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

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Message-Id: <20130729135743.c04224fb5d8e64b2730d8263@linux-foundation.org>
Date:	Mon, 29 Jul 2013 13:57:43 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	dave.hansen@...el.com, kosaki.motohiro@...fujitsu.com,
	kamezawa.hiroyu@...fujitsu.com, bp@...e.de,
	Dave Hansen <dave@...ux.vnet.ibm.com>
Subject: Re: [PATCH resend] drop_caches: add some documentation and info
 message

On Fri, 26 Jul 2013 14:44:29 +0200 Michal Hocko <mhocko@...e.cz> wrote:

> ...
>
> : 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.
> 
> ...
>
> --- a/fs/drop_caches.c
> +++ b/fs/drop_caches.c
> @@ -59,6 +59,8 @@ int drop_caches_sysctl_handler(ctl_table *table, int write,
>  	if (ret)
>  		return ret;
>  	if (write) {
> +		printk(KERN_INFO "%s (%d): dropped kernel caches: %d\n",
> +		       current->comm, task_pid_nr(current), sysctl_drop_caches);
>  		if (sysctl_drop_caches & 1)
>  			iterate_supers(drop_pagecache_sb, NULL);
>  		if (sysctl_drop_caches & 2)

How about we do

	if (!(sysctl_drop_caches & 4))
		printk(....)

so people can turn it off if it's causing problems?
--
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