From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx190.postini.com [74.125.245.190]) by kanga.kvack.org (Postfix) with SMTP id A14686B0089 for ; Mon, 8 Apr 2013 05:14:19 -0400 (EDT) Message-ID: <51628A88.2090002@parallels.com> Date: Mon, 8 Apr 2013 13:14:48 +0400 From: Glauber Costa MIME-Version: 1.0 Subject: Re: [PATCH v2 03/28] dcache: convert dentry_stat.nr_unused to per-cpu counters References: <1364548450-28254-1-git-send-email-glommer@parallels.com> <1364548450-28254-4-git-send-email-glommer@parallels.com> <20130405011506.GG12011@dastard> In-Reply-To: <20130405011506.GG12011@dastard> Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Dave Chinner Cc: Greg Thelen , linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, containers@lists.linux-foundation.org, Michal Hocko , Johannes Weiner , kamezawa.hiroyu@jp.fujitsu.com, Andrew Morton , hughd@google.com, yinghan@google.com, Dave Chinner On 04/05/2013 05:15 AM, Dave Chinner wrote: > On Thu, Apr 04, 2013 at 06:09:31PM -0700, Greg Thelen wrote: >> On Fri, Mar 29 2013, Glauber Costa wrote: >> >>> From: Dave Chinner >>> >>> Before we split up the dcache_lru_lock, the unused dentry counter >>> needs to be made independent of the global dcache_lru_lock. Convert >>> it to per-cpu counters to do this. >>> >>> Signed-off-by: Dave Chinner >>> Reviewed-by: Christoph Hellwig >>> --- >>> fs/dcache.c | 17 ++++++++++++++--- >>> 1 file changed, 14 insertions(+), 3 deletions(-) >>> >>> diff --git a/fs/dcache.c b/fs/dcache.c >>> index fbfae008..f1196f2 100644 >>> --- a/fs/dcache.c >>> +++ b/fs/dcache.c >>> @@ -118,6 +118,7 @@ struct dentry_stat_t dentry_stat = { >>> }; >>> >>> static DEFINE_PER_CPU(unsigned int, nr_dentry); >>> +static DEFINE_PER_CPU(unsigned int, nr_dentry_unused); >>> >>> #if defined(CONFIG_SYSCTL) && defined(CONFIG_PROC_FS) >>> static int get_nr_dentry(void) >>> @@ -129,10 +130,20 @@ static int get_nr_dentry(void) >>> return sum < 0 ? 0 : sum; >>> } >>> >>> +static int get_nr_dentry_unused(void) >>> +{ >>> + int i; >>> + int sum = 0; >>> + for_each_possible_cpu(i) >>> + sum += per_cpu(nr_dentry_unused, i); >>> + return sum < 0 ? 0 : sum; >>> +} >> >> Just checking... If cpu x is removed, then its per cpu nr_dentry_unused >> count survives so we don't leak nr_dentry_unused. Right? I see code in >> percpu_counter_sum_positive() to explicitly handle this case and I want >> to make sure we don't need it here. > > DEFINE_PER_CPU() gives a variable per possible CPU, and we sum for > all possible CPUs. Therefore online/offline CPUs just don't matter. > > The percpu_counter code uses for_each_online_cpu(), and so it has to > be aware of hotplug operations so taht it doesn't leak counts. > It is an unsigned quantity, however. Can't we go negative if it becomes unused in one cpu, but used in another? Ex: nr_unused/0: 0 nr_unused/1: 0 dentry goes to the LRU at cpu 1: nr_unused/0: 0 nr_unused/1: 1 CPU 1 goes down: nr_unused/0: 0 dentry goes out of the LRU at cpu 0: nr_unused/0: 1 << 32. That would easily be fixed by using a normal signed long, and is in fact what the percpu code does in its internal operations. Any reason not to do it? Something I am not seeing? -- 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