On Wed, Jan 31, 2024 at 12:05:08PM -0300, Fabiano Rosas wrote:
> pet...@redhat.com writes:
>
> > From: Peter Xu <pet...@redhat.com>
> >
> > Multifd send side has two fields to indicate error quits:
> >
> > - MultiFDSendParams.quit
> > - &multifd_send_state->exiting
> >
> > Merge them into the global one. The replacement is done by changing all
> > p->quit checks into the global var check. The global check doesn't need
> > any lock.
> >
> > A few more things done on top of this altogether:
> >
> > - multifd_send_terminate_threads()
> >
> > Moving the xchg() of &multifd_send_state->exiting upper, so as to cover
> > the tracepoint, migrate_set_error() and migrate_set_state().
>
> Good.
>
> >
> > - multifd_send_sync_main()
> >
> > In the 2nd loop, add one more check over the global var to make sure we
> > don't keep the looping if QEMU already decided to quit.
>
> Yes, also because we don't necessarily enter at multifd_send_page()
> every time.
>
> >
> > - multifd_tls_outgoing_handshake()
> >
> > Use multifd_send_terminate_threads() to set the error state. That has
> > a benefit of updating MigrationState.error to that error too, so we can
> > persist that 1st error we hit in that specific channel.
>
> Makes sense.
>
> >
> > - multifd_new_send_channel_async()
> >
> > Take similar approach like above, drop the migrate_set_error() because
> > multifd_send_terminate_threads() already covers that. Unwrap the helper
> > multifd_new_send_channel_cleanup() along the way; not really needed.
> >
> > Signed-off-by: Peter Xu <pet...@redhat.com>
> > ---
> > migration/multifd.h | 2 --
> > migration/multifd.c | 85 ++++++++++++++++++---------------------------
> > 2 files changed, 33 insertions(+), 54 deletions(-)
> >
> > diff --git a/migration/multifd.h b/migration/multifd.h
> > index 35d11f103c..7c040cb85a 100644
> > --- a/migration/multifd.h
> > +++ b/migration/multifd.h
> > @@ -95,8 +95,6 @@ typedef struct {
> > QemuMutex mutex;
> > /* is this channel thread running */
> > bool running;
> > - /* should this thread finish */
> > - bool quit;
> > /* multifd flags for each packet */
> > uint32_t flags;
> > /* global number of generated multifd packets */
> > diff --git a/migration/multifd.c b/migration/multifd.c
> > index b8d2c96533..2c98023d67 100644
> > --- a/migration/multifd.c
> > +++ b/migration/multifd.c
> > @@ -372,6 +372,11 @@ struct {
> > MultiFDMethods *ops;
> > } *multifd_send_state;
> >
> > +static bool multifd_send_should_exit(void)
> > +{
> > + return qatomic_read(&multifd_send_state->exiting);
> > +}
> > +
> > /*
> > * The migration thread can wait on either of the two semaphores. This
> > * function can be used to kick the main thread out of waiting on either of
> > @@ -409,7 +414,7 @@ static int multifd_send_pages(void)
> > MultiFDSendParams *p = NULL; /* make happy gcc */
> > MultiFDPages_t *pages = multifd_send_state->pages;
> >
> > - if (qatomic_read(&multifd_send_state->exiting)) {
> > + if (multifd_send_should_exit()) {
> > return -1;
> > }
> >
> > @@ -421,14 +426,11 @@ static int multifd_send_pages(void)
> > */
> > next_channel %= migrate_multifd_channels();
> > for (i = next_channel;; i = (i + 1) % migrate_multifd_channels()) {
> > - p = &multifd_send_state->params[i];
> > -
> > - qemu_mutex_lock(&p->mutex);
> > - if (p->quit) {
> > - error_report("%s: channel %d has already quit!", __func__, i);
> > - qemu_mutex_unlock(&p->mutex);
> > + if (multifd_send_should_exit()) {
> > return -1;
> > }
> > + p = &multifd_send_state->params[i];
> > + qemu_mutex_lock(&p->mutex);
> > if (!p->pending_job) {
> > p->pending_job++;
> > next_channel = (i + 1) % migrate_multifd_channels();
>
> Hm, I'm not sure it's correct to check 'exiting' outside of the
> lock. While it is an atomic operation, it is not atomic in relation to
> pending_job...
>
> ... looking closer, it seems that we can do what you suggest because
> p->pending_job is not touched by the multifd_send_thread in case of
> error, which means this function will indeed miss the 'exiting' flag,
> but pending_job > 0 means it will loop to the next channel and _then_ it
> will see the 'exiting' flag.
It could still be the last channel we iterate, then IIUC we can still try
to assign a job to a thread even if a concurrent error is set there.
However IMHO it's okay; the error in the sender thread should ultimately
set migrate_set_error() and the main thread should detect that in the
migration loop, then we'll still quit. The extra queued job shouldn't
matter, IMHO.
>
> > @@ -483,6 +485,16 @@ static void multifd_send_terminate_threads(Error *err)
> > {
> > int i;
> >
> > + /*
> > + * We don't want to exit each threads twice. Depending on where
> > + * we get the error, or if there are two independent errors in two
> > + * threads at the same time, we can end calling this function
> > + * twice.
> > + */
> > + if (qatomic_xchg(&multifd_send_state->exiting, 1)) {
> > + return;
> > + }
> > +
> > trace_multifd_send_terminate_threads(err != NULL);
> >
> > if (err) {
> > @@ -497,26 +509,13 @@ static void multifd_send_terminate_threads(Error *err)
> > }
> > }
> >
> > - /*
> > - * We don't want to exit each threads twice. Depending on where
> > - * we get the error, or if there are two independent errors in two
> > - * threads at the same time, we can end calling this function
> > - * twice.
> > - */
> > - if (qatomic_xchg(&multifd_send_state->exiting, 1)) {
> > - return;
> > - }
> > -
> > for (i = 0; i < migrate_multifd_channels(); i++) {
> > MultiFDSendParams *p = &multifd_send_state->params[i];
> >
> > - qemu_mutex_lock(&p->mutex);
> > - p->quit = true;
>
> Now that you removed this, we decoupled kicking the threads from setting
> the exit/error, so this function could be split in two.
>
> We could set the exiting flag at the places the error occurred (multifd
> threads, thread creation, etc) and "terminate the threads" at
> multifd_save_cleanup(). That second part we already do actually:
>
> void multifd_save_cleanup(void) {
> ...
> multifd_send_terminate_threads(NULL);
> ^see?
> for (i = 0; i < migrate_multifd_channels(); i++) {
> MultiFDSendParams *p = &multifd_send_state->params[i];
>
> if (p->running) {
> qemu_thread_join(&p->thread);
> }
> }
> ...
> }
>
> I think there's no reason anymore for the channels to kick each
> other. They would all be waiting at p->sem and multifd_send_cleanup()
> would kick + join them.
Sounds good here.
I'll attach one patch like this, feel free to have an early look:
=====
>From f9a3d63d5cca0068daaea4c72392803f4b29dcb5 Mon Sep 17 00:00:00 2001
From: Peter Xu <pet...@redhat.com>
Date: Thu, 1 Feb 2024 17:01:54 +0800
Subject: [PATCH] migration/multifd: Split multifd_send_terminate_threads()
Split multifd_send_terminate_threads() into two functions:
- multifd_send_set_error(): used when an error happened on the sender
side, set error and quit state only
- multifd_send_terminate_threads(): used only by the main thread to kick
all multifd send threads out of sleep, for the last recycling.
Use multifd_send_set_error() in the three old call sites where only the
error will be set.
Use multifd_send_terminate_threads() in the last one where the main thread
will kick the multifd threads at last in multifd_save_cleanup().
Both helpers will need to set quitting=1.
Suggested-by: Fabiano Rosas <faro...@suse.de>
Signed-off-by: Peter Xu <pet...@redhat.com>
---
migration/multifd.c | 27 ++++++++++++++++++---------
migration/trace-events | 2 +-
2 files changed, 19 insertions(+), 10 deletions(-)
diff --git a/migration/multifd.c b/migration/multifd.c
index c71e74b101..95dc29c8c7 100644
--- a/migration/multifd.c
+++ b/migration/multifd.c
@@ -536,10 +536,9 @@ int multifd_queue_page(RAMBlock *block, ram_addr_t offset)
return 1;
}
-static void multifd_send_terminate_threads(Error *err)
+/* Multifd send side hit an error; remember it and prepare to quit */
+static void multifd_send_set_error(Error *err)
{
- int i;
-
/*
* We don't want to exit each threads twice. Depending on where
* we get the error, or if there are two independent errors in two
@@ -550,8 +549,6 @@ static void multifd_send_terminate_threads(Error *err)
return;
}
- trace_multifd_send_terminate_threads(err != NULL);
-
if (err) {
MigrationState *s = migrate_get_current();
migrate_set_error(s, err);
@@ -563,7 +560,19 @@ static void multifd_send_terminate_threads(Error *err)
MIGRATION_STATUS_FAILED);
}
}
+}
+
+static void multifd_send_terminate_threads(void)
+{
+ int i;
+
+ trace_multifd_send_terminate_threads();
+ /*
+ * Tell everyone we're quitting. No xchg() needed here; we simply
+ * always set it.
+ */
+ qatomic_set(&multifd_send_state->exiting, 1);
for (i = 0; i < migrate_multifd_channels(); i++) {
MultiFDSendParams *p = &multifd_send_state->params[i];
@@ -586,7 +595,7 @@ void multifd_save_cleanup(void)
if (!migrate_multifd()) {
return;
}
- multifd_send_terminate_threads(NULL);
+ multifd_send_terminate_threads();
for (i = 0; i < migrate_multifd_channels(); i++) {
MultiFDSendParams *p = &multifd_send_state->params[i];
@@ -778,7 +787,7 @@ out:
if (ret) {
assert(local_err);
trace_multifd_send_error(p->id);
- multifd_send_terminate_threads(local_err);
+ multifd_send_set_error(local_err);
multifd_send_kick_main(p);
error_free(local_err);
}
@@ -814,7 +823,7 @@ static void multifd_tls_outgoing_handshake(QIOTask *task,
trace_multifd_tls_outgoing_handshake_error(ioc, error_get_pretty(err));
- multifd_send_terminate_threads(err);
+ multifd_send_set_error(err);
multifd_send_kick_main(p);
error_free(err);
}
@@ -896,7 +905,7 @@ static void multifd_new_send_channel_async(QIOTask *task,
gpointer opaque)
}
trace_multifd_new_send_channel_async_error(p->id, local_err);
- multifd_send_terminate_threads(local_err);
+ multifd_send_set_error(local_err);
multifd_send_kick_main(p);
object_unref(OBJECT(ioc));
error_free(local_err);
diff --git a/migration/trace-events b/migration/trace-events
index de4a743c8a..298ad2b0dd 100644
--- a/migration/trace-events
+++ b/migration/trace-events
@@ -141,7 +141,7 @@ multifd_send_error(uint8_t id) "channel %u"
multifd_send_sync_main(long packet_num) "packet num %ld"
multifd_send_sync_main_signal(uint8_t id) "channel %u"
multifd_send_sync_main_wait(uint8_t id) "channel %u"
-multifd_send_terminate_threads(bool error) "error %d"
+multifd_send_terminate_threads(void) ""
multifd_send_thread_end(uint8_t id, uint64_t packets, uint64_t normal_pages)
"channel %u packets %" PRIu64 " normal pages %" PRIu64
multifd_send_thread_start(uint8_t id) "%u"
multifd_tls_outgoing_handshake_start(void *ioc, void *tioc, const char
*hostname) "ioc=%p tioc=%p hostname=%s"
--
2.43.0
--
Peter Xu
