Date: Fri, 11 Jul 2014 11:04:05 -0500 (CDT)
From: Christoph Lameter <cl@gentwo.org>
To: Tejun Heo <tj@kernel.org>
cc: Jiang Liu <jiang.liu@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@suse.de>, David Rientjes <rientjes@google.com>,
        Mike Galbraith <umgwanakikbuti@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        Vladimir Davydov <vdavydov@parallels.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Rik van Riel <riel@redhat.com>, Wanpeng Li <liwanp@linux.vnet.ibm.com>,
        Zhang Yanfei <zhangyanfei@cn.fujitsu.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Jianyu Zhan <nasa4836@gmail.com>, malc <av1474@comtv.ru>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Fabian Frederick <fabf@skynet.be>, Tony Luck <tony.luck@intel.com>,
        linux-mm@kvack.org, linux-hotplug@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [RFC Patch V1 07/30] mm: Use cpu_to_mem()/numa_mem_id() to
 support memoryless node
In-Reply-To: <20140711155838.GB30865@htj.dyndns.org>
Message-ID: <alpine.DEB.2.11.1407111101070.27434@gentwo.org>
References: <1405064267-11678-1-git-send-email-jiang.liu@linux.intel.com> <1405064267-11678-8-git-send-email-jiang.liu@linux.intel.com> <20140711144205.GA27706@htj.dyndns.org> <alpine.DEB.2.11.1407111012210.25527@gentwo.org> <20140711152156.GB29137@htj.dyndns.org>
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Sender: linux-kernel-owner@vger.kernel.org

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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