From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S934554AbbIWAmX (ORCPT ); Tue, 22 Sep 2015 20:42:23 -0400 Received: from mail-pa0-f52.google.com ([209.85.220.52]:32921 "EHLO mail-pa0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759871AbbIWAmV (ORCPT ); Tue, 22 Sep 2015 20:42:21 -0400 From: Greg Thelen To: Andrew Morton Cc: Johannes Weiner , Michal Hocko , Cgroups , "linux-mm\@kvack.org" , "linux-kernel\@vger.kernel.org" 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: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: 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