Date: Mon, 10 Feb 2014 12:51:02 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Johannes Weiner <hannes@cmpxchg.org>
Cc: Rik van Riel <riel@redhat.com>, Dave Hansen <dave@sr71.net>,
        Michal Hocko <mhocko@suse.cz>,
        KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [patch] drop_caches: add some documentation and info message
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In-Reply-To: <20140207212601.GI6963@cmpxchg.org>
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	<20140207181332.GG6963@cmpxchg.org>
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	<20140207212601.GI6963@cmpxchg.org>
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On Fri, 7 Feb 2014 16:26:01 -0500 Johannes Weiner <hannes@cmpxchg.org> wrote:

> On Fri, Feb 07, 2014 at 12:31:29PM -0800, Andrew Morton wrote:
> > On Fri, 7 Feb 2014 13:13:32 -0500 Johannes Weiner <hannes@cmpxchg.org> wrote:
> > 
> > > @@ -63,6 +64,9 @@ int drop_caches_sysctl_handler(ctl_table *table, int write,
> > >  			iterate_supers(drop_pagecache_sb, NULL);
> > >  		if (sysctl_drop_caches & 2)
> > >  			drop_slab();
> > > +		printk_ratelimited(KERN_INFO "%s (%d): dropped kernel caches: %d\n",
> > > +				   current->comm, task_pid_nr(current),
> > > +				   sysctl_drop_caches);
> > >  	}
> > >  	return 0;
> > >  }
> > 
> > My concern with this is that there may be people whose
> > other-party-provided software uses drop_caches.  Their machines will
> > now sit there emitting log messages and there's nothing they can do
> > about it, apart from whining at their vendors.
> 
> Ironically, we have a customer that is complaining that we currently
> do not log these events, and they want to know who in their stack is
> being idiotic.

Right.  But if we release a kernel which goes blah on every write to
drop_caches, that customer has logs full of blahs which they are
now totally uninterested in.

> > We could do something like this?
> 
> They can already change the log level.

Suppressing unrelated things...

>  The below will suppress
> valuable debugging information in a way that still results in
> inconspicuous looking syslog excerpts, which somewhat undermines the
> original motivation for this change.

Yes, somewhat.  It is a compromise. You can see my concern here?


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