rcu: Check cond_resched_rcu_qs() state less often to reduce GP overhead

Commit 4a81e8328d37 ("rcu: Reduce overhead of cond_resched() checks
for RCU") moved quiescent-state generation out of cond_resched()
and commit bde6c3aa9930 ("rcu: Provide cond_resched_rcu_qs() to force
quiescent states in long loops") introduced cond_resched_rcu_qs(), and
commit 5cd37193ce85 ("rcu: Make cond_resched_rcu_qs() apply to normal RCU
flavors") introduced the per-CPU rcu_qs_ctr variable, which is frequently
polled by the RCU core state machine.

This frequent polling can increase grace-period rate, which in turn
increases grace-period overhead, which is visible in some benchmarks
(for example, the "open1" benchmark in Anton Blanchard's "will it scale"
suite).  This commit therefore reduces the rate at which rcu_qs_ctr
is polled by moving that polling into the force-quiescent-state (FQS)
machinery, and by further polling it only after the grace period has
been in effect for at least jiffies_till_sched_qs jiffies.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index 9d4f9b3a2b7b26ddedbd5b61d2add58c9a94cdab..e3facb356838c912e86e1e4c7a9f25c2f10555c2 100644 (file)
--- a/include/trace/events/rcu.h
+++ b/include/trace/events/rcu.h
@@ -385,11 +385,11 @@ TRACE_EVENT(rcu_quiescent_state_report,
 
 /*
  * Tracepoint for quiescent states detected by force_quiescent_state().
- * These trace events include the type of RCU, the grace-period number
- * that was blocked by the CPU, the CPU itself, and the type of quiescent
- * state, which can be "dti" for dyntick-idle mode, "ofl" for CPU offline,
- * or "kick" when kicking a CPU that has been in dyntick-idle mode for
- * too long.
+ * These trace events include the type of RCU, the grace-period number that
+ * was blocked by the CPU, the CPU itself, and the type of quiescent state,
+ * which can be "dti" for dyntick-idle mode, "ofl" for CPU offline, "kick"
+ * when kicking a CPU that has been in dyntick-idle mode for too long, or
+ * "rqc" if the CPU got a quiescent state via its rcu_qs_ctr.
  */
 TRACE_EVENT(rcu_fqs,
 

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 8b970319c75bd668e6400f71e1d4ed56554563e9..d8245cbd08f94506053c9baec623e39305c39cdd 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1232,7 +1232,10 @@ static int dyntick_save_progress_counter(struct rcu_data *rdp,
 static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
                                    bool *isidle, unsigned long *maxj)
 {
+       unsigned long jtsq;
        int *rcrmp;
+       unsigned long rjtsc;
+       struct rcu_node *rnp;
 
        /*
         * If the CPU passed through or entered a dynticks idle phase with
@@ -1248,6 +1251,31 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
                return 1;
        }
 
+       /* Compute and saturate jiffies_till_sched_qs. */
+       jtsq = jiffies_till_sched_qs;
+       rjtsc = rcu_jiffies_till_stall_check();
+       if (jtsq > rjtsc / 2) {
+               WRITE_ONCE(jiffies_till_sched_qs, rjtsc);
+               jtsq = rjtsc / 2;
+       } else if (jtsq < 1) {
+               WRITE_ONCE(jiffies_till_sched_qs, 1);
+               jtsq = 1;
+       }
+
+       /*
+        * Has this CPU encountered a cond_resched_rcu_qs() since the
+        * beginning of the grace period?  For this to be the case,
+        * the CPU has to have noticed the current grace period.  This
+        * might not be the case for nohz_full CPUs looping in the kernel.
+        */
+       rnp = rdp->mynode;
+       if (time_after(jiffies, rdp->rsp->gp_start + jtsq) &&
+           READ_ONCE(rdp->rcu_qs_ctr_snap) != per_cpu(rcu_qs_ctr, rdp->cpu) &&
+           READ_ONCE(rdp->gpnum) == rnp->gpnum && !rdp->gpwrap) {
+               trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("rqc"));
+               return 1;
+       }
+
        /*
         * Check for the CPU being offline, but only if the grace period
         * is old enough.  We don't need to worry about the CPU changing
@@ -1290,9 +1318,8 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
         * warning delay.
         */
        rcrmp = &per_cpu(rcu_sched_qs_mask, rdp->cpu);
-       if (ULONG_CMP_GE(jiffies,
-                        rdp->rsp->gp_start + jiffies_till_sched_qs) ||
-           ULONG_CMP_GE(jiffies, rdp->rsp->jiffies_resched)) {
+       if (time_after(jiffies, rdp->rsp->gp_start + jtsq) ||
+           time_after(jiffies, rdp->rsp->jiffies_resched)) {
                if (!(READ_ONCE(*rcrmp) & rdp->rsp->flavor_mask)) {
                        WRITE_ONCE(rdp->cond_resched_completed,
                                   READ_ONCE(rdp->mynode->completed));
@@ -2550,10 +2577,8 @@ rcu_report_qs_rdp(int cpu, struct rcu_state *rsp, struct rcu_data *rdp)
 
        rnp = rdp->mynode;
        raw_spin_lock_irqsave_rcu_node(rnp, flags);
-       if ((rdp->cpu_no_qs.b.norm &&
-            rdp->rcu_qs_ctr_snap == __this_cpu_read(rcu_qs_ctr)) ||
-           rdp->gpnum != rnp->gpnum || rnp->completed == rnp->gpnum ||
-           rdp->gpwrap) {
+       if (rdp->cpu_no_qs.b.norm || rdp->gpnum != rnp->gpnum ||
+           rnp->completed == rnp->gpnum || rdp->gpwrap) {
 
                /*
                 * The grace period in which this quiescent state was
@@ -2608,8 +2633,7 @@ rcu_check_quiescent_state(struct rcu_state *rsp, struct rcu_data *rdp)
         * Was there a quiescent state since the beginning of the grace
         * period? If no, then exit and wait for the next call.
         */
-       if (rdp->cpu_no_qs.b.norm &&
-           rdp->rcu_qs_ctr_snap == __this_cpu_read(rcu_qs_ctr))
+       if (rdp->cpu_no_qs.b.norm)
                return;
 
        /*
@@ -3563,9 +3587,7 @@ static int __rcu_pending(struct rcu_state *rsp, struct rcu_data *rdp)
            rdp->core_needs_qs && rdp->cpu_no_qs.b.norm &&
            rdp->rcu_qs_ctr_snap == __this_cpu_read(rcu_qs_ctr)) {
                rdp->n_rp_core_needs_qs++;
-       } else if (rdp->core_needs_qs &&
-                  (!rdp->cpu_no_qs.b.norm ||
-                   rdp->rcu_qs_ctr_snap != __this_cpu_read(rcu_qs_ctr))) {
+       } else if (rdp->core_needs_qs && !rdp->cpu_no_qs.b.norm) {
                rdp->n_rp_report_qs++;
                return 1;
        }