On 2025/11/07 23:00, Peter Xu wrote:
On Fri, Nov 07, 2025 at 10:47:35AM +0900, Akihiko Odaki wrote:
On 2025/11/07 6:52, Peter Xu wrote:
On Thu, Nov 06, 2025 at 10:40:52AM +0900, Akihiko Odaki wrote:
+ /*
+ * Ensure that the forced variable has not been set after fetching
+ * rcu_call_count; otherwise we may get confused by a force quiescent
+ * state request for an element later than n.
+ */
+ while (qatomic_xchg(&forced, false)) {
+ sleep = false;
+ n = qatomic_read(&rcu_call_count);
}
This is pretty tricky, and I wonder if it will make the code easier to read
if we convert the sync_event to be a semaphore instead. When as a sem, it
will take account of whatever kick to it, either a call_rcu1() or an
enforced rcu flush, so that we don't need to reset it. Meanwhile, we don't
worry on an slightly outdated "n" read because the 2nd round of sem_wait()
will catch that new "n".
Instead, worst case is rcu thread runs one more round without seeing
callbacks on the queue.
I'm not sure if that could help simplying code, maybe also make it less
error-prone.
Semaphore is not applicable here because it will not de-duplicate concurrent
kicks of RCU threads.
Why concurrent kicks of rcu threads is a problem? QemuSemaphore is
thread-safe itself, meanwhile IIUC it only still causes call_rcu_thread()
loops some more rounds reading "n", which looks all safe. No?
It is safe but incurs overheads and confusing. QemuEvent represents the
boolean semantics better.
I also have difficulty to understand how converting sync_event to a
semaphore simplifies the code. Perhaps some (pseudo)code to show how the
code will look like may be useful.
I prepared a patch on top of your current patchset to show what I meant. I
also added comments and some test results showing why I think it might be
fine that the sem overhead should be small.
In short, I tested a VM with 8 vCPUs and 4G mem, booting Linux and properly
poweroff, I only saw <1000 rcu_call1 users in total. That should be the
max-bound of sem overhead on looping in rcu thread.
It's in patch format but still treat it as a comment instead to discuss
with you. Attaching it is just easier for me.
===8<===
From 71f15ed19050a973088352a8d71b6cc6b7b5f7cf Mon Sep 17 00:00:00 2001
From: Peter Xu <[email protected]>
Date: Thu, 6 Nov 2025 16:03:00 -0500
Subject: [PATCH] rcu: Make sync_event a semaphore
It could simply all reset logic, especially after enforced rcu is
introduced we'll also need a tweak to re-read "n", which can be avoided too
when with a sem.
However, the sem can introduce an overhead in high frequecy rcu frees.
This patch is drafted with the assumption that rcu free is at least very
rare in QEMU, hence it's not a problem.
When I tested with this command:
qemu-system-x86_64 -M q35,kernel-irqchip=split,suppress-vmdesc=on -smp 8 \
-m 4G -msg timestamp=on -name peter-vm,debug-threads=on -cpu Nehalem \
-accel kvm -qmp unix:/tmp/peter.sock,server,nowait -nographic \
-monitor telnet::6666,server,nowait -netdev
user,id=net0,hostfwd=tcp::5555-:22
-device e1000,netdev=net0 -device virtio-balloon $DISK
I booted a pre-installed Linux, login and poweroff, wait until VM
completely shutdowns. I captured less than 1000 rcu_free1() calls in
summary. It means for the whole lifetime of such VM the max overhead of
the call_rcu_thread() loop reading rcu_call_count will be 1000 loops.
Signed-off-by: Peter Xu <[email protected]>
---
util/rcu.c | 36 ++++++++----------------------------
1 file changed, 8 insertions(+), 28 deletions(-)
diff --git a/util/rcu.c b/util/rcu.c
index 85f9333f5d..dfe031a5c9 100644
--- a/util/rcu.c
+++ b/util/rcu.c
@@ -54,7 +54,7 @@ static int rcu_call_count;
static QemuMutex rcu_registry_lock;
/* Set when the forced variable is set or rcu_call_count becomes non-zero. */
-static QemuEvent sync_event;
+static QemuSemaphore sync_event;
/*
* Check whether a quiescent state was crossed between the beginning of
@@ -80,7 +80,7 @@ static ThreadList registry = QLIST_HEAD_INITIALIZER(registry);
void force_rcu(void)
{
qatomic_set(&forced, true);
- qemu_event_set(&sync_event);
+ qemu_sem_post(&sync_event);
}
/* Wait for previous parity/grace period to be empty of readers. */
@@ -148,7 +148,7 @@ static void wait_for_readers(bool sleep)
*/
qemu_event_reset(&rcu_gp_event);
} else if (qatomic_read(&rcu_call_count) >= RCU_CALL_MIN_SIZE ||
- !sleeps || qemu_event_timedwait(&sync_event, 10)) {
+ !sleeps || qemu_sem_timedwait(&sync_event, 10)) {
/*
* Now one of the following heuristical conditions is satisfied:
* - A decent number of callbacks piled up.
@@ -286,7 +286,6 @@ static void *call_rcu_thread(void *opaque)
rcu_register_thread();
for (;;) {
- bool sleep = true;
int n;
/*
@@ -294,7 +293,6 @@ static void *call_rcu_thread(void *opaque)
* added before enter_qs() starts.
*/
for (;;) {
- qemu_event_reset(&sync_event);
n = qatomic_read(&rcu_call_count);
if (n) {
break;
@@ -303,36 +301,19 @@ static void *call_rcu_thread(void *opaque)
#if defined(CONFIG_MALLOC_TRIM)
malloc_trim(4 * 1024 * 1024);
#endif
- qemu_event_wait(&sync_event);
+ qemu_sem_wait(&sync_event);
}
- /*
- * Ensure that an event for a rcu_call_count change will not interrupt
- * wait_for_readers().
- */
- qemu_event_reset(&sync_event);
-
- /*
- * Ensure that the forced variable has not been set after fetching
- * rcu_call_count; otherwise we may get confused by a force quiescent
- * state request for an element later than n.
- */
- while (qatomic_xchg(&forced, false)) {
- sleep = false;
- n = qatomic_read(&rcu_call_count);
- }
-
- enter_qs(sleep);
+ enter_qs(!qatomic_xchg(&forced, false));
This is not OK; the forced variable may be set after rcu_call_count is
fetched. In that case, we should avoid unsetting the force quiescent state
request for the elements later than "n" or refetch "n".
Indeed I missed that part, but it should be trivial to fix, on top of my
previous patch:
===8<===
diff --git a/util/rcu.c b/util/rcu.c
index dfe031a5c9..aff98d9ee2 100644
--- a/util/rcu.c
+++ b/util/rcu.c
@@ -286,6 +286,7 @@ static void *call_rcu_thread(void *opaque)
rcu_register_thread();
for (;;) {
+ bool sleep;
int n;
/*
@@ -293,6 +294,7 @@ static void *call_rcu_thread(void *opaque)
* added before enter_qs() starts.
*/
for (;;) {
+ sleep = !qatomic_xchg(&forced, false);
This doesn't work either; the following sequence may happen (assume
forced is false at beginning):
qatomic_xchg(&forced, false) |
| call_rcu1()
| qatomic_fetch_inc(&rcu_call_count)
| force_rcu()
| qatomic_set(&forced, true)
qatomic_read(&rcu_call_count)
We need to enter the force quiescent state for the node added with the
call_rcu1() call in this sequence, but this code doesn't.
n = qatomic_read(&rcu_call_count);
if (n) {
break;
@@ -304,7 +306,7 @@ static void *call_rcu_thread(void *opaque)
qemu_sem_wait(&sync_event);
}
- enter_qs(!qatomic_xchg(&forced, false));
+ enter_qs(sleep);
qatomic_sub(&rcu_call_count, n);
bql_lock();
while (n > 0) {
===8<===
The idea is still the same, using semaphore can avoid explicit resets and a
lot of other ordering constraints like reading call_count, etc.
E.g. even before this series, we still need to properly reset at explicit
time to make sure we can capture a set() correct. When with sem, all these
issues are gone simply because we won't miss post() when it's a counter not
boolean.
Also, would you please also have a look at other comments I left in the
same email (after the patch I attached)?
https://lore.kernel.org/qemu-devel/[email protected]/
Can search "When I was having a closer look, I found some other issues".
I have just replied to the email. My mailer ignored the part after "--".
Regards,
Akihiko Odaki
