Re: [PATCH v2 12/17] migration/multifd: Device state transfer support - send side

[Fabiano Rosas]

"Maciej S. Szmigiero" <mail@maciej.szmigiero.name> writes:

> From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
>
> A new function multifd_queue_device_state() is provided for device to queue
> its state for transmission via a multifd channel.
>
> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
> ---
>  include/migration/misc.h         |  4 ++
>  migration/meson.build            |  1 +
>  migration/multifd-device-state.c | 99 ++++++++++++++++++++++++++++++++
>  migration/multifd-nocomp.c       |  6 +-
>  migration/multifd-qpl.c          |  2 +-
>  migration/multifd-uadk.c         |  2 +-
>  migration/multifd-zlib.c         |  2 +-
>  migration/multifd-zstd.c         |  2 +-
>  migration/multifd.c              | 65 +++++++++++++++------
>  migration/multifd.h              | 29 +++++++++-
>  10 files changed, 184 insertions(+), 28 deletions(-)
>  create mode 100644 migration/multifd-device-state.c
>
> diff --git a/include/migration/misc.h b/include/migration/misc.h
> index bfadc5613bac..7266b1b77d1f 100644
> --- a/include/migration/misc.h
> +++ b/include/migration/misc.h
> @@ -111,4 +111,8 @@ bool migration_in_bg_snapshot(void);
>  /* migration/block-dirty-bitmap.c */
>  void dirty_bitmap_mig_init(void);
>  
> +/* migration/multifd-device-state.c */
> +bool multifd_queue_device_state(char *idstr, uint32_t instance_id,
> +                                char *data, size_t len);
> +
>  #endif
> diff --git a/migration/meson.build b/migration/meson.build
> index 77f3abf08eb1..00853595894f 100644
> --- a/migration/meson.build
> +++ b/migration/meson.build
> @@ -21,6 +21,7 @@ system_ss.add(files(
>    'migration-hmp-cmds.c',
>    'migration.c',
>    'multifd.c',
> +  'multifd-device-state.c',
>    'multifd-nocomp.c',
>    'multifd-zlib.c',
>    'multifd-zero-page.c',
> diff --git a/migration/multifd-device-state.c 
> b/migration/multifd-device-state.c
> new file mode 100644
> index 000000000000..c9b44f0b5ab9
> --- /dev/null
> +++ b/migration/multifd-device-state.c
> @@ -0,0 +1,99 @@
> +/*
> + * Multifd device state migration
> + *
> + * Copyright (C) 2024 Oracle and/or its affiliates.
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2 or later.
> + * See the COPYING file in the top-level directory.
> + */
> +
> +#include "qemu/osdep.h"
> +#include "qemu/lockable.h"
> +#include "migration/misc.h"
> +#include "multifd.h"
> +
> +static QemuMutex queue_job_mutex;
> +
> +static MultiFDSendData *device_state_send;
> +
> +size_t multifd_device_state_payload_size(void)
> +{
> +    return sizeof(MultiFDDeviceState_t);
> +}

This will not be necessary because the payload size is the same as the
data type. We only need it for the special case where the MultiFDPages_t
is smaller than the total ram payload size.

> +
> +void multifd_device_state_save_setup(void)

s/save/send/. The ram ones are only called "save" because they're called
from ram_save_setup(), but we then have the proper nocomp_send_setup
hook.

> +{
> +    qemu_mutex_init(&queue_job_mutex);
> +
> +    device_state_send = multifd_send_data_alloc();
> +}
> +
> +void multifd_device_state_clear(MultiFDDeviceState_t *device_state)
> +{
> +    g_clear_pointer(&device_state->idstr, g_free);
> +    g_clear_pointer(&device_state->buf, g_free);
> +}
> +
> +void multifd_device_state_save_cleanup(void)

s/save/send/

> +{
> +    g_clear_pointer(&device_state_send, multifd_send_data_free);
> +
> +    qemu_mutex_destroy(&queue_job_mutex);
> +}
> +
> +static void multifd_device_state_fill_packet(MultiFDSendParams *p)
> +{
> +    MultiFDDeviceState_t *device_state = &p->data->u.device_state;
> +    MultiFDPacketDeviceState_t *packet = p->packet_device_state;
> +
> +    packet->hdr.flags = cpu_to_be32(p->flags);
> +    strncpy(packet->idstr, device_state->idstr, sizeof(packet->idstr));
> +    packet->instance_id = cpu_to_be32(device_state->instance_id);
> +    packet->next_packet_size = cpu_to_be32(p->next_packet_size);
> +}
> +
> +void multifd_device_state_send_prepare(MultiFDSendParams *p)
> +{
> +    MultiFDDeviceState_t *device_state = &p->data->u.device_state;
> +
> +    assert(multifd_payload_device_state(p->data));
> +
> +    multifd_send_prepare_header_device_state(p);
> +
> +    assert(!(p->flags & MULTIFD_FLAG_SYNC));
> +
> +    p->next_packet_size = device_state->buf_len;
> +    if (p->next_packet_size > 0) {
> +        p->iov[p->iovs_num].iov_base = device_state->buf;
> +        p->iov[p->iovs_num].iov_len = p->next_packet_size;
> +        p->iovs_num++;
> +    }
> +
> +    p->flags |= MULTIFD_FLAG_NOCOMP | MULTIFD_FLAG_DEVICE_STATE;
> +
> +    multifd_device_state_fill_packet(p);
> +}
> +
> +bool multifd_queue_device_state(char *idstr, uint32_t instance_id,
> +                                char *data, size_t len)
> +{
> +    /* Device state submissions can come from multiple threads */
> +    QEMU_LOCK_GUARD(&queue_job_mutex);
> +    MultiFDDeviceState_t *device_state;
> +
> +    assert(multifd_payload_empty(device_state_send));
> +
> +    multifd_set_payload_type(device_state_send, 
> MULTIFD_PAYLOAD_DEVICE_STATE);
> +    device_state = &device_state_send->u.device_state;
> +    device_state->idstr = g_strdup(idstr);
> +    device_state->instance_id = instance_id;
> +    device_state->buf = g_memdup2(data, len);
> +    device_state->buf_len = len;
> +
> +    if (!multifd_send(&device_state_send)) {
> +        multifd_send_data_clear(device_state_send);
> +        return false;
> +    }
> +
> +    return true;
> +}
> diff --git a/migration/multifd-nocomp.c b/migration/multifd-nocomp.c
> index 39eb77c9b3b7..0b7b543f44db 100644
> --- a/migration/multifd-nocomp.c
> +++ b/migration/multifd-nocomp.c
> @@ -116,13 +116,13 @@ static int 
> multifd_nocomp_send_prepare(MultiFDSendParams *p, Error **errp)
>           * Only !zerocopy needs the header in IOV; zerocopy will
>           * send it separately.
>           */
> -        multifd_send_prepare_header(p);
> +        multifd_send_prepare_header_ram(p);
>      }
>  
>      multifd_send_prepare_iovs(p);
>      p->flags |= MULTIFD_FLAG_NOCOMP;
>  
> -    multifd_send_fill_packet(p);
> +    multifd_send_fill_packet_ram(p);
>  
>      if (use_zero_copy_send) {
>          /* Send header first, without zerocopy */
> @@ -371,7 +371,7 @@ bool multifd_send_prepare_common(MultiFDSendParams *p)
>          return false;
>      }
>  
> -    multifd_send_prepare_header(p);
> +    multifd_send_prepare_header_ram(p);
>  
>      return true;
>  }
> diff --git a/migration/multifd-qpl.c b/migration/multifd-qpl.c
> index 75041a4c4dfe..bd6b5b6a3868 100644
> --- a/migration/multifd-qpl.c
> +++ b/migration/multifd-qpl.c
> @@ -490,7 +490,7 @@ static int multifd_qpl_send_prepare(MultiFDSendParams *p, 
> Error **errp)
>  
>  out:
>      p->flags |= MULTIFD_FLAG_QPL;
> -    multifd_send_fill_packet(p);
> +    multifd_send_fill_packet_ram(p);
>      return 0;
>  }
>  
> diff --git a/migration/multifd-uadk.c b/migration/multifd-uadk.c
> index db2549f59bfe..6e2d26010742 100644
> --- a/migration/multifd-uadk.c
> +++ b/migration/multifd-uadk.c
> @@ -198,7 +198,7 @@ static int multifd_uadk_send_prepare(MultiFDSendParams 
> *p, Error **errp)
>      }
>  out:
>      p->flags |= MULTIFD_FLAG_UADK;
> -    multifd_send_fill_packet(p);
> +    multifd_send_fill_packet_ram(p);
>      return 0;
>  }
>  
> diff --git a/migration/multifd-zlib.c b/migration/multifd-zlib.c
> index 6787538762d2..62a1fe59ad3e 100644
> --- a/migration/multifd-zlib.c
> +++ b/migration/multifd-zlib.c
> @@ -156,7 +156,7 @@ static int multifd_zlib_send_prepare(MultiFDSendParams 
> *p, Error **errp)
>  
>  out:
>      p->flags |= MULTIFD_FLAG_ZLIB;
> -    multifd_send_fill_packet(p);
> +    multifd_send_fill_packet_ram(p);
>      return 0;
>  }
>  
> diff --git a/migration/multifd-zstd.c b/migration/multifd-zstd.c
> index 1576b1e2adc6..f98b07e7f9f5 100644
> --- a/migration/multifd-zstd.c
> +++ b/migration/multifd-zstd.c
> @@ -143,7 +143,7 @@ static int multifd_zstd_send_prepare(MultiFDSendParams 
> *p, Error **errp)
>  
>  out:
>      p->flags |= MULTIFD_FLAG_ZSTD;
> -    multifd_send_fill_packet(p);
> +    multifd_send_fill_packet_ram(p);
>      return 0;
>  }
>  
> diff --git a/migration/multifd.c b/migration/multifd.c
> index a74e8a5cc891..bebe5b5a9b9c 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -12,6 +12,7 @@
>  
>  #include "qemu/osdep.h"
>  #include "qemu/cutils.h"
> +#include "qemu/iov.h"
>  #include "qemu/rcu.h"
>  #include "exec/target_page.h"
>  #include "sysemu/sysemu.h"
> @@ -19,6 +20,7 @@
>  #include "qemu/error-report.h"
>  #include "qapi/error.h"
>  #include "file.h"
> +#include "migration/misc.h"
>  #include "migration.h"
>  #include "migration-stats.h"
>  #include "savevm.h"
> @@ -107,7 +109,9 @@ MultiFDSendData *multifd_send_data_alloc(void)
>       * added to the union in the future are larger than
>       * (MultiFDPages_t + flex array).
>       */
> -    max_payload_size = MAX(multifd_ram_payload_size(), 
> sizeof(MultiFDPayload));
> +    max_payload_size = MAX(multifd_ram_payload_size(),
> +                           multifd_device_state_payload_size());

This is not needed, the sizeof(MultiFDPayload) below already has the
same effect.

> +    max_payload_size = MAX(max_payload_size, sizeof(MultiFDPayload));
>  
>      /*
>       * Account for any holes the compiler might insert. We can't pack
> @@ -126,6 +130,9 @@ void multifd_send_data_clear(MultiFDSendData *data)
>      }
>  
>      switch (data->type) {
> +    case MULTIFD_PAYLOAD_DEVICE_STATE:
> +        multifd_device_state_clear(&data->u.device_state);
> +        break;
>      default:
>          /* Nothing to do */
>          break;
> @@ -228,7 +235,7 @@ static int multifd_recv_initial_packet(QIOChannel *c, 
> Error **errp)
>      return msg.id;
>  }
>  
> -void multifd_send_fill_packet(MultiFDSendParams *p)
> +void multifd_send_fill_packet_ram(MultiFDSendParams *p)

Do we need this change if there's no counterpart for device_state? It
might be less confusing to just leave this one as it is.

>  {
>      MultiFDPacket_t *packet = p->packet;
>      uint64_t packet_num;
> @@ -397,20 +404,16 @@ bool multifd_send(MultiFDSendData **send_data)
>  
>          p = &multifd_send_state->params[i];
>          /*
> -         * Lockless read to p->pending_job is safe, because only multifd
> -         * sender thread can clear it.
> +         * Lockless RMW on p->pending_job_preparing is safe, because only 
> multifd
> +         * sender thread can clear it after it had seen p->pending_job being 
> set.
> +         *
> +         * Pairs with qatomic_store_release() in multifd_send_thread().
>           */
> -        if (qatomic_read(&p->pending_job) == false) {
> +        if (qatomic_cmpxchg(&p->pending_job_preparing, false, true) == 
> false) {

What's the motivation for this change? It would be better to have it in
a separate patch with a proper justification.

>              break;
>          }
>      }
>  
> -    /*
> -     * Make sure we read p->pending_job before all the rest.  Pairs with
> -     * qatomic_store_release() in multifd_send_thread().
> -     */
> -    smp_mb_acquire();
> -
>      assert(multifd_payload_empty(p->data));
>  
>      /*
> @@ -534,6 +537,7 @@ static bool 
> multifd_send_cleanup_channel(MultiFDSendParams *p, Error **errp)
>      p->name = NULL;
>      g_clear_pointer(&p->data, multifd_send_data_free);
>      p->packet_len = 0;
> +    g_clear_pointer(&p->packet_device_state, g_free);
>      g_free(p->packet);
>      p->packet = NULL;
>      multifd_send_state->ops->send_cleanup(p, errp);
> @@ -545,6 +549,7 @@ static void multifd_send_cleanup_state(void)
>  {
>      file_cleanup_outgoing_migration();
>      socket_cleanup_outgoing_migration();
> +    multifd_device_state_save_cleanup();
>      qemu_sem_destroy(&multifd_send_state->channels_created);
>      qemu_sem_destroy(&multifd_send_state->channels_ready);
>      g_free(multifd_send_state->params);
> @@ -670,19 +675,29 @@ static void *multifd_send_thread(void *opaque)
>           * qatomic_store_release() in multifd_send().
>           */
>          if (qatomic_load_acquire(&p->pending_job)) {
> +            bool is_device_state = multifd_payload_device_state(p->data);
> +            size_t total_size;
> +
>              p->flags = 0;
>              p->iovs_num = 0;
>              assert(!multifd_payload_empty(p->data));
>  
> -            ret = multifd_send_state->ops->send_prepare(p, &local_err);
> -            if (ret != 0) {
> -                break;
> +            if (is_device_state) {
> +                multifd_device_state_send_prepare(p);
> +            } else {
> +                ret = multifd_send_state->ops->send_prepare(p, &local_err);
> +                if (ret != 0) {
> +                    break;
> +                }
>              }
>  
>              if (migrate_mapped_ram()) {
> +                assert(!is_device_state);
> +
>                  ret = file_write_ramblock_iov(p->c, p->iov, p->iovs_num,
>                                                &p->data->u.ram, &local_err);
>              } else {
> +                total_size = iov_size(p->iov, p->iovs_num);
>                  ret = qio_channel_writev_full_all(p->c, p->iov, p->iovs_num,
>                                                    NULL, 0, p->write_flags,
>                                                    &local_err);
> @@ -692,18 +707,27 @@ static void *multifd_send_thread(void *opaque)
>                  break;
>              }
>  
> -            stat64_add(&mig_stats.multifd_bytes,
> -                       p->next_packet_size + p->packet_len);
> +            if (is_device_state) {
> +                stat64_add(&mig_stats.multifd_bytes, total_size);
> +            } else {
> +                /*
> +                 * Can't just always add total_size since IOVs do not include
> +                 * packet header in the zerocopy RAM case.
> +                 */
> +                stat64_add(&mig_stats.multifd_bytes,
> +                           p->next_packet_size + p->packet_len);

You could set total_size for both branches after send_prepare and use it
here unconditionally.

> +            }
>  
>              p->next_packet_size = 0;
>              multifd_send_data_clear(p->data);
>  
>              /*
>               * Making sure p->data is published before saying "we're
> -             * free".  Pairs with the smp_mb_acquire() in
> +             * free".  Pairs with the qatomic_cmpxchg() in
>               * multifd_send().
>               */
>              qatomic_store_release(&p->pending_job, false);
> +            qatomic_store_release(&p->pending_job_preparing, false);
>          } else {
>              /*
>               * If not a normal job, must be a sync request.  Note that
> @@ -714,7 +738,7 @@ static void *multifd_send_thread(void *opaque)
>  
>              if (use_packets) {
>                  p->flags = MULTIFD_FLAG_SYNC;
> -                multifd_send_fill_packet(p);
> +                multifd_send_fill_packet_ram(p);
>                  ret = qio_channel_write_all(p->c, (void *)p->packet,
>                                              p->packet_len, &local_err);
>                  if (ret != 0) {
> @@ -910,6 +934,9 @@ bool multifd_send_setup(void)
>              p->packet_len = sizeof(MultiFDPacket_t)
>                            + sizeof(uint64_t) * page_count;
>              p->packet = g_malloc0(p->packet_len);
> +            p->packet_device_state = 
> g_malloc0(sizeof(*p->packet_device_state));
> +            p->packet_device_state->hdr.magic = cpu_to_be32(MULTIFD_MAGIC);
> +            p->packet_device_state->hdr.version = 
> cpu_to_be32(MULTIFD_VERSION);
>          }
>          p->name = g_strdup_printf("mig/src/send_%d", i);
>          p->write_flags = 0;
> @@ -944,6 +971,8 @@ bool multifd_send_setup(void)
>          }
>      }
>  
> +    multifd_device_state_save_setup();
> +
>      return true;
>  
>  err:
> diff --git a/migration/multifd.h b/migration/multifd.h
> index a0853622153e..c15c83104c8b 100644
> --- a/migration/multifd.h
> +++ b/migration/multifd.h
> @@ -120,10 +120,12 @@ typedef struct {
>  typedef enum {
>      MULTIFD_PAYLOAD_NONE,
>      MULTIFD_PAYLOAD_RAM,
> +    MULTIFD_PAYLOAD_DEVICE_STATE,
>  } MultiFDPayloadType;
>  
>  typedef union MultiFDPayload {
>      MultiFDPages_t ram;
> +    MultiFDDeviceState_t device_state;
>  } MultiFDPayload;
>  
>  struct MultiFDSendData {
> @@ -136,6 +138,11 @@ static inline bool multifd_payload_empty(MultiFDSendData 
> *data)
>      return data->type == MULTIFD_PAYLOAD_NONE;
>  }
>  
> +static inline bool multifd_payload_device_state(MultiFDSendData *data)
> +{
> +    return data->type == MULTIFD_PAYLOAD_DEVICE_STATE;
> +}
> +
>  static inline void multifd_set_payload_type(MultiFDSendData *data,
>                                              MultiFDPayloadType type)
>  {
> @@ -182,13 +189,15 @@ typedef struct {
>       * cleared by the multifd sender threads.
>       */
>      bool pending_job;
> +    bool pending_job_preparing;
>      bool pending_sync;
>      MultiFDSendData *data;
>  
>      /* thread local variables. No locking required */
>  
> -    /* pointer to the packet */
> +    /* pointers to the possible packet types */
>      MultiFDPacket_t *packet;
> +    MultiFDPacketDeviceState_t *packet_device_state;
>      /* size of the next packet that contains pages */
>      uint32_t next_packet_size;
>      /* packets sent through this channel */
> @@ -276,18 +285,25 @@ typedef struct {
>  } MultiFDMethods;
>  
>  void multifd_register_ops(int method, MultiFDMethods *ops);
> -void multifd_send_fill_packet(MultiFDSendParams *p);
> +void multifd_send_fill_packet_ram(MultiFDSendParams *p);
>  bool multifd_send_prepare_common(MultiFDSendParams *p);
>  void multifd_send_zero_page_detect(MultiFDSendParams *p);
>  void multifd_recv_zero_page_process(MultiFDRecvParams *p);
>  
> -static inline void multifd_send_prepare_header(MultiFDSendParams *p)
> +static inline void multifd_send_prepare_header_ram(MultiFDSendParams *p)

This could instead go to multifd-nocomp.c and become multifd_ram_prepare_header.

>  {
>      p->iov[0].iov_len = p->packet_len;
>      p->iov[0].iov_base = p->packet;
>      p->iovs_num++;
>  }
>  
> +static inline void 
> multifd_send_prepare_header_device_state(MultiFDSendParams *p)

Seem like this could also move to multifd-device-state.c and drop the
"send" part.

> +{
> +    p->iov[0].iov_len = sizeof(*p->packet_device_state);
> +    p->iov[0].iov_base = p->packet_device_state;
> +    p->iovs_num++;
> +}
> +
>  void multifd_channel_connect(MultiFDSendParams *p, QIOChannel *ioc);
>  bool multifd_send(MultiFDSendData **send_data);
>  MultiFDSendData *multifd_send_data_alloc(void);
> @@ -310,4 +326,11 @@ int multifd_ram_flush_and_sync(void);
>  size_t multifd_ram_payload_size(void);
>  void multifd_ram_fill_packet(MultiFDSendParams *p);
>  int multifd_ram_unfill_packet(MultiFDRecvParams *p, Error **errp);
> +
> +size_t multifd_device_state_payload_size(void);
> +void multifd_device_state_save_setup(void);
> +void multifd_device_state_clear(MultiFDDeviceState_t *device_state);
> +void multifd_device_state_save_cleanup(void);
> +void multifd_device_state_send_prepare(MultiFDSendParams *p);
> +
>  #endif