Re: [PATCH 03/14] migration/multifd: Drop MultiFDSendParams.quit, cleanup error paths

[Peter Xu <peterx@redhat.com>]

On Wed, Jan 31, 2024 at 12:05:08PM -0300, Fabiano Rosas wrote:
> peterx@redhat.com writes:
> 
> > From: Peter Xu <peterx@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 <peterx@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 <peterx@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 <farosas@suse.de>
Signed-off-by: Peter Xu <peterx@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