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Date: Wed, 9 Dec 2015 15:03:02 -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 v2 tip/core/rcu 05/11] rcu: Reduce expedited GP memory contention via per-CPU variables
Currently, the piggybacked-work checks carried out by sync_exp_work_done()
atomically increment a small set of variables (the ->expedited_workdone0,
->expedited_workdone1, ->expedited_workdone2, ->expedited_workdone3
fields in the rcu_state structure), which will form a memory-contention
bottleneck given a sufficiently large number of CPUs concurrently invoking
either synchronize_rcu_expedited() or synchronize_sched_expedited().
This commit therefore moves these for fields to the per-CPU rcu_data
structure, eliminating the memory contention. The show_rcuexp() function
also changes to sum up each field in the rcu_data structures.
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
kernel/rcu/tree.c | 11 +++++------
kernel/rcu/tree.h | 8 ++++----
kernel/rcu/tree_trace.c | 18 ++++++++++++------
3 files changed, 21 insertions(+), 16 deletions(-)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 00f07d6436ce..33d7e2551165 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3585,7 +3585,7 @@ static bool sync_exp_work_done(struct rcu_state *rsp, struct rcu_node *rnp,
*/
static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
{
- struct rcu_data *rdp;
+ struct rcu_data *rdp = per_cpu_ptr(rsp->rda, raw_smp_processor_id());
struct rcu_node *rnp0;
struct rcu_node *rnp1 = NULL;
@@ -3599,7 +3599,7 @@ static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
if (!mutex_is_locked(&rnp0->exp_funnel_mutex)) {
if (mutex_trylock(&rnp0->exp_funnel_mutex)) {
if (sync_exp_work_done(rsp, rnp0, NULL,
- &rsp->expedited_workdone0, s))
+ &rdp->expedited_workdone0, s))
return NULL;
return rnp0;
}
@@ -3613,14 +3613,13 @@ static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
* can be inexact, as it is just promoting locality and is not
* strictly needed for correctness.
*/
- rdp = per_cpu_ptr(rsp->rda, raw_smp_processor_id());
- if (sync_exp_work_done(rsp, NULL, NULL, &rsp->expedited_workdone1, s))
+ if (sync_exp_work_done(rsp, NULL, NULL, &rdp->expedited_workdone1, s))
return NULL;
mutex_lock(&rdp->exp_funnel_mutex);
rnp0 = rdp->mynode;
for (; rnp0 != NULL; rnp0 = rnp0->parent) {
if (sync_exp_work_done(rsp, rnp1, rdp,
- &rsp->expedited_workdone2, s))
+ &rdp->expedited_workdone2, s))
return NULL;
mutex_lock(&rnp0->exp_funnel_mutex);
if (rnp1)
@@ -3630,7 +3629,7 @@ static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
rnp1 = rnp0;
}
if (sync_exp_work_done(rsp, rnp1, rdp,
- &rsp->expedited_workdone3, s))
+ &rdp->expedited_workdone3, s))
return NULL;
return rnp1;
}
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 8151971a8978..6cbec31f99d6 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -386,6 +386,10 @@ struct rcu_data {
struct rcu_head oom_head;
#endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */
struct mutex exp_funnel_mutex;
+ atomic_long_t expedited_workdone0; /* # done by others #0. */
+ atomic_long_t expedited_workdone1; /* # done by others #1. */
+ atomic_long_t expedited_workdone2; /* # done by others #2. */
+ atomic_long_t expedited_workdone3; /* # done by others #3. */
/* 7) Callback offloading. */
#ifdef CONFIG_RCU_NOCB_CPU
@@ -500,10 +504,6 @@ struct rcu_state {
/* End of fields guarded by barrier_mutex. */
unsigned long expedited_sequence; /* Take a ticket. */
- atomic_long_t expedited_workdone0; /* # done by others #0. */
- atomic_long_t expedited_workdone1; /* # done by others #1. */
- atomic_long_t expedited_workdone2; /* # done by others #2. */
- atomic_long_t expedited_workdone3; /* # done by others #3. */
atomic_long_t expedited_normal; /* # fallbacks to normal. */
atomic_t expedited_need_qs; /* # CPUs left to check in. */
wait_queue_head_t expedited_wq; /* Wait for check-ins. */
diff --git a/kernel/rcu/tree_trace.c b/kernel/rcu/tree_trace.c
index 8efaba870d96..d43649450ea4 100644
--- a/kernel/rcu/tree_trace.c
+++ b/kernel/rcu/tree_trace.c
@@ -183,14 +183,20 @@ static const struct file_operations rcudata_fops = {
static int show_rcuexp(struct seq_file *m, void *v)
{
+ int cpu;
struct rcu_state *rsp = (struct rcu_state *)m->private;
-
+ struct rcu_data *rdp;
+ unsigned long s0 = 0, s1 = 0, s2 = 0, s3 = 0;
+
+ for_each_possible_cpu(cpu) {
+ rdp = per_cpu_ptr(rsp->rda, cpu);
+ s0 += atomic_long_read(&rdp->expedited_workdone0);
+ s1 += atomic_long_read(&rdp->expedited_workdone1);
+ s2 += atomic_long_read(&rdp->expedited_workdone2);
+ s3 += atomic_long_read(&rdp->expedited_workdone3);
+ }
seq_printf(m, "s=%lu wd0=%lu wd1=%lu wd2=%lu wd3=%lu n=%lu enq=%d sc=%lu\n",
- rsp->expedited_sequence,
- atomic_long_read(&rsp->expedited_workdone0),
- atomic_long_read(&rsp->expedited_workdone1),
- atomic_long_read(&rsp->expedited_workdone2),
- atomic_long_read(&rsp->expedited_workdone3),
+ rsp->expedited_sequence, s0, s1, s2, s3,
atomic_long_read(&rsp->expedited_normal),
atomic_read(&rsp->expedited_need_qs),
rsp->expedited_sequence / 2);
--
2.5.2
--
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