Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751920Ab0KWI6p (ORCPT ); Tue, 23 Nov 2010 03:58:45 -0500 Received: from smtp-out.google.com ([74.125.121.35]:50688 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751216Ab0KWI6n convert rfc822-to-8bit (ORCPT ); Tue, 23 Nov 2010 03:58:43 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=google.com; s=beta; h=mime-version:sender:in-reply-to:references:date :x-google-sender-auth:message-id:subject:from:to:cc:content-type :content-transfer-encoding; b=UNkEltHRdz/s5Q2bozq2sM4+CiSUtqGRtsF6sZk5krQtWtmQJo/rDWQybQJDTXKcM4 mknEYHHFMOUtZFzzmDhw== MIME-Version: 1.0 In-Reply-To: <4CEB77E0.10202@cn.fujitsu.com> References: <1290398767-15230-1-git-send-email-ccross@android.com> <4CEB77E0.10202@cn.fujitsu.com> Date: Tue, 23 Nov 2010 00:58:39 -0800 X-Google-Sender-Auth: l6uVw_byqRD7PqB8Uiuqir1kPbw Message-ID: Subject: Re: [PATCH] cgroup: Convert synchronize_rcu to call_rcu in cgroup_attach_task From: Colin Cross To: Li Zefan Cc: linux-kernel@vger.kernel.org, Paul Menage , containers@lists.linux-foundation.org Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8BIT X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2642 Lines: 62 On Tue, Nov 23, 2010 at 12:14 AM, Li Zefan wrote: > 12:06, Colin Cross wrote: >> The synchronize_rcu call in cgroup_attach_task can be very >> expensive. ?All fastpath accesses to task->cgroups that expect >> task->cgroups not to change already use task_lock() or >> cgroup_lock() to protect against updates, and, in cgroup.c, >> only the CGROUP_DEBUG files have RCU read-side critical >> sections. >> >> sched.c uses RCU read-side-critical sections on task->cgroups, >> but only to ensure that a dereference of task->cgroups does >> not become invalid, not that it doesn't change. >> > > Other cgroup subsystems also use rcu_read_lock to access task->cgroups, > for example net_cls cgroup and device cgroup. I believe the same comment applies as sched.c, I'll update the commit message. > I don't think the performance of task attaching is so critically > important that we have to use call_rcu() instead of synchronize_rcu()? On my desktop, moving a task between cgroups averages 100 ms, and on an Tegra2 SMP ARM platform it takes 20 ms. Moving a task with many threads can take hundreds of milliseconds or more. With this patch it takes 50 microseconds to move one task, a 400x improvement. >> This patch adds a function put_css_set_rcu, which delays the >> put until after a grace period has elapsed. ?This ensures that >> any RCU read-side critical sections that dereferenced >> task->cgroups in sched.c have completed before the css_set is >> deleted. ?The synchronize_rcu()/put_css_set() combo in >> cgroup_attach_task() can then be replaced with >> put_css_set_rcu(). >> > >> Also converts the CGROUP_DEBUG files that access >> current->cgroups to use task_lock(current) instead of >> rcu_read_lock(). >> > > What for? What do we gain from doing this for those debug > interfaces? Left over from the previous patch that incorrectly dropped RCU completely. I'll put the rcu_read_locks back. >> Signed-off-by: Colin Cross >> >> --- >> >> This version fixes the problems with the previous patch by >> keeping the use of RCU in cgroup_attach_task, but allowing >> cgroup_attach_task to return immediately by deferring the >> final put_css_reg to an rcu callback. >> >> ?include/linux/cgroup.h | ? ?4 +++ >> ?kernel/cgroup.c ? ? ? ?| ? 58 ++++++++++++++++++++++++++++++++++++++---------- >> ?2 files changed, 50 insertions(+), 12 deletions(-) > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/