From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753768AbeEWDKG (ORCPT ); Tue, 22 May 2018 23:10:06 -0400 Received: from mail-pg0-f67.google.com ([74.125.83.67]:36263 "EHLO mail-pg0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753165AbeEWDKF (ORCPT ); Tue, 22 May 2018 23:10:05 -0400 X-Google-Smtp-Source: AB8JxZoaTN0vno4Yj2Cr6s7MUqREjb9XxwkO9/5KV2E5gkW4xEjhi8GRuB1a/c5fFFqt+uci/g12qw== Date: Tue, 22 May 2018 20:10:03 -0700 From: Joel Fernandes To: Steven Rostedt Cc: "Paul E. McKenney" , 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: <20180523031003.GA60111@joelaf.mtv.corp.google.com> References: <20180518183623.GA163151@joelaf.mtv.corp.google.com> <20180519022918.GV3803@linux.vnet.ibm.com> <20180519225905.GB134184@joelaf.mtv.corp.google.com> <20180520004938.GZ3803@linux.vnet.ibm.com> <20180520112843.57079857@grimm.local.home> <20180520191846.GA248075@joelaf.mtv.corp.google.com> <20180521215951.2d6abfcb@gandalf.local.home> <20180522045414.GG40541@joelaf.mtv.corp.google.com> <20180522083832.45353d5f@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180522083832.45353d5f@gandalf.local.home> 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 Tue, May 22, 2018 at 08:38:32AM -0400, Steven Rostedt wrote: > On Mon, 21 May 2018 21:54:14 -0700 > Joel Fernandes wrote: > > > > Yes, lets brain storm this if you like. One way I was thinking if we can > > manually check every CPU and see what state its in (usermode, kernel, idle > > etc) using an IPI mechanism. Once all CPUs have been seen to be in usermode, > > or idle atleast once - then we are done. You have probably already thought > > Nope, it has nothing to do with CPUs, it really has to do with tasks. > > CPU0 > ---- > task 1: (pinned to CPU 0) > call func_tracer_trampoline > [on trampoline] > [timer tick, schedule ] > > task 2: (higher priority, also pinned to CPU 0) > goes to user space > [ Runs for along time ] > > We cannot free the trampoline until task 2 releases the CPU and lets > task 1 run again to get off the CPU. Right. I totally missed that. Ofcourse, its not sufficient to see if a CPU is in usermode. :( > > about this so feel free to say why its not a good idea, but to me there are 3 > > places that a tasks quiescent state is recorded: during the timer tick, > > during task sleep and during rcu_note_voluntary_context_switch in > > cond_resched_rcu_qs. Of these, I feel only the cond_resched_rcu_qs case isn't > > trackable with IPI mechanism which may make the detection a bit slower, but > > tasks-RCU in mainline is slow right now anyway (~ 1 second delay if any task > > was held). > > > The way I was originally going to handle this was with a per task > counter, where it can be incremented at certain points via tracepoints. > > Thus my synchronize tasks, would have connected to a bunch of > tracepoints at known quiescent states that would increment the counter, > and then check each task until they all pass a certain point, or are in > a quiescent state (userspace or idle). But this would be doing much of > what RCU does today, and that is why we decided to hook with the RCU > infrastructure. > > I have to ask, what's your motivation for getting rid of RCU tasks? Again, my motivation was just to see if we could use RCU-preempt. Linus was talking about unifying RCU variants/API better. I believe Paul is well on top of that task and I have been helping as I could with his recent series for that. Since RCU-preempt doesn't suit this job, looks like we have keep RCU tasks around. That said I sent you a patch to speed up RCU-tasks a bit, could you a take a look? Paul was asking for your input: https://lkml.org/lkml/2018/5/20/263 Welcome back from your vacation, hope it was fun! thanks, - Joel