linux-kernel - Re: [PATCH] add some drop_caches documentation and info messsge

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Message-ID: <CAHGf_=rhjUQieH6sqqbt0ST6cbsTYLnZtwizKObBWV6w3Mff4g@mail.gmail.com>
Date:	Fri, 12 Oct 2012 14:54:08 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	linux-mm@...ck.org, Dave Hansen <dave@...ux.vnet.ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] add some drop_caches documentation and info messsge

On Fri, Oct 12, 2012 at 8:57 AM, Michal Hocko <mhocko@...e.cz> wrote:
> Hi,
> I would like to resurrect the following Dave's patch. The last time it
> has been posted was here https://lkml.org/lkml/2010/9/16/250 and there
> didn't seem to be any strong opposition.
> Kosaki was worried about possible excessive logging when somebody drops
> caches too often (but then he claimed he didn't have a strong opinion
> on that) but I would say opposite. If somebody does that then I would
> really like to know that from the log when supporting a system because
> it almost for sure means that there is something fishy going on. It is
> also worth mentioning that only root can write drop caches so this is
> not an flooding attack vector.
> I am bringing that up again because this can be really helpful when
> chasing strange performance issues which (surprise surprise) turn out to
> be related to artificially dropped caches done because the admin thinks
> this would help...
>
> I have just refreshed the original patch on top of the current mm tree
> but I could live with KERN_INFO as well if people think that KERN_NOTICE
> is too hysterical.
> ---
> From 1f4058be9b089bc9d43d71bc63989335d7637d8d Mon Sep 17 00:00:00 2001
> From: Dave Hansen <dave@...ux.vnet.ibm.com>
> Date: Fri, 12 Oct 2012 14:30:54 +0200
> Subject: [PATCH] add some drop_caches documentation and info messsge
>
> There is plenty of anecdotal evidence and a load of blog posts
> suggesting that using "drop_caches" periodically keeps your system
> running in "tip top shape".  Perhaps adding some kernel
> documentation will increase the amount of accurate data on its use.
>
> If we are not shrinking caches effectively, then we have real bugs.
> Using drop_caches will simply mask the bugs and make them harder
> to find, but certainly does not fix them, nor is it an appropriate
> "workaround" to limit the size of the caches.
>
> It's a great debugging tool, and is really handy for doing things
> like repeatable benchmark runs.  So, add a bit more documentation
> about it, and add a little KERN_NOTICE.  It should help developers
> who are chasing down reclaim-related bugs.
>
> [mhocko@...e.cz: refreshed to current -mm tree]
> Signed-off-by: Dave Hansen <dave@...ux.vnet.ibm.com>
> Reviewed-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> Acked-by: Michal Hocko <mhocko@...e.cz>

Looks fine.

Acked-by: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
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