RCU callback processing currently uses rcu_is_nocb_cpu() to determine
whether or not the current CPU's callbacks are to be offloaded.
This works, but it is not so good for cache locality. Plus use of
->cblist for offloaded callbacks will greatly increase the frequency
of these checks. This commit therefore adds a ->offloaded flag to the
rcu_segcblist structure to provide a more flexible and cache-friendly
means of checking for callback offloading.
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
---
include/linux/rcu_segcblist.h | 1 +
kernel/rcu/rcu_segcblist.c | 12 ++++++++++++
kernel/rcu/rcu_segcblist.h | 7 +++++++
kernel/rcu/tree.c | 10 ++++++----
kernel/rcu/tree_plugin.h | 11 +++++++----
5 files changed, 33 insertions(+), 8 deletions(-)
diff --git a/include/linux/rcu_segcblist.h b/include/linux/rcu_segcblist.h
index f48888040332..82977726da29 100644
--- a/include/linux/rcu_segcblist.h
+++ b/include/linux/rcu_segcblist.h
@@ -68,6 +68,7 @@ struct rcu_segcblist {
long len;
long len_lazy;
u8 enabled;
+ u8 offloaded;
};
#define RCU_SEGCBLIST_INITIALIZER(n) \
diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
index b305dcac34c9..700779f4c0cb 100644
--- a/kernel/rcu/rcu_segcblist.c
+++ b/kernel/rcu/rcu_segcblist.c
@@ -73,6 +73,18 @@ void rcu_segcblist_disable(struct rcu_segcblist *rsclp)
rsclp->enabled = 0;
}
+/*
+ * Mark the specified rcu_segcblist structure as offloaded. This
+ * structure must be empty.
+ */
+void rcu_segcblist_offload(struct rcu_segcblist *rsclp)
+{
+ WARN_ON_ONCE(!rcu_segcblist_empty(rsclp));
+ WARN_ON_ONCE(rcu_segcblist_n_cbs(rsclp));
+ WARN_ON_ONCE(rcu_segcblist_n_lazy_cbs(rsclp));
+ rsclp->offloaded = 1;
+}
+
/*
* Does the specified rcu_segcblist structure contain callbacks that
* are ready to be invoked?
diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
index b2de7b32da29..8f3783391075 100644
--- a/kernel/rcu/rcu_segcblist.h
+++ b/kernel/rcu/rcu_segcblist.h
@@ -66,6 +66,12 @@ static inline bool rcu_segcblist_is_enabled(struct rcu_segcblist *rsclp)
return rsclp->enabled;
}
+/* Is the specified rcu_segcblist offloaded? */
+static inline bool rcu_segcblist_is_offloaded(struct rcu_segcblist *rsclp)
+{
+ return rsclp->offloaded;
+}
+
/*
* Are all segments following the specified segment of the specified
* rcu_segcblist structure empty of callbacks? (The specified
@@ -78,6 +84,7 @@ static inline bool rcu_segcblist_restempty(struct rcu_segcblist *rsclp, int seg)
void rcu_segcblist_init(struct rcu_segcblist *rsclp);
void rcu_segcblist_disable(struct rcu_segcblist *rsclp);
+void rcu_segcblist_offload(struct rcu_segcblist *rsclp);
bool rcu_segcblist_ready_cbs(struct rcu_segcblist *rsclp);
bool rcu_segcblist_pend_cbs(struct rcu_segcblist *rsclp);
struct rcu_head *rcu_segcblist_first_cb(struct rcu_segcblist *rsclp);
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a14e5fbbea46..6f5c96c4f9a3 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2858,10 +2858,11 @@ void rcu_barrier(void)
* corresponding CPU's preceding callbacks have been invoked.
*/
for_each_possible_cpu(cpu) {
- if (!cpu_online(cpu) && !rcu_is_nocb_cpu(cpu))
- continue;
rdp = per_cpu_ptr(&rcu_data, cpu);
- if (rcu_is_nocb_cpu(cpu)) {
+ if (!cpu_online(cpu) &&
+ !rcu_segcblist_is_offloaded(&rdp->cblist))
+ continue;
+ if (rcu_segcblist_is_offloaded(&rdp->cblist)) {
if (!rcu_nocb_cpu_needs_barrier(cpu)) {
rcu_barrier_trace(TPS("OfflineNoCB"), cpu,
rcu_state.barrier_sequence);
@@ -3155,7 +3156,8 @@ void rcutree_migrate_callbacks(int cpu)
struct rcu_node *rnp_root = rcu_get_root();
bool needwake;
- if (rcu_is_nocb_cpu(cpu) || rcu_segcblist_empty(&rdp->cblist))
+ if (rcu_segcblist_is_offloaded(&rdp->cblist) ||
+ rcu_segcblist_empty(&rdp->cblist))
return; /* No callbacks to migrate. */
local_irq_save(flags);
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index b8a43cf9bb4e..fc6133eed50a 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1382,7 +1382,7 @@ static void rcu_prepare_for_idle(void)
int tne;
lockdep_assert_irqs_disabled();
- if (rcu_is_nocb_cpu(smp_processor_id()))
+ if (rcu_segcblist_is_offloaded(&rdp->cblist))
return;
/* Handle nohz enablement switches conservatively. */
@@ -1431,8 +1431,10 @@ static void rcu_prepare_for_idle(void)
*/
static void rcu_cleanup_after_idle(void)
{
+ struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
+
lockdep_assert_irqs_disabled();
- if (rcu_is_nocb_cpu(smp_processor_id()))
+ if (rcu_segcblist_is_offloaded(&rdp->cblist))
return;
if (rcu_try_advance_all_cbs())
invoke_rcu_core();
@@ -1694,7 +1696,7 @@ static bool __call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *rhp,
bool lazy, unsigned long flags)
{
- if (!rcu_is_nocb_cpu(rdp->cpu))
+ if (!rcu_segcblist_is_offloaded(&rdp->cblist))
return false;
__call_rcu_nocb_enqueue(rdp, rhp, &rhp->next, 1, lazy, flags);
if (__is_kfree_rcu_offset((unsigned long)rhp->func))
@@ -1729,7 +1731,7 @@ static bool __maybe_unused rcu_nocb_adopt_orphan_cbs(struct rcu_data *my_rdp,
unsigned long flags)
{
lockdep_assert_irqs_disabled();
- if (!rcu_is_nocb_cpu(smp_processor_id()))
+ if (!rcu_segcblist_is_offloaded(&my_rdp->cblist))
return false; /* Not NOCBs CPU, caller must migrate CBs. */
__call_rcu_nocb_enqueue(my_rdp, rcu_segcblist_head(&rdp->cblist),
rcu_segcblist_tail(&rdp->cblist),
@@ -2192,6 +2194,7 @@ static bool init_nocb_callback_list(struct rcu_data *rdp)
}
rcu_segcblist_init(&rdp->cblist);
rcu_segcblist_disable(&rdp->cblist);
+ rcu_segcblist_offload(&rdp->cblist);
return true;
}
--
2.17.1
