From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f169.google.com (mail-wi0-f169.google.com [209.85.212.169]) by kanga.kvack.org (Postfix) with ESMTP id 28E9882A12 for ; Fri, 22 May 2015 19:13:30 -0400 (EDT) Received: by wicmc15 with SMTP id mc15so2480488wic.1 for ; Fri, 22 May 2015 16:13:29 -0700 (PDT) Received: from gum.cmpxchg.org (gum.cmpxchg.org. [85.214.110.215]) by mx.google.com with ESMTPS id fy6si419868wib.38.2015.05.22.16.13.28 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 22 May 2015 16:13:28 -0700 (PDT) Date: Fri, 22 May 2015 19:12:49 -0400 From: Johannes Weiner Subject: Re: [PATCH 01/19] memcg: make mem_cgroup_read_{stat|event}() iterate possible cpus instead of online Message-ID: <20150522231249.GA6485@cmpxchg.org> References: <1432333416-6221-1-git-send-email-tj@kernel.org> <1432333416-6221-2-git-send-email-tj@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1432333416-6221-2-git-send-email-tj@kernel.org> Sender: owner-linux-mm@kvack.org List-ID: To: Tejun Heo Cc: axboe@kernel.dk, linux-kernel@vger.kernel.org, jack@suse.cz, hch@infradead.org, linux-fsdevel@vger.kernel.org, vgoyal@redhat.com, lizefan@huawei.com, cgroups@vger.kernel.org, linux-mm@kvack.org, mhocko@suse.cz, clm@fb.com, fengguang.wu@intel.com, david@fromorbit.com, gthelen@google.com, khlebnikov@yandex-team.ru On Fri, May 22, 2015 at 06:23:18PM -0400, Tejun Heo wrote: > cpu_possible_mask represents the CPUs which are actually possible > during that boot instance. For systems which don't support CPU > hotplug, this will match cpu_online_mask exactly in most cases. Even > for systems which support CPU hotplug, the number of possible CPU > slots is highly unlikely to diverge greatly from the number of online > CPUs. The only cases where the difference between possible and online > caused problems were when the boot code failed to initialize the > possible mask and left it fully set at NR_CPUS - 1. > > As such, most per-cpu constructs allocate for all possible CPUs and > often iterate over the possibles, which also has the benefit of > avoiding the blocking CPU hotplug synchronization. > > memcg open codes per-cpu stat counting for mem_cgroup_read_stat() and > mem_cgroup_read_events(), which iterates over online CPUs and handles > CPU hotplug operations explicitly. This complexity doesn't actually > buy anything. Switch to iterating over the possibles and drop the > explicit CPU hotplug handling. > > Eventually, we want to convert memcg to use percpu_counter instead of > its own custom implementation which also benefits from quick access > w/o summing for cases where larger error margin is acceptable. > > This will allow mem_cgroup_read_stat() to be called from non-sleepable > contexts which will be used by cgroup writeback. > > Signed-off-by: Tejun Heo > Cc: Johannes Weiner > Cc: Michal Hocko Acked-by: Johannes Weiner -- 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