On Sat, Nov 08, 2025 at 10:47:16AM +0900, Akihiko Odaki wrote:
> 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 worry on an slightly outdated "n" read because the 2nd round of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) <----------
> [1]
> 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.
We don't necessarily need to identify "this sequence" or "next sequence",
but what we want to make sure when forced rcu triggered, rcu thread doesn't
sleep until flushing the callback injected right before force_rcu(), right?
IOW, at [1] above after setting forced, we still will post to sem, so IIUC
what will happen is both the normal call_rcu1() and force_rcu() will start
to post to the sem, hence the value of sem can be 2. If I add the whole
process into above picture, there must be a pre-existing call_rcu1() that
kicks the rcu thread first at [0] otherwise rcu thread should be asleep,
then with the help of the 2nd post at [2] it should guarantee the rcu
callback of force_rcu() be finally invoked without sleep:
rcu thread other thread
---------- ------------
call_rcu1() |
qemu_sem_post(&sync_event)| <----------
[0]
|
... rcu thread waked up ...
|
qatomic_xchg(&forced, false) |
| call_rcu1() <----------
[a]
| qatomic_fetch_inc(&rcu_call_count)
| ... assume count>0 already, no post ...
| force_rcu()
| qatomic_set(&forced, true) <----------
[1]
| ... force_rcu() always post ...
| qemu_sem_post(&sync_event) <----------
[2]
qatomic_read(&rcu_call_count) |
enter_qs() |
wait_for_readers(sleep=N) |
... finish without explicit sleep in wait_for_readers(), due to [2] above...
(even if sleep=N)
... invoked the rcu callback injected at [a] ...
... the next sequence of rcu will see force=true, but it's fine to see
force=true (false positive in this case when [a] callback is
already flushed) ...
IOW, IIUC there're two things needed to make sure the rcu thread finish
asap, one is the set of force=true, the other is the event_set(sync_event).
IMHO it's the same here when using sem, just that sem can remember more
than 1 count so it'll be able to identify concurrent call_rcu1() and
force_rcu().
Thanks,
>
> > 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
>
--
Peter Xu
