From: Andrew Morton <akpm@linux-foundation.org>
To: Michal Hocko <mhocko@suse.cz>
Cc: linux-mm@kvack.org, Dave Hansen <dave@linux.vnet.ibm.com>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] add some drop_caches documentation and info messsge
Date: Tue, 23 Oct 2012 16:45:46 -0700 [thread overview]
Message-ID: <20121023164546.747e90f6.akpm@linux-foundation.org> (raw)
In-Reply-To: <20121012125708.GJ10110@dhcp22.suse.cz>
On Fri, 12 Oct 2012 14:57:08 +0200
Michal Hocko <mhocko@suse.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@linux.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.
>
> ...
>
> + printk(KERN_NOTICE "%s (%d): dropped kernel caches: %d\n",
> + current->comm, task_pid_nr(current), sysctl_drop_caches);
urgh. Are we really sure we want to do this? The system operators who
are actually using this thing will hate us :(
More friendly alternatives might be:
- Taint the kernel. But that will only become apparent with an oops
trace or similar.
- Add a drop_caches counter and make that available in /proc/vmstat,
show_mem() output and perhaps other places.
I suspect the /proc/vmstat counter will suffice - if someone is having
vm issues, we'll be seeing their /proc/vmstat at some stage and if the
drop_caches counter is high, that's enough to get suspicious?
next prev parent reply other threads:[~2012-10-23 23:45 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-12 12:57 [PATCH] add some drop_caches documentation and info messsge Michal Hocko
2012-10-12 18:54 ` KOSAKI Motohiro
2012-10-15 9:04 ` Kamezawa Hiroyuki
2012-10-15 14:05 ` Dave Hansen
2012-10-23 23:45 ` Andrew Morton [this message]
2012-10-24 6:29 ` Michal Hocko
2012-10-24 19:54 ` Andrew Morton
2012-10-24 20:28 ` Dave Hansen
2012-10-24 20:48 ` Andrew Morton
2012-10-24 21:06 ` Borislav Petkov
2012-10-24 21:13 ` Andrew Morton
2012-10-24 22:04 ` Rafael J. Wysocki
2012-10-25 1:17 ` Andrew Morton
2012-10-25 20:16 ` Rafael J. Wysocki
2012-10-29 8:59 ` Jiri Kosina
2012-10-29 9:58 ` Borislav Petkov
2012-10-29 10:01 ` Jiri Kosina
2012-10-29 10:11 ` Borislav Petkov
2012-10-31 17:31 ` Pavel Machek
2012-10-31 17:46 ` Borislav Petkov
2012-10-24 22:35 ` KOSAKI Motohiro
2012-10-25 14:21 ` Michal Hocko
2012-10-24 21:18 ` Dave Hansen
2012-10-24 22:48 ` Borislav Petkov
2012-10-24 22:57 ` Dave Hansen
2012-10-25 0:56 ` KOSAKI Motohiro
2012-10-25 9:24 ` Borislav Petkov
2012-10-25 11:57 ` Dave Hansen
2012-10-25 14:24 ` Borislav Petkov
2012-10-25 14:25 ` Michal Hocko
2012-10-26 7:45 ` Mika Boström
2012-11-01 20:26 ` Pavel Machek
2012-10-25 14:09 ` Michal Hocko
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