Re: [PATCH 17/20] migration: Postcopy preemption preparation on channel creation

[Peter Xu]

On Mon, Feb 21, 2022 at 06:39:36PM +0000, Dr. David Alan Gilbert wrote:
> * Peter Xu (peterx@redhat.com) wrote:
> > Create a new socket for postcopy to be prepared to send postcopy requested
> > pages via this specific channel, so as to not get blocked by precopy pages.
> > 
> > A new thread is also created on dest qemu to receive data from this new 
> > channel
> > based on the ram_load_postcopy() routine.
> > 
> > The ram_load_postcopy(POSTCOPY) branch and the thread has not started to
> > function, and that'll be done in follow up patches.
> > 
> > Cleanup the new sockets on both src/dst QEMUs, meanwhile look after the new
> > thread too to make sure it'll be recycled properly.
> > 
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> >  migration/migration.c    | 62 ++++++++++++++++++++++++----
> >  migration/migration.h    |  8 ++++
> >  migration/postcopy-ram.c | 88 ++++++++++++++++++++++++++++++++++++++--
> >  migration/postcopy-ram.h | 10 +++++
> >  migration/ram.c          | 25 ++++++++----
> >  migration/ram.h          |  4 +-
> >  migration/socket.c       | 22 +++++++++-
> >  migration/socket.h       |  1 +
> >  migration/trace-events   |  3 ++
> >  9 files changed, 203 insertions(+), 20 deletions(-)
> > 
> > diff --git a/migration/migration.c b/migration/migration.c
> > index 4c22bad304..3d7f897b72 100644
> > --- a/migration/migration.c
> > +++ b/migration/migration.c
> > @@ -321,6 +321,12 @@ void migration_incoming_state_destroy(void)
> >          mis->page_requested = NULL;
> >      }
> >  
> > +    if (mis->postcopy_qemufile_dst) {
> > +        
> > migration_ioc_unregister_yank_from_file(mis->postcopy_qemufile_dst);
> > +        qemu_fclose(mis->postcopy_qemufile_dst);
> > +        mis->postcopy_qemufile_dst = NULL;
> > +    }
> > +
> >      yank_unregister_instance(MIGRATION_YANK_INSTANCE);
> >  }
> >  
> > @@ -714,15 +720,21 @@ void migration_fd_process_incoming(QEMUFile *f, Error 
> > **errp)
> >      migration_incoming_process();
> >  }
> >  
> > +static bool migration_needs_multiple_sockets(void)
> > +{
> > +    return migrate_use_multifd() || migrate_postcopy_preempt();
> > +}
> > +
> >  void migration_ioc_process_incoming(QIOChannel *ioc, Error **errp)
> >  {
> >      MigrationIncomingState *mis = migration_incoming_get_current();
> >      Error *local_err = NULL;
> >      bool start_migration;
> > +    QEMUFile *f;
> >  
> >      if (!mis->from_src_file) {
> >          /* The first connection (multifd may have multiple) */
> > -        QEMUFile *f = qemu_fopen_channel_input(ioc);
> > +        f = qemu_fopen_channel_input(ioc);
> >  
> >          if (!migration_incoming_setup(f, errp)) {
> >              return;
> > @@ -730,13 +742,18 @@ void migration_ioc_process_incoming(QIOChannel *ioc, 
> > Error **errp)
> >  
> >          /*
> >           * Common migration only needs one channel, so we can start
> > -         * right now.  Multifd needs more than one channel, we wait.
> > +         * right now.  Some features need more than one channel, we wait.
> >           */
> > -        start_migration = !migrate_use_multifd();
> > +        start_migration = !migration_needs_multiple_sockets();
> >      } else {
> >          /* Multiple connections */
> > -        assert(migrate_use_multifd());
> > -        start_migration = multifd_recv_new_channel(ioc, &local_err);
> > +        assert(migration_needs_multiple_sockets());
> > +        if (migrate_use_multifd()) {
> > +            start_migration = multifd_recv_new_channel(ioc, &local_err);
> > +        } else if (migrate_postcopy_preempt()) {
> > +            f = qemu_fopen_channel_input(ioc);
> > +            start_migration = postcopy_preempt_new_channel(mis, f);
> > +        }
> >          if (local_err) {
> >              error_propagate(errp, local_err);
> >              return;
> > @@ -761,11 +778,20 @@ void migration_ioc_process_incoming(QIOChannel *ioc, 
> > Error **errp)
> >  bool migration_has_all_channels(void)
> >  {
> >      MigrationIncomingState *mis = migration_incoming_get_current();
> > -    bool all_channels;
> >  
> > -    all_channels = multifd_recv_all_channels_created();
> > +    if (!mis->from_src_file) {
> > +        return false;
> > +    }
> > +
> > +    if (migrate_use_multifd()) {
> > +        return multifd_recv_all_channels_created();
> > +    }
> > +
> > +    if (migrate_postcopy_preempt()) {
> > +        return mis->postcopy_qemufile_dst != NULL;
> > +    }
> >  
> > -    return all_channels && mis->from_src_file != NULL;
> > +    return true;
> >  }
> >  
> >  /*
> > @@ -1858,6 +1884,12 @@ static void migrate_fd_cleanup(MigrationState *s)
> >          qemu_fclose(tmp);
> >      }
> >  
> > +    if (s->postcopy_qemufile_src) {
> > +        migration_ioc_unregister_yank_from_file(s->postcopy_qemufile_src);
> > +        qemu_fclose(s->postcopy_qemufile_src);
> > +        s->postcopy_qemufile_src = NULL;
> > +    }
> > +
> >      assert(!migration_is_active(s));
> >  
> >      if (s->state == MIGRATION_STATUS_CANCELLING) {
> > @@ -3233,6 +3265,11 @@ static void migration_completion(MigrationState *s)
> >          qemu_savevm_state_complete_postcopy(s->to_dst_file);
> >          qemu_mutex_unlock_iothread();
> >  
> > +        /* Shutdown the postcopy fast path thread */
> > +        if (migrate_postcopy_preempt()) {
> > +            postcopy_preempt_shutdown_file(s);
> > +        }
> > +
> >          trace_migration_completion_postcopy_end_after_complete();
> >      } else {
> >          goto fail;
> > @@ -4120,6 +4157,15 @@ void migrate_fd_connect(MigrationState *s, Error 
> > *error_in)
> >          }
> >      }
> >  
> > +    /* This needs to be done before resuming a postcopy */
> > +    if (postcopy_preempt_setup(s, &local_err)) {
> > +        error_report_err(local_err);
> > +        migrate_set_state(&s->state, MIGRATION_STATUS_SETUP,
> > +                          MIGRATION_STATUS_FAILED);
> > +        migrate_fd_cleanup(s);
> > +        return;
> > +    }
> > +
> >      if (resume) {
> >          /* Wakeup the main migration thread to do the recovery */
> >          migrate_set_state(&s->state, MIGRATION_STATUS_POSTCOPY_PAUSED,
> > diff --git a/migration/migration.h b/migration/migration.h
> > index af4bcb19c2..caa910d956 100644
> > --- a/migration/migration.h
> > +++ b/migration/migration.h
> > @@ -23,6 +23,7 @@
> >  #include "io/channel-buffer.h"
> >  #include "net/announce.h"
> >  #include "qom/object.h"
> > +#include "postcopy-ram.h"
> >  
> >  struct PostcopyBlocktimeContext;
> >  
> > @@ -112,6 +113,11 @@ struct MigrationIncomingState {
> >       * enabled.
> >       */
> >      unsigned int postcopy_channels;
> > +    /* QEMUFile for postcopy only; it'll be handled by a separate thread */
> > +    QEMUFile *postcopy_qemufile_dst;
> > +    /* Postcopy priority thread is used to receive postcopy requested 
> > pages */
> > +    QemuThread postcopy_prio_thread;
> > +    bool postcopy_prio_thread_created;
> >      /*
> >       * An array of temp host huge pages to be used, one for each postcopy
> >       * channel.
> > @@ -192,6 +198,8 @@ struct MigrationState {
> >      QEMUBH *cleanup_bh;
> >      /* Protected by qemu_file_lock */
> >      QEMUFile *to_dst_file;
> > +    /* Postcopy specific transfer channel */
> > +    QEMUFile *postcopy_qemufile_src;
> >      QIOChannelBuffer *bioc;
> >      /*
> >       * Protects to_dst_file/from_dst_file pointers.  We need to make sure 
> > we
> > diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> > index 738cc55fa6..30eddaacd1 100644
> > --- a/migration/postcopy-ram.c
> > +++ b/migration/postcopy-ram.c
> > @@ -32,6 +32,9 @@
> >  #include "trace.h"
> >  #include "hw/boards.h"
> >  #include "exec/ramblock.h"
> > +#include "socket.h"
> > +#include "qemu-file-channel.h"
> > +#include "yank_functions.h"
> >  
> >  /* Arbitrary limit on size of each discard command,
> >   * keeps them around ~200 bytes
> > @@ -566,6 +569,11 @@ int 
> > postcopy_ram_incoming_cleanup(MigrationIncomingState *mis)
> >  {
> >      trace_postcopy_ram_incoming_cleanup_entry();
> >  
> > +    if (mis->postcopy_prio_thread_created) {
> > +        qemu_thread_join(&mis->postcopy_prio_thread);
> > +        mis->postcopy_prio_thread_created = false;
> > +    }
> > +
> >      if (mis->have_fault_thread) {
> >          Error *local_err = NULL;
> >  
> > @@ -1101,8 +1109,13 @@ static int 
> > postcopy_temp_pages_setup(MigrationIncomingState *mis)
> >      int err, i, channels;
> >      void *temp_page;
> >  
> > -    /* TODO: will be boosted when enable postcopy preemption */
> > -    mis->postcopy_channels = 1;
> > +    if (migrate_postcopy_preempt()) {
> > +        /* If preemption enabled, need extra channel for urgent requests */
> > +        mis->postcopy_channels = RAM_CHANNEL_MAX;
> > +    } else {
> > +        /* Both precopy/postcopy on the same channel */
> > +        mis->postcopy_channels = 1;
> > +    }
> >  
> >      channels = mis->postcopy_channels;
> >      mis->postcopy_tmp_pages = g_malloc0_n(sizeof(PostcopyTmpPage), 
> > channels);
> > @@ -1169,7 +1182,7 @@ int 
> > postcopy_ram_incoming_setup(MigrationIncomingState *mis)
> >          return -1;
> >      }
> >  
> > -    postcopy_thread_create(mis, &mis->fault_thread, "postcopy/fault",
> > +    postcopy_thread_create(mis, &mis->fault_thread, "qemu/fault-default",
> 
> That name is still too long; I'd lose the 'qemu/'

Sure.

> 
> >                             postcopy_ram_fault_thread, 
> > QEMU_THREAD_JOINABLE);
> >      mis->have_fault_thread = true;
> >  
> > @@ -1184,6 +1197,16 @@ int 
> > postcopy_ram_incoming_setup(MigrationIncomingState *mis)
> >          return -1;
> >      }
> >  
> > +    if (migrate_postcopy_preempt()) {
> > +        /*
> > +         * This thread needs to be created after the temp pages because 
> > it'll fetch
> > +         * RAM_CHANNEL_POSTCOPY PostcopyTmpPage immediately.
> > +         */
> > +        postcopy_thread_create(mis, &mis->postcopy_prio_thread, 
> > "qemu/fault-fast",
> 
> and that one.

OK.

> 
> > +                               postcopy_preempt_thread, 
> > QEMU_THREAD_JOINABLE);
> > +        mis->postcopy_prio_thread_created = true;
> > +    }
> > +
> >      trace_postcopy_ram_enable_notify();
> >  
> >      return 0;
> > @@ -1513,3 +1536,62 @@ void postcopy_unregister_shared_ufd(struct 
> > PostCopyFD *pcfd)
> >          }
> >      }
> >  }
> > +
> > +bool postcopy_preempt_new_channel(MigrationIncomingState *mis, QEMUFile 
> > *file)
> > +{
> > +    mis->postcopy_qemufile_dst = file;
> > +
> > +    trace_postcopy_preempt_new_channel();
> > +
> > +    /* Start the migration immediately */
> > +    return true;
> > +}
> > +
> > +int postcopy_preempt_setup(MigrationState *s, Error **errp)
> > +{
> > +    QIOChannel *ioc;
> > +
> > +    if (!migrate_postcopy_preempt()) {
> > +        return 0;
> > +    }
> > +
> > +    if (!migrate_multi_channels_is_allowed()) {
> > +        error_setg(errp, "Postcopy preempt is not supported as current "
> > +                   "migration stream does not support multi-channels.");
> > +        return -1;
> > +    }
> > +
> > +    ioc = socket_send_channel_create_sync(errp);
> 
> How do we get away with doing this sync here, but have to use async for
> multifd?

Ah right..  I think both will work?  But async (as what multifd is doing)
is definitely more elegant because synced version can block the main thread
for unexpected times.

I'll add a new patch to support async channel creations (assuming that's
better than squashing the change).

-- 
Peter Xu