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Date: Wed, 19 Apr 2017 09:58:30 -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, bobby.prani@...il.com,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH v3 tip/core/rcu 33/40] srcu: Crude control of expedited grace periods
SRCU's implementation of expedited grace periods has always assumed
that the SRCU instance is idle when the expedited request arrives.
This commit improves this a bit by maintaining a count of the number
of outstanding expedited requests, thus allowing prior non-expedited
grace periods accommodate these requests by shifting to expedited mode.
However, any non-expedited wait already in progress will still wait for
the full duration.
Improved control of expedited grace periods is planned, but one step
at a time.
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
include/linux/srcu.h | 1 +
kernel/rcu/srcu.c | 84 ++++++++++++++++++++++++++++------------------------
2 files changed, 47 insertions(+), 38 deletions(-)
diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index e7dbc01b61a1..73a1b6296224 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -42,6 +42,7 @@ struct srcu_array {
struct srcu_struct {
unsigned long completed;
unsigned long srcu_gp_seq;
+ atomic_t srcu_exp_cnt;
struct srcu_array __percpu *per_cpu_ref;
spinlock_t queue_lock; /* protect ->srcu_cblist */
struct rcu_segcblist srcu_cblist;
diff --git a/kernel/rcu/srcu.c b/kernel/rcu/srcu.c
index 90ffea31b188..3cfcc59bddf3 100644
--- a/kernel/rcu/srcu.c
+++ b/kernel/rcu/srcu.c
@@ -43,6 +43,7 @@ static int init_srcu_struct_fields(struct srcu_struct *sp)
{
sp->completed = 0;
sp->srcu_gp_seq = 0;
+ atomic_set(&sp->srcu_exp_cnt, 0);
spin_lock_init(&sp->queue_lock);
rcu_segcblist_init(&sp->srcu_cblist);
INIT_DELAYED_WORK(&sp->work, process_srcu);
@@ -179,7 +180,6 @@ static bool srcu_readers_active(struct srcu_struct *sp)
return sum;
}
-#define SRCU_CALLBACK_BATCH 10
#define SRCU_INTERVAL 1
/**
@@ -197,6 +197,7 @@ static bool srcu_readers_active(struct srcu_struct *sp)
*/
void cleanup_srcu_struct(struct srcu_struct *sp)
{
+ WARN_ON_ONCE(atomic_read(&sp->srcu_exp_cnt));
if (WARN_ON(srcu_readers_active(sp)))
return; /* Leakage unless caller handles error. */
if (WARN_ON(!rcu_segcblist_empty(&sp->srcu_cblist)))
@@ -244,13 +245,10 @@ EXPORT_SYMBOL_GPL(__srcu_read_unlock);
* We use an adaptive strategy for synchronize_srcu() and especially for
* synchronize_srcu_expedited(). We spin for a fixed time period
* (defined below) to allow SRCU readers to exit their read-side critical
- * sections. If there are still some readers after 10 microseconds,
- * we repeatedly block for 1-millisecond time periods. This approach
- * has done well in testing, so there is no need for a config parameter.
+ * sections. If there are still some readers after a few microseconds,
+ * we repeatedly block for 1-millisecond time periods.
*/
#define SRCU_RETRY_CHECK_DELAY 5
-#define SYNCHRONIZE_SRCU_TRYCOUNT 2
-#define SYNCHRONIZE_SRCU_EXP_TRYCOUNT 12
/*
* Start an SRCU grace period.
@@ -267,16 +265,16 @@ static void srcu_gp_start(struct srcu_struct *sp)
}
/*
- * Wait until all readers counted by array index idx complete, but loop
- * a maximum of trycount times. The caller must ensure that ->completed
- * is not changed while checking.
+ * Wait until all readers counted by array index idx complete, but
+ * loop an additional time if there is an expedited grace period pending.
+ * The caller must ensure that ->completed is not changed while checking.
*/
static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount)
{
for (;;) {
if (srcu_readers_active_idx_check(sp, idx))
return true;
- if (--trycount <= 0)
+ if (--trycount + !!atomic_read(&sp->srcu_exp_cnt) <= 0)
return false;
udelay(SRCU_RETRY_CHECK_DELAY);
}
@@ -364,7 +362,7 @@ static void srcu_reschedule(struct srcu_struct *sp, unsigned long delay);
/*
* Helper function for synchronize_srcu() and synchronize_srcu_expedited().
*/
-static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
+static void __synchronize_srcu(struct srcu_struct *sp)
{
struct rcu_synchronize rcu;
struct rcu_head *head = &rcu.head;
@@ -401,6 +399,32 @@ static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
}
/**
+ * synchronize_srcu_expedited - Brute-force SRCU grace period
+ * @sp: srcu_struct with which to synchronize.
+ *
+ * Wait for an SRCU grace period to elapse, but be more aggressive about
+ * spinning rather than blocking when waiting.
+ *
+ * Note that synchronize_srcu_expedited() has the same deadlock and
+ * memory-ordering properties as does synchronize_srcu().
+ */
+void synchronize_srcu_expedited(struct srcu_struct *sp)
+{
+ bool do_norm = rcu_gp_is_normal();
+
+ if (!do_norm) {
+ atomic_inc(&sp->srcu_exp_cnt);
+ smp_mb__after_atomic(); /* increment before GP. */
+ }
+ __synchronize_srcu(sp);
+ if (!do_norm) {
+ smp_mb__before_atomic(); /* GP before decrement. */
+ atomic_dec(&sp->srcu_exp_cnt);
+ }
+}
+EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
+
+/**
* synchronize_srcu - wait for prior SRCU read-side critical-section completion
* @sp: srcu_struct with which to synchronize.
*
@@ -441,29 +465,14 @@ static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
*/
void synchronize_srcu(struct srcu_struct *sp)
{
- __synchronize_srcu(sp, (rcu_gp_is_expedited() && !rcu_gp_is_normal())
- ? SYNCHRONIZE_SRCU_EXP_TRYCOUNT
- : SYNCHRONIZE_SRCU_TRYCOUNT);
+ if (rcu_gp_is_expedited())
+ synchronize_srcu_expedited(sp);
+ else
+ __synchronize_srcu(sp);
}
EXPORT_SYMBOL_GPL(synchronize_srcu);
/**
- * synchronize_srcu_expedited - Brute-force SRCU grace period
- * @sp: srcu_struct with which to synchronize.
- *
- * Wait for an SRCU grace period to elapse, but be more aggressive about
- * spinning rather than blocking when waiting.
- *
- * Note that synchronize_srcu_expedited() has the same deadlock and
- * memory-ordering properties as does synchronize_srcu().
- */
-void synchronize_srcu_expedited(struct srcu_struct *sp)
-{
- __synchronize_srcu(sp, SYNCHRONIZE_SRCU_EXP_TRYCOUNT);
-}
-EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
-
-/**
* srcu_barrier - Wait until all in-flight call_srcu() callbacks complete.
* @sp: srcu_struct on which to wait for in-flight callbacks.
*/
@@ -490,7 +499,7 @@ EXPORT_SYMBOL_GPL(srcu_batches_completed);
* Core SRCU state machine. Advance callbacks from ->batch_check0 to
* ->batch_check1 and then to ->batch_done as readers drain.
*/
-static void srcu_advance_batches(struct srcu_struct *sp, int trycount)
+static void srcu_advance_batches(struct srcu_struct *sp)
{
int idx;
@@ -521,8 +530,8 @@ static void srcu_advance_batches(struct srcu_struct *sp, int trycount)
if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_SCAN1) {
idx = 1 ^ (sp->completed & 1);
- if (!try_check_zero(sp, idx, trycount))
- return; /* readers present, retry after SRCU_INTERVAL */
+ if (!try_check_zero(sp, idx, 1))
+ return; /* readers present, retry later. */
srcu_flip(sp);
rcu_seq_set_state(&sp->srcu_gp_seq, SRCU_STATE_SCAN2);
}
@@ -534,9 +543,8 @@ static void srcu_advance_batches(struct srcu_struct *sp, int trycount)
* so check at least twice in quick succession after a flip.
*/
idx = 1 ^ (sp->completed & 1);
- trycount = trycount < 2 ? 2 : trycount;
- if (!try_check_zero(sp, idx, trycount))
- return; /* readers present, retry after SRCU_INTERVAL */
+ if (!try_check_zero(sp, idx, 2))
+ return; /* readers present, retry after later. */
srcu_gp_end(sp);
}
}
@@ -602,8 +610,8 @@ void process_srcu(struct work_struct *work)
sp = container_of(work, struct srcu_struct, work.work);
- srcu_advance_batches(sp, 1);
+ srcu_advance_batches(sp);
srcu_invoke_callbacks(sp);
- srcu_reschedule(sp, SRCU_INTERVAL);
+ srcu_reschedule(sp, atomic_read(&sp->srcu_exp_cnt) ? 0 : SRCU_INTERVAL);
}
EXPORT_SYMBOL_GPL(process_srcu);
--
2.5.2
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