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

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Date:	Mon, 14 Oct 2013 19:25:55 -0700
From:	Yinghai Lu <yinghai@...nel.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Zhang Yanfei <zhangyanfei.yes@...il.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.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

On Mon, Oct 14, 2013 at 1:55 PM, Tejun Heo <tj@...nel.org> 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?

Even we try to go minimum range instead of range that whole range on boot node,
without parsing srat at first, the minimum range could be crossed the boundary
of nodes.

>
>> 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.

no, do not assume 4k page. even we are using 1GB mapping,  we will still have
chance to have one node to take 512G RAM, that means we can have one 4k page
on local node ram.

>
> * Memory hotplug is at NUMA node granularity instead of device.

Yes.

>
>> > 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?

On x86 system with intel new cpus there is memory controller built-in.,
could have hotplug modules (with socket and memory) and those hotplug modules
will be serviced as one single point. Just nowadays like we have pcie
card hotplugable.

I don't see where is the " a clear performance trade-off".

Yinghai
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