From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f41.google.com (mail-pa0-f41.google.com [209.85.220.41]) by kanga.kvack.org (Postfix) with ESMTP id 9E7F26B0253 for ; Tue, 22 Sep 2015 20:42:21 -0400 (EDT) Received: by pacbt3 with SMTP id bt3so5543416pac.3 for ; Tue, 22 Sep 2015 17:42:21 -0700 (PDT) Received: from mail-pa0-x231.google.com (mail-pa0-x231.google.com. [2607:f8b0:400e:c03::231]) by mx.google.com with ESMTPS id tq7si6311333pbc.242.2015.09.22.17.42.20 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 22 Sep 2015 17:42:20 -0700 (PDT) Received: by pacfv12 with SMTP id fv12so24175716pac.2 for ; Tue, 22 Sep 2015 17:42:20 -0700 (PDT) From: Greg Thelen Subject: Re: [PATCH] memcg: make mem_cgroup_read_stat() unsigned Date: Tue, 22 Sep 2015 17:42:13 -0700 Message-ID: MIME-Version: 1.0 Content-Type: text/plain Sender: owner-linux-mm@kvack.org List-ID: To: Andrew Morton Cc: Johannes Weiner , Michal Hocko , Cgroups , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" Andrew Morton wrote: > On Tue, 22 Sep 2015 15:16:32 -0700 Greg Thelen wrote: > >> mem_cgroup_read_stat() returns a page count by summing per cpu page >> counters. The summing is racy wrt. updates, so a transient negative sum >> is possible. Callers don't want negative values: >> - mem_cgroup_wb_stats() doesn't want negative nr_dirty or nr_writeback. >> - oom reports and memory.stat shouldn't show confusing negative usage. >> - tree_usage() already avoids negatives. >> >> Avoid returning negative page counts from mem_cgroup_read_stat() and >> convert it to unsigned. > > Someone please remind me why this code doesn't use the existing > percpu_counter library which solved this problem years ago. > >> for_each_possible_cpu(cpu) > > and which doesn't iterate across offlined CPUs. I found [1] and [2] discussing memory layout differences between: a) existing memcg hand rolled per cpu arrays of counters vs b) array of generic percpu_counter The current approach was claimed to have lower memory overhead and better cache behavior. I assume it's pretty straightforward to create generic percpu_counter_array routines which memcg could use. Possibly something like this could be made general enough could be created to satisfy vmstat, but less clear. [1] http://www.spinics.net/lists/cgroups/msg06216.html [2] https://lkml.org/lkml/2014/9/11/1057 -- 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