On Tue, Feb 20, 2024 at 07:41:23PM -0300, Fabiano Rosas wrote:
> Currently multifd does not need to have knowledge of pages on the
> receiving side because all the information needed is within the
> packets that come in the stream.
>
> We're about to add support to fixed-ram migration, which cannot use
> packets because it expects the ramblock section in the migration file
> to contain only the guest pages data.
>
> Add a data structure to transfer pages between the ram migration code
> and the multifd receiving threads.
>
> We don't want to reuse MultiFDPages_t for two reasons:
>
> a) multifd threads don't really need to know about the data they're
> receiving.
>
> b) the receiving side has to be stopped to load the pages, which means
> we can experiment with larger granularities than page size when
> transferring data.
>
> Signed-off-by: Fabiano Rosas <faro...@suse.de>
> ---
> @Peter: a 'quit' flag cannot be used instead of pending_job. The
> receiving thread needs know there's no more data coming. If the
> migration thread sets a 'quit' flag, the multifd thread would see the
> flag right away and exit.
Hmm.. isn't this exactly what we want? I'll comment for this inline below.
> The only way is to clear pending_job on the
> thread and spin once more.
> ---
> migration/file.c | 1 +
> migration/multifd.c | 122 +++++++++++++++++++++++++++++++++++++++++---
> migration/multifd.h | 15 ++++++
> 3 files changed, 131 insertions(+), 7 deletions(-)
>
> diff --git a/migration/file.c b/migration/file.c
> index 5d4975f43e..22d052a71f 100644
> --- a/migration/file.c
> +++ b/migration/file.c
> @@ -6,6 +6,7 @@
> */
>
> #include "qemu/osdep.h"
> +#include "exec/ramblock.h"
> #include "qemu/cutils.h"
> #include "qapi/error.h"
> #include "channel.h"
> diff --git a/migration/multifd.c b/migration/multifd.c
> index 0a5279314d..45a0c7aaa8 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -81,9 +81,15 @@ struct {
>
> struct {
> MultiFDRecvParams *params;
> + MultiFDRecvData *data;
> /* number of created threads */
> int count;
> - /* syncs main thread and channels */
> + /*
> + * For sockets: this is posted once for each MULTIFD_FLAG_SYNC flag.
> + *
> + * For files: this is only posted at the end of the file load to mark
> + * completion of the load process.
> + */
> QemuSemaphore sem_sync;
> /* global number of generated multifd packets */
> uint64_t packet_num;
> @@ -1110,6 +1116,53 @@ bool multifd_send_setup(void)
> return true;
> }
>
> +bool multifd_recv(void)
> +{
> + int i;
> + static int next_recv_channel;
> + MultiFDRecvParams *p = NULL;
> + MultiFDRecvData *data = multifd_recv_state->data;
[1]
> +
> + /*
> + * next_channel can remain from a previous migration that was
> + * using more channels, so ensure it doesn't overflow if the
> + * limit is lower now.
> + */
> + next_recv_channel %= migrate_multifd_channels();
> + for (i = next_recv_channel;; i = (i + 1) % migrate_multifd_channels()) {
> + if (multifd_recv_should_exit()) {
> + return false;
> + }
> +
> + p = &multifd_recv_state->params[i];
> +
> + /*
> + * Safe to read atomically without a lock because the flag is
> + * only set by this function below. Reading an old value of
> + * true is not an issue because it would only send us looking
> + * for the next idle channel.
> + */
> + if (qatomic_read(&p->pending_job) == false) {
> + next_recv_channel = (i + 1) % migrate_multifd_channels();
> + break;
> + }
> + }
IIUC you'll need an smp_mb_acquire() here. The ordering of "reading
pending_job" and below must be guaranteed, similar to the sender side.
> +
> + assert(!p->data->size);
> + multifd_recv_state->data = p->data;
[2]
> + p->data = data;
> +
> + qatomic_set(&p->pending_job, true);
Then here:
qatomic_store_release(&p->pending_job, true);
Please consider add comment above all acquire/releases pairs like sender
too.
> + qemu_sem_post(&p->sem);
> +
> + return true;
> +}
> +
> +MultiFDRecvData *multifd_get_recv_data(void)
> +{
> + return multifd_recv_state->data;
> +}
Can also use it above [1].
I'm thinking maybe we can do something like:
#define MULTIFD_RECV_DATA_GLOBAL (multifd_recv_state->data)
Then we can also use it at [2], and replace multifd_get_recv_data()?
> +
> static void multifd_recv_terminate_threads(Error *err)
> {
> int i;
> @@ -1134,11 +1187,26 @@ static void multifd_recv_terminate_threads(Error *err)
> MultiFDRecvParams *p = &multifd_recv_state->params[i];
>
> /*
> - * multifd_recv_thread may hung at MULTIFD_FLAG_SYNC handle code,
> - * however try to wakeup it without harm in cleanup phase.
> + * The migration thread and channels interact differently
> + * depending on the presence of packets.
> */
> if (multifd_use_packets()) {
> + /*
> + * The channel receives as long as there are packets. When
> + * packets end (i.e. MULTIFD_FLAG_SYNC is reached), the
> + * channel waits for the migration thread to sync. If the
> + * sync never happens, do it here.
> + */
> qemu_sem_post(&p->sem_sync);
> + } else {
> + /*
> + * The channel waits for the migration thread to give it
> + * work. When the migration thread runs out of work, it
> + * releases the channel and waits for any pending work to
> + * finish. If we reach here (e.g. due to error) before the
> + * work runs out, release the channel.
> + */
> + qemu_sem_post(&p->sem);
> }
>
> /*
> @@ -1167,6 +1235,7 @@ static void
> multifd_recv_cleanup_channel(MultiFDRecvParams *p)
> p->c = NULL;
> qemu_mutex_destroy(&p->mutex);
> qemu_sem_destroy(&p->sem_sync);
> + qemu_sem_destroy(&p->sem);
> g_free(p->name);
> p->name = NULL;
> p->packet_len = 0;
> @@ -1184,6 +1253,8 @@ static void multifd_recv_cleanup_state(void)
> qemu_sem_destroy(&multifd_recv_state->sem_sync);
> g_free(multifd_recv_state->params);
> multifd_recv_state->params = NULL;
> + g_free(multifd_recv_state->data);
> + multifd_recv_state->data = NULL;
> g_free(multifd_recv_state);
> multifd_recv_state = NULL;
> }
> @@ -1251,11 +1322,11 @@ static void *multifd_recv_thread(void *opaque)
> bool has_data = false;
> p->normal_num = 0;
>
> - if (multifd_recv_should_exit()) {
> - break;
> - }
> -
> if (use_packets) {
> + if (multifd_recv_should_exit()) {
> + break;
> + }
> +
> ret = qio_channel_read_all_eof(p->c, (void *)p->packet,
> p->packet_len, &local_err);
> if (ret == 0 || ret == -1) { /* 0: EOF -1: Error */
> @@ -1274,6 +1345,26 @@ static void *multifd_recv_thread(void *opaque)
> p->flags &= ~MULTIFD_FLAG_SYNC;
> has_data = !!p->normal_num;
> qemu_mutex_unlock(&p->mutex);
> + } else {
> + /*
> + * No packets, so we need to wait for the vmstate code to
> + * give us work.
> + */
> + qemu_sem_wait(&p->sem);
> +
> + if (multifd_recv_should_exit()) {
> + break;
> + }
> +
> + /*
> + * Migration thread did not send work, break and signal
> + * sem_sync so it knows we're not lagging behind.
> + */
> + if (!qatomic_read(&p->pending_job)) {
> + break;
> + }
In reality, this _must_ be true when reaching here, right? Since AFAIU
recv side p->sem is posted only in two conditions:
1) when there is work (pending_job==true)
2) when terminating threads (multifd_recv_should_exit==true)
Then if 2) is checked above, I assume 1) must be the case here?
> +
> + has_data = !!p->data->size;
> }
>
> if (has_data) {
> @@ -1288,9 +1379,17 @@ static void *multifd_recv_thread(void *opaque)
> qemu_sem_post(&multifd_recv_state->sem_sync);
> qemu_sem_wait(&p->sem_sync);
> }
> + } else {
> + p->total_normal_pages += p->data->size / qemu_target_page_size();
> + p->data->size = 0;
> + qatomic_set(&p->pending_job, false);
I think it needs to be:
qatomic_store_release(&p->pending_job, false);
?
So as to guarantee when the other side sees pending_job==false, size must
already have been reset.
> }
> }
>
> + if (!use_packets) {
> + qemu_sem_post(&p->sem_sync);
> + }
> +
> if (local_err) {
> multifd_recv_terminate_threads(local_err);
> error_free(local_err);
> @@ -1320,6 +1419,10 @@ int multifd_recv_setup(Error **errp)
> thread_count = migrate_multifd_channels();
> multifd_recv_state = g_malloc0(sizeof(*multifd_recv_state));
> multifd_recv_state->params = g_new0(MultiFDRecvParams, thread_count);
> +
> + multifd_recv_state->data = g_new0(MultiFDRecvData, 1);
> + multifd_recv_state->data->size = 0;
> +
> qatomic_set(&multifd_recv_state->count, 0);
> qatomic_set(&multifd_recv_state->exiting, 0);
> qemu_sem_init(&multifd_recv_state->sem_sync, 0);
> @@ -1330,8 +1433,13 @@ int multifd_recv_setup(Error **errp)
>
> qemu_mutex_init(&p->mutex);
> qemu_sem_init(&p->sem_sync, 0);
> + qemu_sem_init(&p->sem, 0);
> + p->pending_job = false;
> p->id = i;
>
> + p->data = g_new0(MultiFDRecvData, 1);
> + p->data->size = 0;
> +
> if (use_packets) {
> p->packet_len = sizeof(MultiFDPacket_t)
> + sizeof(uint64_t) * page_count;
> diff --git a/migration/multifd.h b/migration/multifd.h
> index 9a6a7a72df..19188815a3 100644
> --- a/migration/multifd.h
> +++ b/migration/multifd.h
> @@ -13,6 +13,8 @@
> #ifndef QEMU_MIGRATION_MULTIFD_H
> #define QEMU_MIGRATION_MULTIFD_H
>
> +typedef struct MultiFDRecvData MultiFDRecvData;
> +
> bool multifd_send_setup(void);
> void multifd_send_shutdown(void);
> int multifd_recv_setup(Error **errp);
> @@ -23,6 +25,8 @@ void multifd_recv_new_channel(QIOChannel *ioc, Error
> **errp);
> void multifd_recv_sync_main(void);
> int multifd_send_sync_main(void);
> bool multifd_queue_page(RAMBlock *block, ram_addr_t offset);
> +bool multifd_recv(void);
> +MultiFDRecvData *multifd_get_recv_data(void);
>
> /* Multifd Compression flags */
> #define MULTIFD_FLAG_SYNC (1 << 0)
> @@ -63,6 +67,13 @@ typedef struct {
> RAMBlock *block;
> } MultiFDPages_t;
>
> +struct MultiFDRecvData {
> + void *opaque;
> + size_t size;
> + /* for preadv */
> + off_t file_offset;
> +};
> +
> typedef struct {
> /* Fields are only written at creating/deletion time */
> /* No lock required for them, they are read only */
> @@ -154,6 +165,8 @@ typedef struct {
>
> /* syncs main thread and channels */
> QemuSemaphore sem_sync;
> + /* sem where to wait for more work */
> + QemuSemaphore sem;
>
> /* this mutex protects the following parameters */
> QemuMutex mutex;
> @@ -163,6 +176,8 @@ typedef struct {
> uint32_t flags;
> /* global number of generated multifd packets */
> uint64_t packet_num;
> + int pending_job;
> + MultiFDRecvData *data;
>
> /* thread local variables. No locking required */
>
> --
> 2.35.3
>
--
Peter Xu
