From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756266Ab2ICJ4L (ORCPT ); Mon, 3 Sep 2012 05:56:11 -0400 Received: from relay3-d.mail.gandi.net ([217.70.183.195]:39880 "EHLO relay3-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756186Ab2ICJ4K (ORCPT ); Mon, 3 Sep 2012 05:56:10 -0400 X-Originating-IP: 217.70.178.130 X-Originating-IP: 50.43.46.74 Date: Mon, 3 Sep 2012 02:56:01 -0700 From: Josh Triplett To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu, patches@linaro.org, "Paul E. McKenney" Subject: Re: [PATCH tip/core/rcu 23/23] rcu: Simplify quiescent-state detection Message-ID: <20120903095601.GL5574@leaf> References: <20120830181811.GA29154@linux.vnet.ibm.com> <1346350718-30937-1-git-send-email-paulmck@linux.vnet.ibm.com> <1346350718-30937-23-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: <1346350718-30937-23-git-send-email-paulmck@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 30, 2012 at 11:18:38AM -0700, Paul E. McKenney wrote: > From: "Paul E. McKenney" > > The current quiescent-state detection algorithm is needlessly > complex. It records the grace-period number corresponding to > the quiescent state at the time of the quiescent state, which > works, but it seems better to simply erase any record of previous > quiescent states at the time that the CPU notices the new grace > period. This has the further advantage of removing another piece > of RCU for which lockless reasoning is required. > > Therefore, this commit makes this change. > > Signed-off-by: Paul E. McKenney > Signed-off-by: Paul E. McKenney Reviewed-by: Josh Triplett > kernel/rcutree.c | 27 +++++++++++---------------- > kernel/rcutree.h | 2 -- > kernel/rcutree_plugin.h | 2 -- > kernel/rcutree_trace.c | 12 +++++------- > 4 files changed, 16 insertions(+), 27 deletions(-) > > diff --git a/kernel/rcutree.c b/kernel/rcutree.c > index 44609c3..d39ad5c 100644 > --- a/kernel/rcutree.c > +++ b/kernel/rcutree.c > @@ -176,8 +176,6 @@ void rcu_sched_qs(int cpu) > { > struct rcu_data *rdp = &per_cpu(rcu_sched_data, cpu); > > - rdp->passed_quiesce_gpnum = rdp->gpnum; > - barrier(); > if (rdp->passed_quiesce == 0) > trace_rcu_grace_period("rcu_sched", rdp->gpnum, "cpuqs"); > rdp->passed_quiesce = 1; > @@ -187,8 +185,6 @@ void rcu_bh_qs(int cpu) > { > struct rcu_data *rdp = &per_cpu(rcu_bh_data, cpu); > > - rdp->passed_quiesce_gpnum = rdp->gpnum; > - barrier(); > if (rdp->passed_quiesce == 0) > trace_rcu_grace_period("rcu_bh", rdp->gpnum, "cpuqs"); > rdp->passed_quiesce = 1; > @@ -897,12 +893,8 @@ static void __note_new_gpnum(struct rcu_state *rsp, struct rcu_node *rnp, struct > */ > rdp->gpnum = rnp->gpnum; > trace_rcu_grace_period(rsp->name, rdp->gpnum, "cpustart"); > - if (rnp->qsmask & rdp->grpmask) { > - rdp->qs_pending = 1; > - rdp->passed_quiesce = 0; > - } else { > - rdp->qs_pending = 0; > - } > + rdp->passed_quiesce = 0; > + rdp->qs_pending = !!(rnp->qsmask & rdp->grpmask); > zero_cpu_stall_ticks(rdp); > } > } > @@ -982,10 +974,13 @@ __rcu_process_gp_end(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_dat > * our behalf. Catch up with this state to avoid noting > * spurious new grace periods. If another grace period > * has started, then rnp->gpnum will have advanced, so > - * we will detect this later on. > + * we will detect this later on. Of course, any quiescent > + * states we found for the old GP are now invalid. > */ > - if (ULONG_CMP_LT(rdp->gpnum, rdp->completed)) > + if (ULONG_CMP_LT(rdp->gpnum, rdp->completed)) { > rdp->gpnum = rdp->completed; > + rdp->passed_quiesce = 0; > + } > > /* > * If RCU does not need a quiescent state from this CPU, > @@ -1357,7 +1352,7 @@ rcu_report_qs_rnp(unsigned long mask, struct rcu_state *rsp, > * based on quiescent states detected in an earlier grace period! > */ > static void > -rcu_report_qs_rdp(int cpu, struct rcu_state *rsp, struct rcu_data *rdp, long lastgp) > +rcu_report_qs_rdp(int cpu, struct rcu_state *rsp, struct rcu_data *rdp) > { > unsigned long flags; > unsigned long mask; > @@ -1365,7 +1360,8 @@ rcu_report_qs_rdp(int cpu, struct rcu_state *rsp, struct rcu_data *rdp, long las > > rnp = rdp->mynode; > raw_spin_lock_irqsave(&rnp->lock, flags); > - if (lastgp != rnp->gpnum || rnp->completed == rnp->gpnum) { > + if (rdp->passed_quiesce == 0 || rdp->gpnum != rnp->gpnum || > + rnp->completed == rnp->gpnum) { > > /* > * The grace period in which this quiescent state was > @@ -1424,7 +1420,7 @@ rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp) > * Tell RCU we are done (but rcu_report_qs_rdp() will be the > * judge of that). > */ > - rcu_report_qs_rdp(rdp->cpu, rsp, rdp, rdp->passed_quiesce_gpnum); > + rcu_report_qs_rdp(rdp->cpu, rsp, rdp); > } > > #ifdef CONFIG_HOTPLUG_CPU > @@ -2599,7 +2595,6 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible) > rdp->completed = rnp->completed; > rdp->passed_quiesce = 0; > rdp->qs_pending = 0; > - rdp->passed_quiesce_gpnum = rnp->gpnum - 1; > trace_rcu_grace_period(rsp->name, rdp->gpnum, "cpuonl"); > } > raw_spin_unlock(&rnp->lock); /* irqs already disabled. */ > diff --git a/kernel/rcutree.h b/kernel/rcutree.h > index 8f0293c..935dd4c 100644 > --- a/kernel/rcutree.h > +++ b/kernel/rcutree.h > @@ -246,8 +246,6 @@ struct rcu_data { > /* in order to detect GP end. */ > unsigned long gpnum; /* Highest gp number that this CPU */ > /* is aware of having started. */ > - unsigned long passed_quiesce_gpnum; > - /* gpnum at time of quiescent state. */ > bool passed_quiesce; /* User-mode/idle loop etc. */ > bool qs_pending; /* Core waits for quiesc state. */ > bool beenonline; /* CPU online at least once. */ > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h > index f1e06f6..4bc190a 100644 > --- a/kernel/rcutree_plugin.h > +++ b/kernel/rcutree_plugin.h > @@ -137,8 +137,6 @@ static void rcu_preempt_qs(int cpu) > { > struct rcu_data *rdp = &per_cpu(rcu_preempt_data, cpu); > > - rdp->passed_quiesce_gpnum = rdp->gpnum; > - barrier(); > if (rdp->passed_quiesce == 0) > trace_rcu_grace_period("rcu_preempt", rdp->gpnum, "cpuqs"); > rdp->passed_quiesce = 1; > diff --git a/kernel/rcutree_trace.c b/kernel/rcutree_trace.c > index f54f0ce..bd4df13 100644 > --- a/kernel/rcutree_trace.c > +++ b/kernel/rcutree_trace.c > @@ -86,12 +86,11 @@ static void print_one_rcu_data(struct seq_file *m, struct rcu_data *rdp) > { > if (!rdp->beenonline) > return; > - seq_printf(m, "%3d%cc=%lu g=%lu pq=%d pgp=%lu qp=%d", > + seq_printf(m, "%3d%cc=%lu g=%lu pq=%d qp=%d", > rdp->cpu, > cpu_is_offline(rdp->cpu) ? '!' : ' ', > rdp->completed, rdp->gpnum, > - rdp->passed_quiesce, rdp->passed_quiesce_gpnum, > - rdp->qs_pending); > + rdp->passed_quiesce, rdp->qs_pending); > seq_printf(m, " dt=%d/%llx/%d df=%lu", > atomic_read(&rdp->dynticks->dynticks), > rdp->dynticks->dynticks_nesting, > @@ -150,12 +149,11 @@ static void print_one_rcu_data_csv(struct seq_file *m, struct rcu_data *rdp) > { > if (!rdp->beenonline) > return; > - seq_printf(m, "%d,%s,%lu,%lu,%d,%lu,%d", > + seq_printf(m, "%d,%s,%lu,%lu,%d,%d", > rdp->cpu, > cpu_is_offline(rdp->cpu) ? "\"N\"" : "\"Y\"", > rdp->completed, rdp->gpnum, > - rdp->passed_quiesce, rdp->passed_quiesce_gpnum, > - rdp->qs_pending); > + rdp->passed_quiesce, rdp->qs_pending); > seq_printf(m, ",%d,%llx,%d,%lu", > atomic_read(&rdp->dynticks->dynticks), > rdp->dynticks->dynticks_nesting, > @@ -186,7 +184,7 @@ static int show_rcudata_csv(struct seq_file *m, void *unused) > int cpu; > struct rcu_state *rsp; > > - seq_puts(m, "\"CPU\",\"Online?\",\"c\",\"g\",\"pq\",\"pgp\",\"pq\","); > + seq_puts(m, "\"CPU\",\"Online?\",\"c\",\"g\",\"pq\",\"pq\","); > seq_puts(m, "\"dt\",\"dt nesting\",\"dt NMI nesting\",\"df\","); > seq_puts(m, "\"of\",\"qll\",\"ql\",\"qs\""); > #ifdef CONFIG_RCU_BOOST > -- > 1.7.8 >