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

[Fabiano Rosas]

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.

> @@ -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.

>          qemu_sem_post(&p->sem);
>          if (p->c) {
>              qio_channel_shutdown(p->c, QIO_CHANNEL_SHUTDOWN_BOTH, NULL);
>          }
> -        qemu_mutex_unlock(&p->mutex);
>      }
>  }
>  
> @@ -615,16 +614,13 @@ int multifd_send_sync_main(void)
>      for (i = 0; i < migrate_multifd_channels(); i++) {
>          MultiFDSendParams *p = &multifd_send_state->params[i];
>  
> -        trace_multifd_send_sync_main_signal(p->id);
> -
> -        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;
>          }
>  
> +        trace_multifd_send_sync_main_signal(p->id);
> +
> +        qemu_mutex_lock(&p->mutex);
>          p->packet_num = multifd_send_state->packet_num++;
>          p->flags |= MULTIFD_FLAG_SYNC;
>          p->pending_job++;
> @@ -634,6 +630,10 @@ int multifd_send_sync_main(void)
>      for (i = 0; i < migrate_multifd_channels(); i++) {
>          MultiFDSendParams *p = &multifd_send_state->params[i];
>  
> +        if (multifd_send_should_exit()) {
> +            return -1;
> +        }
> +
>          qemu_sem_wait(&multifd_send_state->channels_ready);
>          trace_multifd_send_sync_main_wait(p->id);
>          qemu_sem_wait(&p->sem_sync);
> @@ -671,7 +671,7 @@ static void *multifd_send_thread(void *opaque)
>          qemu_sem_post(&multifd_send_state->channels_ready);
>          qemu_sem_wait(&p->sem);
>  
> -        if (qatomic_read(&multifd_send_state->exiting)) {
> +        if (multifd_send_should_exit()) {
>              break;
>          }
>          qemu_mutex_lock(&p->mutex);
> @@ -786,12 +786,7 @@ static void multifd_tls_outgoing_handshake(QIOTask *task,
>  
>      trace_multifd_tls_outgoing_handshake_error(ioc, error_get_pretty(err));
>  
> -    migrate_set_error(migrate_get_current(), err);
> -    /*
> -     * Error happen, mark multifd_send_thread status as 'quit' although it
> -     * is not created, and then tell who pay attention to me.
> -     */
> -    p->quit = true;
> +    multifd_send_terminate_threads(err);
>      multifd_send_kick_main(p);
>      error_free(err);
>  }
> @@ -857,22 +852,6 @@ static bool multifd_channel_connect(MultiFDSendParams *p,
>      return true;
>  }
>  
> -static void multifd_new_send_channel_cleanup(MultiFDSendParams *p,
> -                                             QIOChannel *ioc, Error *err)
> -{
> -     migrate_set_error(migrate_get_current(), err);
> -     /* Error happen, we need to tell who pay attention to me */
> -     multifd_send_kick_main(p);
> -     /*
> -      * Although multifd_send_thread is not created, but main migration
> -      * thread need to judge whether it is running, so we need to mark
> -      * its status.
> -      */
> -     p->quit = true;
> -     object_unref(OBJECT(ioc));
> -     error_free(err);
> -}
> -
>  static void multifd_new_send_channel_async(QIOTask *task, gpointer opaque)
>  {
>      MultiFDSendParams *p = opaque;
> @@ -889,7 +868,10 @@ static void multifd_new_send_channel_async(QIOTask 
> *task, gpointer opaque)
>      }
>  
>      trace_multifd_new_send_channel_async_error(p->id, local_err);
> -    multifd_new_send_channel_cleanup(p, ioc, local_err);
> +    multifd_send_terminate_threads(local_err);
> +    multifd_send_kick_main(p);
> +    object_unref(OBJECT(ioc));
> +    error_free(local_err);
>  }
>  
>  static void multifd_new_send_channel_create(gpointer opaque)
> @@ -921,7 +903,6 @@ int multifd_save_setup(Error **errp)
>          qemu_mutex_init(&p->mutex);
>          qemu_sem_init(&p->sem, 0);
>          qemu_sem_init(&p->sem_sync, 0);
> -        p->quit = false;
>          p->pending_job = 0;
>          p->id = i;
>          p->pages = multifd_pages_init(page_count);