From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH v15 04/13] task_isolation: add initial support Date: Tue, 27 Sep 2016 07:48:00 -0700 Message-ID: <20160927144800.GE14933@linux.vnet.ibm.com> References: <20160830075854.GZ10153@twins.programming.kicks-ass.net> <3f84f736-ed7f-adff-d5f0-4f7db664208f@mellanox.com> <20160927142219.GC6242@lerouge> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20160927142219.GC6242@lerouge> Sender: owner-linux-mm@kvack.org To: Frederic Weisbecker Cc: Andy Lutomirski , Chris Metcalf , Thomas Gleixner , "linux-doc@vger.kernel.org" , Christoph Lameter , Michal Hocko , Gilad Ben Yossef , Andrew Morton , Viresh Kumar , Linux API , Steven Rostedt , Ingo Molnar , Tejun Heo , Rik van Riel , Will Deacon , "linux-mm@kvack.org" , "linux-kernel@vger.kernel.org" , Catalin Marinas , Peter Zijlstra List-Id: linux-api@vger.kernel.org On Tue, Sep 27, 2016 at 04:22:20PM +0200, Frederic Weisbecker wrote: > On Fri, Sep 02, 2016 at 10:28:00AM -0700, Andy Lutomirski wrote: > > > > Unless I'm missing something (which is reasonably likely), couldn't > > the isolation code just force or require rcu_nocbs on the isolated > > CPUs to avoid this problem entirely. > > rcu_nocb is already implied by nohz_full. Which means that RCU callbacks > are offlined outside the nohz_full set of CPUs. Indeed, at boot time, RCU makes any nohz_full CPU also be a rcu_nocb CPU. > > I admit I still don't understand why the RCU context tracking code > > can't just run the callback right away instead of waiting however many > > microseconds in general. I feel like paulmck has explained it to me > > at least once, but that doesn't mean I remember the answer. > > The RCU context tracking doesn't take care of callbacks. It's only there > to tell the RCU core whether the CPU runs code that may or may not run > RCU read side critical sections. This is assumed by "kernel may use RCU, > userspace can't". And RCU has to wait for read-side critical sections to complete before invoking callbacks. Thanx, Paul -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org