Please explain
> > what motivated you to make this change.
> 
> Sorry, I think this confusion might actually be my fault and Hoan has just
> been implementing my vague suggestion here:
> 
> https://lore.kernel.org/linux-arm-kernel/20190625101245.s4vxfosoop52gl4e@willie-the-truck/
> 
> If the preference of the mm folks is to leave CONFIG_NODES_SPAN_OTHER_NODES
> as it is, then we can define it for arm64. I just find it a bit weird that
> the majority of NUMA-capable architectures have to add a symbol in the arch
> Kconfig file, for what appears to be a performance optimisation applicable
> only to ia64, mips and sh.
> 
> At the very least we could make the thing selectable.

Hmm, I thought this was selectable. But I am obviously wrong here.
Looking more closely, it seems that this is indeed only about
__early_pfn_to_nid and as such not something that should add a config
symbol. This should have been called out in the changelog though.

Also while at it, does HAVE_MEMBLOCK_NODE_MAP fall into a similar
bucket? Do we have any NUMA architecture that doesn't enable it?

Thanks!
-- 
Michal Hocko
SUSE Labs