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    SubjectRe: [PATCH tip/core/rcu 23/23] rcu: Simplify quiescent-state detection
    On Thu, Aug 30, 2012 at 11:18:38AM -0700, Paul E. McKenney wrote:
    > From: "Paul E. McKenney" <paul.mckenney@linaro.org>
    >
    > 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 <paul.mckenney@linaro.org>
    > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

    Reviewed-by: Josh Triplett <josh@joshtriplett.org>

    > 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
    >


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