From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757237Ab2JRQfq (ORCPT ); Thu, 18 Oct 2012 12:35:46 -0400 Received: from e9.ny.us.ibm.com ([32.97.182.139]:53456 "EHLO e9.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756206Ab2JRQfo (ORCPT ); Thu, 18 Oct 2012 12:35:44 -0400 Date: Thu, 18 Oct 2012 09:35:25 -0700 From: "Paul E. McKenney" To: Eric Dumazet Cc: linux-kernel@vger.kernel.org Subject: Re: [PATCH] rcu: restore correct batch limiting Message-ID: <20121018163525.GJ2518@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1350458052.26103.31.camel@edumazet-glaptop> <20121018115859.GH2518@linux.vnet.ibm.com> <1350564290.26103.1496.camel@edumazet-glaptop> <20121018152543.GI2518@linux.vnet.ibm.com> <1350576628.32642.14.camel@edumazet-glaptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1350576628.32642.14.camel@edumazet-glaptop> User-Agent: Mutt/1.5.21 (2010-09-15) X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12101816-7182-0000-0000-000002E005BA Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Oct 18, 2012 at 06:10:28PM +0200, Eric Dumazet wrote: > On Thu, 2012-10-18 at 08:25 -0700, Paul E. McKenney wrote: > > On Thu, Oct 18, 2012 at 02:44:50PM +0200, Eric Dumazet wrote: > > > > > > > Having 2 billions callbacks on one cpu would be problematic, I really > > > hope nobody relies on this ;) > > > > Fair point! ;-) > > > > But just making everything long makes it quite easy to analyze. > > > > > I guess the 10/infinity switch should be smarter. > > > > > > something like the following maybe : > > > > > > rdp->blimit = max(blimit, rdp->qlen >> 6); > > > > > > (if queue is big, dont wait to hit 10000 before allowing more items to > > > be handled per round) > > > > The -rt guys would not be amused. :-( > > > > But for non-realtime use, increasing rcutree.blimit either at boot or > > via sysfs could make sense. It is also likely that I will move callback > > processing to a kthread at some point, which would allow some additional > > flexibility. > > > > Ah, I now realize the loop can exceed blimit, but is it true for BH > variant ? (Not really a problem for 3.6/3.7 kernels, but prior ones) Yep, applies to all the RCU flavors. > if (++count >= bl && > (need_resched() || > (!is_idle_task(current) && !rcu_is_callbacks_kthread()))) > break; > > I wonder if ksoftirqd should be included as well... This would be safe only if ksoftirqd could be guaranteed to be the lowest-priority process on the given CPU, which I do not believe to be the case. The problem is that if ksoftirqd is not the lowest-priority process on the given CPU, we can seriously delay that other process for no good reason. The fact that ksoftirqd does local_bh_disable() means that the scheduler cannot preempt it, either. :-( > > Furthermore, it would be easy to have one default for non-rt and another > > for -rt, if that would help. > > > > > Signed-off-by: Eric Dumazet > > > > > > Please dont forget stable teams. (3.2 + ) > > > > Added both, please see below! > > Seems fine to me, thanks Paul ! Thank you for everything on this one! Thanx, Paul