From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752561AbeESW7J (ORCPT ); Sat, 19 May 2018 18:59:09 -0400 Received: from mail-pg0-f49.google.com ([74.125.83.49]:44877 "EHLO mail-pg0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752045AbeESW7H (ORCPT ); Sat, 19 May 2018 18:59:07 -0400 X-Google-Smtp-Source: AB8JxZoo6qSXqhIHhXQl5iRxVreq64M0b9KxP527uDfC3UGXf4PmtituduHr+1odZrFnFaDLRS3zzQ== Date: Sat, 19 May 2018 15:59:05 -0700 From: Joel Fernandes To: "Paul E. McKenney" Cc: rostedt@goodmis.org, byungchul.park@lge.com, mathieu.desnoyers@efficios.com, Josh Triplett , Lai Jiangshan , linux-kernel@vger.kernel.org, kernel-team@android.com Subject: Re: Tasks RCU vs Preempt RCU Message-ID: <20180519225905.GB134184@joelaf.mtv.corp.google.com> References: <20180518183623.GA163151@joelaf.mtv.corp.google.com> <20180519022918.GV3803@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180519022918.GV3803@linux.vnet.ibm.com> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 18, 2018 at 07:29:18PM -0700, Paul E. McKenney wrote: > On Fri, May 18, 2018 at 11:36:23AM -0700, Joel Fernandes wrote: > > Hi, > > > > I was thinking about tasks-RCU and why its needed. Since preempt-RCU allows > > tasks to be preempted in read-sections, can we not just reuse that mechanism > > for the trampolines since we track all preempted tasks so we would wait on > > all tasks preempted within a trampoline? > > > > I am trying to understand what will _not_ work if we did that.. I'm guessing > > the answer is that that would mean the trampoline has to be wrapped with > > rcu_read_{lock,unlock} which may add some overhead, but please let me know > > if I'm missing something else.. > > > > The advantage I guess is possible elimination of an RCU variant, and also > > possibly eliminating the tasks RCU thread that monitors.. Anyway I was > > thinking more in terms of the effort of reduction of the RCU flavors etc and > > reducing complexity ideas. > > The problem is that if they are preempted while executing in a trampoline, > RCU-preempt doesn't queue them nor does it wait on them. Not if they are wrapped with rcu_read_lock and rcu_read_unlock? From what I can see, you are preparing a list of blocked tasks that would keep the grace period from finishing in rcu_preempt_ctxt_queue? > And the problem with wrapping them with rcu_read_{lock,unlock} is that > there would be a point before the trampoline executed rcu_read_lock() > but while it was on the trampoline. Nothing good comes from this. ;-) Yes, I see what you're saying. The data being protected and freed in this case is the code so relying on it to do the rcu_read_lock seems infeasible. Conceptually atleast, I feel this can be fixed by cleverly implementing trampolines such that the rcu_read_lock isn't done during the trampoline execution. But I am not very experienced with how the trampolines work to say definitely whether it is or isn't possible or worth it. But atleast I felt it was a worthwhile food for thought ;) I actually want to trace out the trampoline executing as it pertains to RCU, with your latest rcu/dev.. I think it will be fun :) thanks! - Joel