Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751585AbaBHL4P (ORCPT ); Sat, 8 Feb 2014 06:56:15 -0500 Received: from mx1.redhat.com ([209.132.183.28]:63788 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751386AbaBHL4N (ORCPT ); Sat, 8 Feb 2014 06:56:13 -0500 Date: Sat, 8 Feb 2014 09:55:57 -0200 From: Rafael Aquini To: Johannes Weiner Cc: Rik van Riel , Andrew Morton , Dave Hansen , Michal Hocko , KOSAKI Motohiro , KAMEZAWA Hiroyuki , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [patch] drop_caches: add some documentation and info message Message-ID: <20140208115556.GB25841@localhost.localdomain> References: <1391794851-11412-1-git-send-email-hannes@cmpxchg.org> <52F51E19.9000406@redhat.com> <20140207181332.GG6963@cmpxchg.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140207181332.GG6963@cmpxchg.org> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 07, 2014 at 01:13:32PM -0500, Johannes Weiner wrote: > On Fri, Feb 07, 2014 at 12:55:37PM -0500, Rik van Riel wrote: > > On 02/07/2014 12:40 PM, Johannes Weiner wrote: > > > > >@@ -59,6 +60,9 @@ int drop_caches_sysctl_handler(ctl_table *table, int write, > > > if (ret) > > > return ret; > > > if (write) { > > >+ printk_ratelimited(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) > > > > > > > Would it be better to print this after the operation > > has completed? > > It would make more sense grammatically :-) Either way is fine with me, > updated below to inform after the fact. > > --- > From: Dave Hansen > Date: Fri, 12 Oct 2012 14:30:54 +0200 > Subject: [patch] drop_caches: add some documentation and info message > > 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. On the contrary, there > have been bug reports on issues that turned out to be misguided use of > cache dropping. > > Dropping caches is a very drastic and disruptive operation that is > good for debugging and running tests, but if it creates bug reports > from production use, kernel developers should be aware of its use. > > Add a bit more documentation about it, and add a little KERN_NOTICE. > > [akpm@linux-foundation.org: checkpatch fixes] > Signed-off-by: Dave Hansen > Signed-off-by: Michal Hocko > Acked-by: KOSAKI Motohiro > Acked-by: KAMEZAWA Hiroyuki > Signed-off-by: Johannes Weiner > --- > Documentation/sysctl/vm.txt | 33 +++++++++++++++++++++++++++------ > fs/drop_caches.c | 4 ++++ > 2 files changed, 31 insertions(+), 6 deletions(-) > > diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt > index d614a9b6a280..36278c610a5f 100644 > --- a/Documentation/sysctl/vm.txt > +++ b/Documentation/sysctl/vm.txt > @@ -175,18 +175,39 @@ Setting this to zero disables periodic writeback altogether. > > drop_caches > > -Writing to this will cause the kernel to drop clean caches, dentries and > -inodes from memory, causing that memory to become free. > +Writing to this will cause the kernel to drop clean caches, as well as > +reclaimable slab objects like dentries and inodes. Once dropped, their > +memory becomes free. > > To free pagecache: > echo 1 > /proc/sys/vm/drop_caches > -To free dentries and inodes: > +To free reclaimable slab objects (includes dentries and inodes): > echo 2 > /proc/sys/vm/drop_caches > -To free pagecache, dentries and inodes: > +To free slab objects and pagecache: > echo 3 > /proc/sys/vm/drop_caches > > -As this is a non-destructive operation and dirty objects are not freeable, the > -user should run `sync' first. > +This is a non-destructive operation and will not free any dirty objects. > +To increase the number of objects freed by this operation, the user may run > +`sync' prior to writing to /proc/sys/vm/drop_caches. This will minimize the > +number of dirty objects on the system and create more candidates to be > +dropped. > + > +This file is not a means to control the growth of the various kernel caches > +(inodes, dentries, pagecache, etc...) These objects are automatically > +reclaimed by the kernel when memory is needed elsewhere on the system. > + > +Use of this file can cause performance problems. Since it discards cached > +objects, it may cost a significant amount of I/O and CPU to recreate the > +dropped objects, especially if they were under heavy use. Because of this, > +use outside of a testing or debugging environment is not recommended. > + > +You may see informational messages in your kernel log when this file is > +used: > + > + cat (1234): dropped kernel caches: 3 > + > +These are informational only. They do not mean that anything is wrong > +with your system. > > ============================================================== > > diff --git a/fs/drop_caches.c b/fs/drop_caches.c > index 9fd702f5bfb2..02ae3386e08f 100644 > --- a/fs/drop_caches.c > +++ b/fs/drop_caches.c > @@ -5,6 +5,7 @@ > #include > #include > #include > +#include > #include > #include > #include > @@ -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", Just a nitpick here: s/printk_ratelimited(KERN_INFO ...)/pr_info_ratelimited(...) Acked-by: Rafael Aquini > + current->comm, task_pid_nr(current), > + sysctl_drop_caches); > } > return 0; > } > -- > 1.8.5.3 > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@kvack.org. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: email@kvack.org -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/