linux-kernel - Re: [RFC][PATCH 2/3] pagemap: export KPF

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Date:	Mon, 19 Dec 2011 15:45:44 -0500
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
CC:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>, linux-mm@...ck.org,
	Andi Kleen <andi@...stfloor.org>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 2/3] pagemap: export KPF_THP

(12/19/11 3:31 PM), Dave Hansen wrote:
> On 12/19/2011 11:24 AM, KOSAKI Motohiro wrote:
>> (12/19/11 1:38 PM), Naoya Horiguchi wrote:
>>> This flag shows that a given pages is a subpage of transparent hugepage.
>>> It does not care about whether it is a head page or a tail page, because
>>> it's clear from pfn of the target page which you should know when you read
>>> /proc/kpageflags.
>>>
>>> Signed-off-by: Naoya Horiguchi<n-horiguchi@...jp.nec.com>
>>
>> NAK.
>>
>> The detail of transparent hugepage are hidden by design. We hope it
>> keep 'transparent'.
>> Until any explain why we should expose KPF_THP, we don't agree it.
> 
> Transparent shouldn't mean "undebuggable", though. :)
> 
> Let's say you profiled a application and the data shows you're missing
> the TLB a bunch, but you're also using THP.  This might give you a shot
> at figuring out which parts of your application are *TRULY* THP-backed
> instead of just the areas you *think* are backed.
> 
> I'm not sure there's another way to figure it out at the moment.

A snapshot status of THP doesn't help your purpose. I think you need
perf or similar profiling subsystem enhancement.

Because of, if you've seen KPF_THP at once, It has no guarantee to keep
hugepages until applications run. Opposite, If you only need rough
statistics, the best way is to add some new stat to
/sys/kernel/mm/transparent_hugepage.

I don't think your usecase and current proposal are matched.

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