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

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Message-Id: <20140210125102.86de67241664da038676af7d@linux-foundation.org>
Date:	Mon, 10 Feb 2014 12:51:02 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Rik van Riel <riel@...hat.com>, Dave Hansen <dave@...1.net>,
	Michal Hocko <mhocko@...e.cz>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [patch] drop_caches: add some documentation and info message

On Fri, 7 Feb 2014 16:26:01 -0500 Johannes Weiner <hannes@...xchg.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@...xchg.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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