From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756745AbbESPRH (ORCPT ); Tue, 19 May 2015 11:17:07 -0400 Received: from casper.infradead.org ([85.118.1.10]:54288 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756728AbbESPRD (ORCPT ); Tue, 19 May 2015 11:17:03 -0400 Date: Tue, 19 May 2015 17:16:59 +0200 From: Peter Zijlstra To: Tejun Heo Cc: lizefan@huawei.com, cgroups@vger.kernel.org, mingo@redhat.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/3] sched, cgroup: replace signal_struct->group_rwsem with a global percpu_rwsem Message-ID: <20150519151659.GF3644@twins.programming.kicks-ass.net> References: <1431549318-16756-1-git-send-email-tj@kernel.org> <1431549318-16756-3-git-send-email-tj@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1431549318-16756-3-git-send-email-tj@kernel.org> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 13, 2015 at 04:35:17PM -0400, Tejun Heo wrote: .gitconfig: [diff "default"] xfuncname = "^[[:alpha:]$_].*[^:]$" Will avoid keying on labels like that and show us this is __cgroup_procs_write(). > @@ -2480,7 +2442,7 @@ retry_find_task: > get_task_struct(tsk); > rcu_read_unlock(); > > - threadgroup_lock(tsk); > + percpu_down_write(&cgroup_threadgroup_rwsem); > if (threadgroup) { > if (!thread_group_leader(tsk)) { > /* > @@ -2490,7 +2452,7 @@ retry_find_task: > * try again; this is > * "double-double-toil-and-trouble-check locking". > */ > - threadgroup_unlock(tsk); > + percpu_up_write(&cgroup_threadgroup_rwsem); > put_task_struct(tsk); > goto retry_find_task; > } > @@ -2703,17 +2665,17 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp) > goto out_finish; > last_task = task; > > - threadgroup_lock(task); > + percpu_down_write(&cgroup_threadgroup_rwsem); > /* raced against de_thread() from another thread? */ > if (!thread_group_leader(task)) { > - threadgroup_unlock(task); > + percpu_up_write(&cgroup_threadgroup_rwsem); > put_task_struct(task); > continue; > } > > ret = cgroup_migrate(src_cset->dfl_cgrp, task, true); > > - threadgroup_unlock(task); > + percpu_up_write(&cgroup_threadgroup_rwsem); > put_task_struct(task); > > if (WARN(ret, "cgroup: failed to update controllers for the default hierarchy (%d), further operations may crash or hang\n", ret)) So my only worry with this patch-set is that these operations will be hugely expensive. Now it looks like the cgroup_update_dfl_csses() thing is very rare, its when you change which controllers are active in a given subtree under the uber-l337-super-comount design. The other one, __cgorup_procs_write() is every /procs, /tasks write to a cgroup, and that does worry me, this could be a somewhat common thing. The Changelog states task migration is a cold path, but is tens of miliseconds per task really no problem?