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rcu: Unify force quiescent state

Browse Source 
Borrow the concept of force quiescent state from Linux to ensure readers
remain fast during normal operation and to avoid stalls.

Background
==========

The previous implementation had four steps to begin reclamation.

1. call_rcu_thread() would wait for the first callback.

2. call_rcu_thread() would periodically poll until a decent number of
   callbacks piled up or it timed out.

3. synchronize_rcu() would statr a grace period (GP).

4. wait_for_readers() would wait for the GP to end. It would also
   trigger the force_rcu notifier to break busy loops in a read-side
   critical section if drain_call_rcu() had been called.

Problem
=======

The separation of waiting logic across these steps led to suboptimal
behavior:

The GP was delayed until call_rcu_thread() stops polling.

force_rcu was not consistently triggered when call_rcu_thread() detected
a high number of pending callbacks or a timeout. This inconsistency
sometimes led to stalls, as reported in a virtio-gpu issue where memory
unmapping was blocked[1].

wait_for_readers() imposed unnecessary overhead in non-urgent cases by
unconditionally executing qatomic_set(&index->waiting, true) and
qemu_event_reset(&rcu_gp_event), which are necessary only for expedited
synchronization.

Solution
========

Move the polling in call_rcu_thread() to wait_for_readers() to prevent
the delay of the GP. Additionally, reorganize wait_for_readers() to
distinguish between two states:

Normal State: it relies exclusively on periodic polling to detect
the end of the GP and maintains the read-side fast path.

Force Quiescent State: Whenever expediting synchronization, it always
triggers force_rcu and executes both qatomic_set(&index->waiting, true)
and qemu_event_reset(&rcu_gp_event). This avoids stalls while confining
the read-side overhead to this state.

This unified approach, inspired by the Linux RCU, ensures consistent and
efficient RCU grace period handling and confirms resolution of the
virtio-gpu issue.

[1] https://lore.kernel.org/qemu-devel/20251014111234.3190346-9-alex.bennee@linaro.org/

Signed-off-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Link: https://lore.kernel.org/r/20251016-force-v1-1-919a82112498@rsg.ci.i.u-tokyo.ac.jp
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Akihiko Odaki
2025-10-16 15:34:41 +09:00
committed by Paolo Bonzini
parent 639a294227
commit 55d98e3ede
1 changed files with 52 additions and 29 deletions
Download Patch File Download Diff File Expand all files Collapse all files

81
util/rcu.c
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@@ -43,10 +43,14 @@
#define RCU_GP_LOCKED (1UL << 0)
#define RCU_GP_CTR (1UL << 1)
#define RCU_CALL_MIN_SIZE 30
unsigned long rcu_gp_ctr = RCU_GP_LOCKED;
QemuEvent rcu_gp_event;
static int in_drain_call_rcu;
static int rcu_call_count;
static QemuMutex rcu_registry_lock;
static QemuMutex rcu_sync_lock;
@@ -76,15 +80,29 @@ static void wait_for_readers(void)
{
ThreadList qsreaders = QLIST_HEAD_INITIALIZER(qsreaders);
struct rcu_reader_data *index, *tmp;
int sleeps = 0;
bool forced = false;
for (;;) {
/* We want to be notified of changes made to rcu_gp_ongoing
* while we walk the list.
/*
* Force the grace period to end and wait for it if any of the
* following heuristical conditions are satisfied:
* - A decent number of callbacks piled up.
* - It timed out.
* - It is in a drain_call_rcu() call.
*
* Otherwise, periodically poll the grace period, hoping it ends
* promptly.
*/
qemu_event_reset(&rcu_gp_event);
if (!forced &&
(qatomic_read(&rcu_call_count) >= RCU_CALL_MIN_SIZE ||
sleeps >= 5 || qatomic_read(&in_drain_call_rcu))) {
forced = true;
QLIST_FOREACH(index, &registry, node) {
qatomic_set(&index->waiting, true);
QLIST_FOREACH(index, &registry, node) {
notifier_list_notify(&index->force_rcu, NULL);
qatomic_set(&index->waiting, true);
}
}
/* Here, order the stores to index->waiting before the loads of
@@ -106,8 +124,6 @@ static void wait_for_readers(void)
* get some extra futex wakeups.
*/
qatomic_set(&index->waiting, false);
} else if (qatomic_read(&in_drain_call_rcu)) {
notifier_list_notify(&index->force_rcu, NULL);
}
}
@@ -115,7 +131,8 @@ static void wait_for_readers(void)
break;
}
/* Wait for one thread to report a quiescent state and try again.
/*
* Sleep for a while and try again.
* Release rcu_registry_lock, so rcu_(un)register_thread() doesn't
* wait too much time.
*
@@ -133,7 +150,20 @@ static void wait_for_readers(void)
* rcu_registry_lock is released.
*/
qemu_mutex_unlock(&rcu_registry_lock);
qemu_event_wait(&rcu_gp_event);
if (forced) {
qemu_event_wait(&rcu_gp_event);
/*
* We want to be notified of changes made to rcu_gp_ongoing
* while we walk the list.
*/
qemu_event_reset(&rcu_gp_event);
} else {
g_usleep(10000);
sleeps++;
}
qemu_mutex_lock(&rcu_registry_lock);
}
@@ -173,15 +203,11 @@ void synchronize_rcu(void)
}
}
#define RCU_CALL_MIN_SIZE 30
/* Multi-producer, single-consumer queue based on urcu/static/wfqueue.h
* from liburcu. Note that head is only used by the consumer.
*/
static struct rcu_head dummy;
static struct rcu_head *head = &dummy, **tail = &dummy.next;
static int rcu_call_count;
static QemuEvent rcu_call_ready_event;
static void enqueue(struct rcu_head *node)
@@ -259,30 +285,27 @@ static void *call_rcu_thread(void *opaque)
rcu_register_thread();
for (;;) {
int tries = 0;
int n = qatomic_read(&rcu_call_count);
int n;
/* Heuristically wait for a decent number of callbacks to pile up.
/*
* Fetch rcu_call_count now, we only must process elements that were
* added before synchronize_rcu() starts.
*/
while (n == 0 || (n < RCU_CALL_MIN_SIZE && ++tries <= 5)) {
g_usleep(10000);
if (n == 0) {
qemu_event_reset(&rcu_call_ready_event);
n = qatomic_read(&rcu_call_count);
if (n == 0) {
#if defined(CONFIG_MALLOC_TRIM)
malloc_trim(4 * 1024 * 1024);
#endif
qemu_event_wait(&rcu_call_ready_event);
}
}
for (;;) {
qemu_event_reset(&rcu_call_ready_event);
n = qatomic_read(&rcu_call_count);
if (n) {
break;
}
#if defined(CONFIG_MALLOC_TRIM)
malloc_trim(4 * 1024 * 1024);
#endif
qemu_event_wait(&rcu_call_ready_event);
}
qatomic_sub(&rcu_call_count, n);
synchronize_rcu();
qatomic_sub(&rcu_call_count, n);
bql_lock();
while (n > 0) {
node = try_dequeue();