On Thu, Aug 30, 2012 at 11:18:38AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mcken...@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.mcken...@linaro.org>
> Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <j...@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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