Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755788AbdDRAIL (ORCPT ); Mon, 17 Apr 2017 20:08:11 -0400 Received: from slow1-d.mail.gandi.net ([217.70.178.86]:34809 "EHLO slow1-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752037AbdDRAIJ (ORCPT ); Mon, 17 Apr 2017 20:08:09 -0400 X-Originating-IP: 50.39.160.18 Date: Mon, 17 Apr 2017 17:07:55 -0700 From: Josh Triplett To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com Subject: Re: [PATCH v2 tip/core/rcu 01/39] rcu: Maintain special bits at bottom of ->dynticks counter Message-ID: <20170418000753.kacd4dc3h7swq3nt@x> References: <20170417234452.GB19013@linux.vnet.ibm.com> <1492472726-3841-1-git-send-email-paulmck@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1492472726-3841-1-git-send-email-paulmck@linux.vnet.ibm.com> User-Agent: NeoMutt/20170306 (1.8.0) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3107 Lines: 71 On Mon, Apr 17, 2017 at 04:44:48PM -0700, Paul E. McKenney wrote: > Currently, IPIs are used to force other CPUs to invalidate their TLBs > in response to a kernel virtual-memory mapping change. This works, but > degrades both battery lifetime (for idle CPUs) and real-time response > (for nohz_full CPUs), and in addition results in unnecessary IPIs due to > the fact that CPUs executing in usermode are unaffected by stale kernel > mappings. It would be better to cause a CPU executing in usermode to > wait until it is entering kernel mode to do the flush, first to avoid > interrupting usemode tasks and second to handle multiple flush requests > with a single flush in the case of a long-running user task. > > This commit therefore reserves a bit at the bottom of the ->dynticks > counter, which is checked upon exit from extended quiescent states. > If it is set, it is cleared and then a new rcu_eqs_special_exit() macro is > invoked, which, if not supplied, is an empty single-pass do-while loop. > If this bottom bit is set on -entry- to an extended quiescent state, > then a WARN_ON_ONCE() triggers. > > This bottom bit may be set using a new rcu_eqs_special_set() function, > which returns true if the bit was set, or false if the CPU turned > out to not be in an extended quiescent state. Please note that this > function refuses to set the bit for a non-nohz_full CPU when that CPU > is executing in usermode because usermode execution is tracked by RCU > as a dyntick-idle extended quiescent state only for nohz_full CPUs. > > Reported-by: Andy Lutomirski > Signed-off-by: Paul E. McKenney Comments below. With those fixed: Reviewed-by: Josh Triplett > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -290,15 +300,20 @@ static DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = { > static void rcu_dynticks_eqs_enter(void) > { > struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks); > - int special; > + int seq; > > /* > * CPUs seeing atomic_inc_return() must see prior RCU read-side > * critical sections, and we also must force ordering with the > * next idle sojourn. > */ > - special = atomic_inc_return(&rdtp->dynticks); > - WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && special & 0x1); > + seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdtp->dynticks); You changed atomic_inc_return to atomic_add_return here, but the comment above still says atomic_inc_return. > @@ -308,15 +323,22 @@ static void rcu_dynticks_eqs_enter(void) > static void rcu_dynticks_eqs_exit(void) > { > struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks); > - int special; > + int seq; > > /* > * CPUs seeing atomic_inc_return() must see prior idle sojourns, > * and we also must force ordering with the next RCU read-side > * critical section. > */ > - special = atomic_inc_return(&rdtp->dynticks); > - WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !(special & 0x1)); > + seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdtp->dynticks); Likewise. - Josh Triplett