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Message-Id: <20230510165832.2187453-4-paulmck@kernel.org>
Date: Wed, 10 May 2023 09:58:28 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: rcu@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, kernel-team@...a.com,
rostedt@...dmis.org, "Paul E. McKenney" <paulmck@...nel.org>,
Domas Mituzas <dmituzas@...a.com>
Subject: [PATCH rcu 4/8] rcu: Employ jiffies-based backstop to callback time limit
Currently, if there are more than 100 ready-to-invoke RCU callbacks queued
on a given CPU, the rcu_do_batch() function sets a timeout for invocation
of the series. This timeout defaulting to three milliseconds, and may
be adjusted using the rcutree.rcu_resched_ns kernel boot parameter.
This timeout is checked using local_clock(), but the overhead of this
function combined with the common-case very small callback-invocation
overhead means that local_clock() is checked every 32nd invocation.
This works well except for longer-than average callbacks. For example,
a series of 500-microsecond-duration callbacks means that local_clock()
is checked only once every 16 milliseconds, which makes it difficult to
enforce a three-millisecond timeout.
This commit therefore adds a Kconfig option RCU_DOUBLE_CHECK_CB_TIME
that enables backup timeout checking using the coarser grained but
lighter weight jiffies. If the jiffies counter detects a timeout,
then local_clock() is consulted even if this is not the 32nd callback.
This prevents the aforementioned 16-millisecond latency blow.
Reported-by: Domas Mituzas <dmituzas@...a.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
kernel/rcu/Kconfig | 18 ++++++++++++++++++
kernel/rcu/tree.c | 28 ++++++++++++++++++++--------
2 files changed, 38 insertions(+), 8 deletions(-)
diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index 9071182b1284..bdd7eadb33d8 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -314,4 +314,22 @@ config RCU_LAZY
To save power, batch RCU callbacks and flush after delay, memory
pressure, or callback list growing too big.
+config RCU_DOUBLE_CHECK_CB_TIME
+ bool "RCU callback-batch backup time check"
+ depends on RCU_EXPERT
+ default n
+ help
+ Use this option to provide more precise enforcement of the
+ rcutree.rcu_resched_ns module parameter in situations where
+ a single RCU callback might run for hundreds of microseconds,
+ thus defeating the 32-callback batching used to amortize the
+ cost of the fine-grained but expensive local_clock() function.
+
+ This option rounds rcutree.rcu_resched_ns up to the next
+ jiffy, and overrides the 32-callback batching if this limit
+ is exceeded.
+
+ Say Y here if you need tighter callback-limit enforcement.
+ Say N here if you are unsure.
+
endmenu # "RCU Subsystem"
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 9a5c160186d1..e2dbea6cee4b 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2047,10 +2047,15 @@ rcu_check_quiescent_state(struct rcu_data *rdp)
}
/* Return true if callback-invocation time limit exceeded. */
-static bool rcu_do_batch_check_time(long count, long tlimit)
+static bool rcu_do_batch_check_time(long count, long tlimit,
+ bool jlimit_check, unsigned long jlimit)
{
// Invoke local_clock() only once per 32 consecutive callbacks.
- return unlikely(tlimit) && !likely(count & 31) && local_clock() >= tlimit;
+ return unlikely(tlimit) &&
+ (!likely(count & 31) ||
+ (IS_ENABLED(CONFIG_RCU_DOUBLE_CHECK_CB_TIME) &&
+ jlimit_check && time_after(jiffies, jlimit))) &&
+ local_clock() >= tlimit;
}
/*
@@ -2059,13 +2064,17 @@ static bool rcu_do_batch_check_time(long count, long tlimit)
*/
static void rcu_do_batch(struct rcu_data *rdp)
{
+ long bl;
+ long count = 0;
int div;
bool __maybe_unused empty;
unsigned long flags;
- struct rcu_head *rhp;
+ unsigned long jlimit;
+ bool jlimit_check = false;
+ long pending;
struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
- long bl, count = 0;
- long pending, tlimit = 0;
+ struct rcu_head *rhp;
+ long tlimit = 0;
/* If no callbacks are ready, just return. */
if (!rcu_segcblist_ready_cbs(&rdp->cblist)) {
@@ -2090,11 +2099,14 @@ static void rcu_do_batch(struct rcu_data *rdp)
div = div < 0 ? 7 : div > sizeof(long) * 8 - 2 ? sizeof(long) * 8 - 2 : div;
bl = max(rdp->blimit, pending >> div);
if ((in_serving_softirq() || rdp->rcu_cpu_kthread_status == RCU_KTHREAD_RUNNING) &&
- unlikely(bl > 100)) {
+ (IS_ENABLED(CONFIG_RCU_DOUBLE_CHECK_CB_TIME) || unlikely(bl > 100))) {
+ const long npj = NSEC_PER_SEC / HZ;
long rrn = READ_ONCE(rcu_resched_ns);
rrn = rrn < NSEC_PER_MSEC ? NSEC_PER_MSEC : rrn > NSEC_PER_SEC ? NSEC_PER_SEC : rrn;
tlimit = local_clock() + rrn;
+ jlimit = jiffies + (rrn + npj + 1) / npj;
+ jlimit_check = true;
}
trace_rcu_batch_start(rcu_state.name,
rcu_segcblist_n_cbs(&rdp->cblist), bl);
@@ -2134,7 +2146,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
* Make sure we don't spend too much time here and deprive other
* softirq vectors of CPU cycles.
*/
- if (rcu_do_batch_check_time(count, tlimit))
+ if (rcu_do_batch_check_time(count, tlimit, jlimit_check, jlimit))
break;
} else {
// In rcuc/rcuoc context, so no worries about
@@ -2147,7 +2159,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
// But rcuc kthreads can delay quiescent-state
// reporting, so check time limits for them.
if (rdp->rcu_cpu_kthread_status == RCU_KTHREAD_RUNNING &&
- rcu_do_batch_check_time(count, tlimit)) {
+ rcu_do_batch_check_time(count, tlimit, jlimit_check, jlimit)) {
rdp->rcu_cpu_has_work = 1;
break;
}
--
2.40.1
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