linux-kernel - Re: [RFC Patch V1 07/30] mm: Use cpu_to_mem()/numa_mem

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Date:	Fri, 11 Jul 2014 11:04:05 -0500 (CDT)
From:	Christoph Lameter <cl@...two.org>
To:	Tejun Heo <tj@...nel.org>
cc:	Jiang Liu <jiang.liu@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Jianyu Zhan <nasa4836@...il.com>, malc <av1474@...tv.ru>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Fabian Frederick <fabf@...net.be>,
	Tony Luck <tony.luck@...el.com>, linux-mm@...ck.org,
	linux-hotplug@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC Patch V1 07/30] mm: Use cpu_to_mem()/numa_mem_id() to
 support memoryless node

On Fri, 11 Jul 2014, Tejun Heo wrote:

> On Fri, Jul 11, 2014 at 10:55:59AM -0500, Christoph Lameter wrote:
> > > Where X is the memless node.  num_mem_id() on X would return either B
> > > or C, right?  If B or C can't satisfy the allocation, the allocator
> > > would fallback to A from B and D for C, both of which aren't optimal.
> > > It should first fall back to C or B respectively, which the allocator
> > > can't do anymoe because the information is lost when the caller side
> > > performs numa_mem_id().
> >
> > True but the advantage is that the numa_mem_id() allows the use of a
> > consitent sort of "local" node which increases allocator performance due
> > to the abillity to cache objects from that node.
>
> But the allocator can do the mapping the same.  I really don't see why
> we'd push the distinction to the individual users.

The "users" (I guess you mean general kernel code/drivers) can use various
memory allocators which will do the right thing internally regarding
GFP_THISNODE. They do not need to worry too much about this unless there
are reasons beyond optimizing NUMA placement to need memory from a
particuylar node (f.e. a device that requires memory from a numa node that
is local to the PCI bus where the hardware resides).

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