linux-kernel - Re: [PATCH part2 v2 0/8] Arrange hotpluggable memory as ZONE

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Date:	Tue, 15 Oct 2013 09:40:10 +0800
From:	Zhang Yanfei <zhangyanfei@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Yinghai Lu <yinghai@...nel.org>,
	Zhang Yanfei <zhangyanfei.yes@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>, Toshi Kani <toshi.kani@...com>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH part2 v2 0/8] Arrange hotpluggable memory as ZONE_MOVABLE

Hello tejun, peter and yinghai

On 10/15/2013 04:55 AM, Tejun Heo wrote:
> Hello,
> 
> On Mon, Oct 14, 2013 at 01:37:20PM -0700, Yinghai Lu wrote:
>> The problem is how to define "amount necessary". If we can parse srat early,
>> then we could just map RAM for all boot nodes one time, instead of try some
>> small and then after SRAT table, expand it cover non-boot nodes.
> 
> Wouldn't that amount be fairly static and restricted?  If you wanna
> chunk memory init anyway, there's no reason to init more than
> necessary until smp stage is reached.  The more you do early, the more
> serialized you're, so wouldn't the goal naturally be initing the
> minimum possible?
> 
>> To keep non-boot numa node hot-removable. we need to page table (and other
>> that we allocate during boot stage) on ram of non boot nodes, or their
>> local node ram.  (share page table always should be on boot nodes).
> 
> The above assumes the followings,
> 
> * 4k page mappings.  It'd be nice to keep everything working for 4k
>   but just following SRAT isn't enough.  What if the non-hotpluggable
>   boot node doesn't stretch high enough and page table reaches down
>   too far?  This won't be an optional behavior, so it is actually
>   *likely* to happen on certain setups.
> 
> * Memory hotplug is at NUMA node granularity instead of device.
> 
>>> Optimizing NUMA boot just requires moving the heavy lifting to
>>> appropriate NUMA nodes.  It doesn't require that early boot phase
>>> should strictly follow NUMA node boundaries.
>>
>> At end of day, I like to see all numa system (ram/cpu/pci) could have
>> non boot nodes to be hot-removed logically. with any boot command
>> line.
> 
> I suppose you mean "without any boot command line"?  Sure, but, first
> of all, there is a clear performance trade-off, and, secondly, don't
> we want something finer grained?  Why would we want to that per-NUMA
> node, which is extremely coarse?
> 

Both ways seem ok enough *currently*. But what tejun always emphasizes
is the trade-off, or benefit / cost ratio. 

Yinghai and peter insist on the long-term plan. But it seems currently
no actual requirements and plans that *must* parse SRAT earlier comparing
to the current approach in this patchset, right?

Should we follow "Make it work first and optimize/beautify it later"?
I think if we have the scene that must parse SRAT earlier, I think tejun
will have no objection to it.

-- 
Thanks.
Zhang Yanfei
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