linux-kernel - Re: [PATCH 0/8] x86, acpi: Move acpi_initrd

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Message-ID: <3ee58764-21c2-4df4-9353-54799a6a3d7b@email.android.com>
Date:	Fri, 23 Aug 2013 15:08:55 +0200
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Tejun Heo <tj@...nel.org>, Toshi Kani <toshi.kani@...com>
CC:	Zhang Yanfei <zhangyanfei.yes@...il.com>,
	Tang Chen <tangchen@...fujitsu.com>, konrad.wilk@...cle.com,
	robert.moore@...el.com, lv.zheng@...el.com, rjw@...k.pl,
	lenb@...nel.org, tglx@...utronix.de, mingo@...e.hu,
	akpm@...ux-foundation.org, trenn@...e.de, yinghai@...nel.org,
	jiang.liu@...wei.com, wency@...fujitsu.com, laijs@...fujitsu.com,
	isimatu.yasuaki@...fujitsu.com, izumi.taku@...fujitsu.com,
	mgorman@...e.de, minchan@...nel.org, mina86@...a86.com,
	gong.chen@...ux.intel.com, vasilis.liaskovitis@...fitbricks.com,
	lwoodman@...hat.com, riel@...hat.com, jweiner@...hat.com,
	prarit@...hat.com, zhangyanfei@...fujitsu.com,
	yanghy@...fujitsu.com, x86@...nel.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-acpi@...r.kernel.org
Subject: Re: [PATCH 0/8] x86, acpi: Move acpi_initrd_override() earlier.

What is the point of 1G+MTRR?  If there are caching differences the TLB will fracture the pages anyway.

Tejun Heo <tj@...nel.org> wrote:
>Hello, Toshi.
>
>On Thu, Aug 22, 2013 at 04:17:41PM -0600, Toshi Kani wrote:
>> I am relatively new to Linux, so I am not a good person to elaborate
>> this.  From my experience on other OS, huge pages helped for the
>kernel,
>> but did not necessarily help user applications.  It depended on
>> applications, which were not niche cases.  But Linux may be
>different,
>> so I asked since you seemed confident.  I'd appreciate if you can
>point
>> us some data that endorses your statement.
>
>We are talking about the kernel linear mapping which is created during
>early boot, so if it's available and useable there's no reason not to
>use it.  Exceptions would be earlier processors which didn't do 1G
>mappings or e820 maps with a lot of holes.  For CPUs used in NUMA
>configurations, the former has been history for a bit now.  Can't be
>sure about the latter but it'd be surprising for that to affect large
>amount of memory in the systems that are of interest here.  Ooh, that
>reminds me that we probably wanna go back to 1G + MTRR mapping under
>4G.  We're currently creating a lot of mapping holes.
>
>> My worry is that the code is unlikely tested with the special logic
>when
>> someone makes code changes to the page tables.  Such code can easily
>be
>> broken in future.
>
>Well, I wouldn't consider flipping the direction of allocation to be
>particularly difficult to get right especially when compared to
>bringing in ACPI tables into the mix.
>
>> To answer your other question/email, I believe Tang's next step is to
>> support local page tables.  This is why we think pursing SRAT earlier
>is
>> the right direction.
>
>Given 1G mappings, is that even a worthwhile effort?  I'm getting even
>more more skeptical.
>
>Thanks.

-- 
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