From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qg0-f48.google.com (mail-qg0-f48.google.com [209.85.192.48]) by kanga.kvack.org (Postfix) with ESMTP id 2F7016B0255 for ; Wed, 23 Sep 2015 00:02:07 -0400 (EDT) Received: by qgev79 with SMTP id v79so10949554qge.0 for ; Tue, 22 Sep 2015 21:02:06 -0700 (PDT) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTPS id 188si4990296qhe.57.2015.09.22.21.02.05 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Tue, 22 Sep 2015 21:02:06 -0700 (PDT) Date: Tue, 22 Sep 2015 21:03:46 -0700 From: Andrew Morton Subject: Re: [PATCH] memcg: make mem_cgroup_read_stat() unsigned Message-Id: <20150922210346.749204fb.akpm@linux-foundation.org> In-Reply-To: References: Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Greg Thelen Cc: Johannes Weiner , Michal Hocko , Cgroups , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" 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... -- 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