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Message-Id: <1485294393-10785-5-git-send-email-paulmck@linux.vnet.ibm.com>
Date: Tue, 24 Jan 2017 13:46:32 -0800
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: linux-kernel@...r.kernel.org
Cc: mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
dvhart@...ux.intel.com, fweisbec@...il.com, oleg@...hat.com,
bobby.prani@...il.com,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH v3 tip/core/rcu 5/6] 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@...ux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@...htriplett.org>
---
include/trace/events/rcu.h | 10 +++++-----
kernel/rcu/tree.c | 46 ++++++++++++++++++++++++++++++++++------------
2 files changed, 39 insertions(+), 17 deletions(-)
diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
index 9d4f9b3a2b7b..e3facb356838 100644
--- 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 8b970319c75b..d8245cbd08f9 100644
--- 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;
}
--
2.5.2
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