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Date: Sun, 22 Apr 2018 20:03:28 -0700
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,
fweisbec@...il.com, oleg@...hat.com, joel.opensrc@...il.com,
torvalds@...ux-foundation.org, npiggin@...il.com,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 05/21] rcu: Make rcu_gp_cleanup() more accurately predict need for new GP
Currently, rcu_gp_cleanup() scans the rcu_node tree in order to reset
state to reflect the end of the grace period. It also checks to see
whether a new grace period is needed, but in a number of cases, rather
than directly cause the new grace period to be immediately started, it
instead leaves the grace-period-needed state where various fail-safes
can find it. This works fine, but results in higher contention on the
root rcu_node structure's ->lock, which is undesirable, and contention
on that lock has recently become noticeable.
This commit therefore makes rcu_gp_cleanup() immediately start a new
grace period if there is any need for one.
It is quite possible that it will later be necessary to throttle the
grace-period rate, but that can be dealt with when and if.
Reported-by: Nicholas Piggin <npiggin@...il.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
kernel/rcu/tree.c | 16 ++++++++++------
kernel/rcu/tree.h | 1 -
kernel/rcu/tree_plugin.h | 17 -----------------
3 files changed, 10 insertions(+), 24 deletions(-)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 497f139056c7..afc5e32f0da4 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1763,14 +1763,14 @@ rcu_start_future_gp(struct rcu_node *rnp, struct rcu_data *rdp,
* Clean up any old requests for the just-ended grace period. Also return
* whether any additional grace periods have been requested.
*/
-static int rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
+static bool rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
{
int c = rnp->completed;
- int needmore;
+ bool needmore;
struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
need_future_gp_element(rnp, c) = 0;
- needmore = need_future_gp_element(rnp, c + 1);
+ needmore = need_any_future_gp(rnp);
trace_rcu_future_gp(rnp, rdp, c,
needmore ? TPS("CleanupMore") : TPS("Cleanup"));
return needmore;
@@ -2113,7 +2113,6 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
{
unsigned long gp_duration;
bool needgp = false;
- int nocb = 0;
struct rcu_data *rdp;
struct rcu_node *rnp = rcu_get_root(rsp);
struct swait_queue_head *sq;
@@ -2152,7 +2151,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
if (rnp == rdp->mynode)
needgp = __note_gp_changes(rsp, rnp, rdp) || needgp;
/* smp_mb() provided by prior unlock-lock pair. */
- nocb += rcu_future_gp_cleanup(rsp, rnp);
+ needgp = rcu_future_gp_cleanup(rsp, rnp) || needgp;
sq = rcu_nocb_gp_get(rnp);
raw_spin_unlock_irq_rcu_node(rnp);
rcu_nocb_gp_cleanup(sq);
@@ -2162,13 +2161,18 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
}
rnp = rcu_get_root(rsp);
raw_spin_lock_irq_rcu_node(rnp); /* Order GP before ->completed update. */
- rcu_nocb_gp_set(rnp, nocb);
/* Declare grace period done. */
WRITE_ONCE(rsp->completed, rsp->gpnum);
trace_rcu_grace_period(rsp->name, rsp->completed, TPS("end"));
rsp->gp_state = RCU_GP_IDLE;
+ /* Check for GP requests since above loop. */
rdp = this_cpu_ptr(rsp->rda);
+ if (need_any_future_gp(rnp)) {
+ trace_rcu_future_gp(rnp, rdp, rsp->completed - 1,
+ TPS("CleanupMore"));
+ needgp = true;
+ }
/* Advance CBs to reduce false positives below. */
needgp = rcu_advance_cbs(rsp, rnp, rdp) || needgp;
if (needgp || cpu_needs_another_gp(rsp, rdp)) {
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 18b091474ffa..bd1103763551 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -469,7 +469,6 @@ static void print_cpu_stall_info_end(void);
static void zero_cpu_stall_ticks(struct rcu_data *rdp);
static void increment_cpu_stall_ticks(void);
static bool rcu_nocb_cpu_needs_barrier(struct rcu_state *rsp, int cpu);
-static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq);
static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp);
static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq);
static void rcu_init_one_nocb(struct rcu_node *rnp);
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 640ea927d8a4..313b77d9cf06 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1780,19 +1780,6 @@ static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq)
swake_up_all(sq);
}
-/*
- * Set the root rcu_node structure's ->need_future_gp field
- * based on the sum of those of all rcu_node structures. This does
- * double-count the root rcu_node structure's requests, but this
- * is necessary to handle the possibility of a rcu_nocb_kthread()
- * having awakened during the time that the rcu_node structures
- * were being updated for the end of the previous grace period.
- */
-static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq)
-{
- need_future_gp_element(rnp, rnp->completed + 1) += nrq;
-}
-
static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp)
{
return &rnp->nocb_gp_wq[rnp->completed & 0x1];
@@ -2495,10 +2482,6 @@ static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq)
{
}
-static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq)
-{
-}
-
static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp)
{
return NULL;
--
2.5.2
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