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rcu: Employ jiffies-based backstop to callback time limit

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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@meta.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Paul E. McKenney 2023-03-31 09:05:56 -07:00
parent fea1c1f010
commit f51164a808
2 changed files with 38 additions and 8 deletions
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18
kernel/rcu/Kconfig
View File

@ -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"

28
kernel/rcu/tree.c
View File

@ -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;
}