From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752810AbbIWHVm (ORCPT ); Wed, 23 Sep 2015 03:21:42 -0400 Received: from mail-pa0-f48.google.com ([209.85.220.48]:35935 "EHLO mail-pa0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752150AbbIWHVk (ORCPT ); Wed, 23 Sep 2015 03:21:40 -0400 References: <20150922210346.749204fb.akpm@linux-foundation.org> 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 In-reply-to: <20150922210346.749204fb.akpm@linux-foundation.org> Date: Wed, 23 Sep 2015 00:21:33 -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 17:42:13 -0700 Greg Thelen wrote: > >> 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 > > That all sounds rather bogus to me. __percpu_counter_add() doesn't > modify struct percpu_counter at all except for when the cpu-local > counter overflows the configured batch size. And for the memcg > application I suspect we can set the batch size to INT_MAX... Nod. The memory usage will be a bit larger, but the code reuse is attractive. I dusted off Vladimir's https://lkml.org/lkml/2014/9/11/710. Next step is to benchmark it before posting.