Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753998AbcKNRZZ (ORCPT ); Mon, 14 Nov 2016 12:25:25 -0500 Received: from relay5-d.mail.gandi.net ([217.70.183.197]:56558 "EHLO relay5-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752158AbcKNRZX (ORCPT ); Mon, 14 Nov 2016 12:25:23 -0500 X-Originating-IP: 50.39.170.172 Date: Mon, 14 Nov 2016 09:25:12 -0800 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, dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com Subject: Re: [PATCH tip/core/rcu 6/7] rcu: Make expedited grace periods recheck dyntick idle state Message-ID: <20161114172512.bcwdy66elesds5t4@jtriplet-mobl2.jf.intel.com> References: <20161114165648.GA15216@linux.vnet.ibm.com> <1479142633-15315-6-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: <1479142633-15315-6-git-send-email-paulmck@linux.vnet.ibm.com> User-Agent: NeoMutt/20161104 (1.7.1) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2774 Lines: 67 On Mon, Nov 14, 2016 at 08:57:12AM -0800, Paul E. McKenney wrote: > Expedited grace periods check dyntick-idle state, and avoid sending > IPIs to idle CPUs, including those running guest OSes, and, on NOHZ_FULL > kernels, nohz_full CPUs. However, the kernel has been observed checking > a CPU while it was non-idle, but sending the IPI after it has gone > idle. This commit therefore rechecks idle state immediately before > sending the IPI, refraining from IPIing CPUs that have since gone idle. > > Reported-by: Rik van Riel > Signed-off-by: Paul E. McKenney atomic_add_return(0, ...) seems odd. Do you actually want that, rather than atomic_read(...)? If so, can you please document exactly why? > kernel/rcu/tree.h | 1 + > kernel/rcu/tree_exp.h | 12 +++++++++++- > 2 files changed, 12 insertions(+), 1 deletion(-) > > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h > index e99a5234d9ed..fe98dd24adf8 100644 > --- a/kernel/rcu/tree.h > +++ b/kernel/rcu/tree.h > @@ -404,6 +404,7 @@ struct rcu_data { > atomic_long_t exp_workdone1; /* # done by others #1. */ > atomic_long_t exp_workdone2; /* # done by others #2. */ > atomic_long_t exp_workdone3; /* # done by others #3. */ > + int exp_dynticks_snap; /* Double-check need for IPI. */ > > /* 7) Callback offloading. */ > #ifdef CONFIG_RCU_NOCB_CPU > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > index 24343eb87b58..d3053e99fdb6 100644 > --- a/kernel/rcu/tree_exp.h > +++ b/kernel/rcu/tree_exp.h > @@ -358,8 +358,10 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp, > struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu); > struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu); > > + rdp->exp_dynticks_snap = > + atomic_add_return(0, &rdtp->dynticks); > if (raw_smp_processor_id() == cpu || > - !(atomic_add_return(0, &rdtp->dynticks) & 0x1) || > + !(rdp->exp_dynticks_snap & 0x1) || > !(rnp->qsmaskinitnext & rdp->grpmask)) > mask_ofl_test |= rdp->grpmask; > } > @@ -377,9 +379,17 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp, > /* IPI the remaining CPUs for expedited quiescent state. */ > for_each_leaf_node_possible_cpu(rnp, cpu) { > unsigned long mask = leaf_node_cpu_bit(rnp, cpu); > + struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu); > + struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu); > + > if (!(mask_ofl_ipi & mask)) > continue; > retry_ipi: > + if (atomic_add_return(0, &rdtp->dynticks) != > + rdp->exp_dynticks_snap) { > + mask_ofl_test |= mask; > + continue; > + } > ret = smp_call_function_single(cpu, func, rsp, 0); > if (!ret) { > mask_ofl_ipi &= ~mask; > -- > 2.5.2 >