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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* 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/'

>                             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.

> +                               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?

Dave

> +    if (ioc == NULL) {
> +        return -1;
> +    }
> +
> +    migration_ioc_register_yank(ioc);
> +    s->postcopy_qemufile_src = qemu_fopen_channel_output(ioc);
> +
> +    trace_postcopy_preempt_new_channel();
> +
> +    return 0;
> +}
> +
> +void *postcopy_preempt_thread(void *opaque)
> +{
> +    MigrationIncomingState *mis = opaque;
> +    int ret;
> +
> +    trace_postcopy_preempt_thread_entry();
> +
> +    rcu_register_thread();
> +
> +    qemu_sem_post(&mis->thread_sync_sem);
> +
> +    /* Sending RAM_SAVE_FLAG_EOS to terminate this thread */
> +    ret = ram_load_postcopy(mis->postcopy_qemufile_dst, RAM_CHANNEL_POSTCOPY);
> +
> +    rcu_unregister_thread();
> +
> +    trace_postcopy_preempt_thread_exit();
> +
> +    return ret == 0 ? NULL : (void *)-1;
> +}
> diff --git a/migration/postcopy-ram.h b/migration/postcopy-ram.h
> index 07684c0e1d..34b1080cde 100644
> --- a/migration/postcopy-ram.h
> +++ b/migration/postcopy-ram.h
> @@ -183,4 +183,14 @@ int postcopy_wake_shared(struct PostCopyFD *pcfd, uint64_t client_addr,
>  int postcopy_request_shared_page(struct PostCopyFD *pcfd, RAMBlock *rb,
>                                   uint64_t client_addr, uint64_t offset);
>  
> +/* Hard-code channels for now for postcopy preemption */
> +enum PostcopyChannels {
> +    RAM_CHANNEL_PRECOPY = 0,
> +    RAM_CHANNEL_POSTCOPY = 1,
> +    RAM_CHANNEL_MAX,
> +};
> +
> +bool postcopy_preempt_new_channel(MigrationIncomingState *mis, QEMUFile *file);
> +int postcopy_preempt_setup(MigrationState *s, Error **errp);
> +
>  #endif
> diff --git a/migration/ram.c b/migration/ram.c
> index f8bc3cd882..36e3da6bb0 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -3643,15 +3643,15 @@ int ram_postcopy_incoming_init(MigrationIncomingState *mis)
>   * rcu_read_lock is taken prior to this being called.
>   *
>   * @f: QEMUFile where to send the data
> + * @channel: the channel to use for loading
>   */
> -int ram_load_postcopy(QEMUFile *f)
> +int ram_load_postcopy(QEMUFile *f, int channel)
>  {
>      int flags = 0, ret = 0;
>      bool place_needed = false;
>      bool matches_target_page_size = false;
>      MigrationIncomingState *mis = migration_incoming_get_current();
> -    /* Currently we only use channel 0.  TODO: use all the channels */
> -    PostcopyTmpPage *tmp_page = &mis->postcopy_tmp_pages[0];
> +    PostcopyTmpPage *tmp_page = &mis->postcopy_tmp_pages[channel];
>  
>      while (!ret && !(flags & RAM_SAVE_FLAG_EOS)) {
>          ram_addr_t addr;
> @@ -3716,10 +3716,10 @@ int ram_load_postcopy(QEMUFile *f)
>              } else if (tmp_page->host_addr !=
>                         host_page_from_ram_block_offset(block, addr)) {
>                  /* not the 1st TP within the HP */
> -                error_report("Non-same host page detected.  Target host page %p, "
> -                             "received host page %p "
> +                error_report("Non-same host page detected on channel %d: "
> +                             "Target host page %p, received host page %p "
>                               "(rb %s offset 0x"RAM_ADDR_FMT" target_pages %d)",
> -                             tmp_page->host_addr,
> +                             channel, tmp_page->host_addr,
>                               host_page_from_ram_block_offset(block, addr),
>                               block->idstr, addr, tmp_page->target_pages);
>                  ret = -EINVAL;
> @@ -4106,7 +4106,12 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
>       */
>      WITH_RCU_READ_LOCK_GUARD() {
>          if (postcopy_running) {
> -            ret = ram_load_postcopy(f);
> +            /*
> +             * Note!  Here RAM_CHANNEL_PRECOPY is the precopy channel of
> +             * postcopy migration, we have another RAM_CHANNEL_POSTCOPY to
> +             * service fast page faults.
> +             */
> +            ret = ram_load_postcopy(f, RAM_CHANNEL_PRECOPY);
>          } else {
>              ret = ram_load_precopy(f);
>          }
> @@ -4268,6 +4273,12 @@ static int ram_resume_prepare(MigrationState *s, void *opaque)
>      return 0;
>  }
>  
> +void postcopy_preempt_shutdown_file(MigrationState *s)
> +{
> +    qemu_put_be64(s->postcopy_qemufile_src, RAM_SAVE_FLAG_EOS);
> +    qemu_fflush(s->postcopy_qemufile_src);
> +}
> +
>  static SaveVMHandlers savevm_ram_handlers = {
>      .save_setup = ram_save_setup,
>      .save_live_iterate = ram_save_iterate,
> diff --git a/migration/ram.h b/migration/ram.h
> index ded0a3a086..5d90945a6e 100644
> --- a/migration/ram.h
> +++ b/migration/ram.h
> @@ -61,7 +61,7 @@ void ram_postcopy_send_discard_bitmap(MigrationState *ms);
>  /* For incoming postcopy discard */
>  int ram_discard_range(const char *block_name, uint64_t start, size_t length);
>  int ram_postcopy_incoming_init(MigrationIncomingState *mis);
> -int ram_load_postcopy(QEMUFile *f);
> +int ram_load_postcopy(QEMUFile *f, int channel);
>  
>  void ram_handle_compressed(void *host, uint8_t ch, uint64_t size);
>  
> @@ -73,6 +73,8 @@ int64_t ramblock_recv_bitmap_send(QEMUFile *file,
>                                    const char *block_name);
>  int ram_dirty_bitmap_reload(MigrationState *s, RAMBlock *rb);
>  bool ramblock_page_is_discarded(RAMBlock *rb, ram_addr_t start);
> +void postcopy_preempt_shutdown_file(MigrationState *s);
> +void *postcopy_preempt_thread(void *opaque);
>  
>  /* ram cache */
>  int colo_init_ram_cache(void);
> diff --git a/migration/socket.c b/migration/socket.c
> index 05705a32d8..a7f345b353 100644
> --- a/migration/socket.c
> +++ b/migration/socket.c
> @@ -26,7 +26,7 @@
>  #include "io/channel-socket.h"
>  #include "io/net-listener.h"
>  #include "trace.h"
> -
> +#include "postcopy-ram.h"
>  
>  struct SocketOutgoingArgs {
>      SocketAddress *saddr;
> @@ -39,6 +39,24 @@ void socket_send_channel_create(QIOTaskFunc f, void *data)
>                                       f, data, NULL, NULL);
>  }
>  
> +QIOChannel *socket_send_channel_create_sync(Error **errp)
> +{
> +    QIOChannelSocket *sioc = qio_channel_socket_new();
> +
> +    if (!outgoing_args.saddr) {
> +        object_unref(OBJECT(sioc));
> +        error_setg(errp, "Initial sock address not set!");
> +        return NULL;
> +    }
> +
> +    if (qio_channel_socket_connect_sync(sioc, outgoing_args.saddr, errp) < 0) {
> +        object_unref(OBJECT(sioc));
> +        return NULL;
> +    }
> +
> +    return QIO_CHANNEL(sioc);
> +}
> +
>  int socket_send_channel_destroy(QIOChannel *send)
>  {
>      /* Remove channel */
> @@ -158,6 +176,8 @@ socket_start_incoming_migration_internal(SocketAddress *saddr,
>  
>      if (migrate_use_multifd()) {
>          num = migrate_multifd_channels();
> +    } else if (migrate_postcopy_preempt()) {
> +        num = RAM_CHANNEL_MAX;
>      }
>  
>      if (qio_net_listener_open_sync(listener, saddr, num, errp) < 0) {
> diff --git a/migration/socket.h b/migration/socket.h
> index 891dbccceb..dc54df4e6c 100644
> --- a/migration/socket.h
> +++ b/migration/socket.h
> @@ -21,6 +21,7 @@
>  #include "io/task.h"
>  
>  void socket_send_channel_create(QIOTaskFunc f, void *data);
> +QIOChannel *socket_send_channel_create_sync(Error **errp);
>  int socket_send_channel_destroy(QIOChannel *send);
>  
>  void socket_start_incoming_migration(const char *str, Error **errp);
> diff --git a/migration/trace-events b/migration/trace-events
> index 1aec580e92..4474a76614 100644
> --- a/migration/trace-events
> +++ b/migration/trace-events
> @@ -278,6 +278,9 @@ postcopy_request_shared_page(const char *sharer, const char *rb, uint64_t rb_off
>  postcopy_request_shared_page_present(const char *sharer, const char *rb, uint64_t rb_offset) "%s already %s offset 0x%"PRIx64
>  postcopy_wake_shared(uint64_t client_addr, const char *rb) "at 0x%"PRIx64" in %s"
>  postcopy_page_req_del(void *addr, int count) "resolved page req %p total %d"
> +postcopy_preempt_new_channel(void) ""
> +postcopy_preempt_thread_entry(void) ""
> +postcopy_preempt_thread_exit(void) ""
>  
>  get_mem_fault_cpu_index(int cpu, uint32_t pid) "cpu: %d, pid: %u"
>  
> -- 
> 2.32.0
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK