On Tue, 2022-08-02 at 08:39 +0200, Juan Quintela wrote:
> Use of flags with respect to locking was incensistant. For the
> sending side:
> - it was set to 0 with mutex held on the multifd channel.
> - MULTIFD_FLAG_SYNC was set with mutex held on the migration thread.
> - Everything else was done without the mutex held on the multifd channel.
>
> On the reception side, it is not used on the migration thread, only on
> the multifd channels threads.
>
> So we move it to the multifd channels thread only variables, and we
> introduce a new bool sync_needed on the send side to pass that information.
>
> Signed-off-by: Juan Quintela <quint...@redhat.com>
> ---
> migration/multifd.h | 10 ++++++----
> migration/multifd.c | 23 +++++++++++++----------
> 2 files changed, 19 insertions(+), 14 deletions(-)
>
> diff --git a/migration/multifd.h b/migration/multifd.h
> index 36f899c56f..a67cefc0a2 100644
> --- a/migration/multifd.h
> +++ b/migration/multifd.h
> @@ -98,12 +98,12 @@ typedef struct {
Just noticed having no name in 'typedef struct' line makes it harder to
understand what is going on.
MultiFDSendParams
> bool running;
> /* should this thread finish */
> bool quit;
> - /* multifd flags for each packet */
> - uint32_t flags;
> /* global number of generated multifd packets */
> uint64_t packet_num;
> /* How many bytes have we sent on the last packet */
> uint64_t sent_bytes;
> + /* Do we need to do an iteration sync */
> + bool sync_needed;
> /* thread has work to do */
> int pending_job;
> /* array of pages to sent.
> @@ -117,6 +117,8 @@ typedef struct {
>
> /* pointer to the packet */
> MultiFDPacket_t *packet;
> + /* multifd flags for each packet */
> + uint32_t flags;
> /* size of the next packet that contains pages */
> uint32_t next_packet_size;
> /* packets sent through this channel */
MultiFDRecvParams
> @@ -163,8 +165,6 @@ typedef struct {
> bool running;
> /* should this thread finish */
> bool quit;
> - /* multifd flags for each packet */
> - uint32_t flags;
> /* global number of generated multifd packets */
> uint64_t packet_num;
>
> @@ -172,6 +172,8 @@ typedef struct {
>
> /* pointer to the packet */
> MultiFDPacket_t *packet;
> + /* multifd flags for each packet */
> + uint32_t flags;
> /* size of the next packet that contains pages */
> uint32_t next_packet_size;
> /* packets sent through this channel */
So, IIUC, the struct member flags got moved down (same struct) to an area
described as thread-local, meaning it does not need locking.
Interesting, I haven't noticed this different areas in the same struct.
> diff --git a/migration/multifd.c b/migration/multifd.c
> index e25b529235..09a40a9135 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -602,7 +602,7 @@ int multifd_send_sync_main(QEMUFile *f)
> }
>
> p->packet_num = multifd_send_state->packet_num++;
> - p->flags |= MULTIFD_FLAG_SYNC;
> + p->sync_needed = true;
> p->pending_job++;
> qemu_mutex_unlock(&p->mutex);
> qemu_sem_post(&p->sem);
> @@ -658,7 +658,11 @@ static void *multifd_send_thread(void *opaque)
>
> if (p->pending_job) {
> uint64_t packet_num = p->packet_num;
> - uint32_t flags = p->flags;
> + p->flags = 0;
> + if (p->sync_needed) {
> + p->flags |= MULTIFD_FLAG_SYNC;
> + p->sync_needed = false;
> + }
Any particular reason why doing p->flags = 0, then p->flags |= MULTIFD_FLAG_SYNC
?
[1] Couldn't it be done without the |= , since it's already being set to zero
before? (becoming "p->flags = MULTIFD_FLAG_SYNC" )
> p->normal_num = 0;
>
> if (use_zero_copy_send) {
> @@ -680,14 +684,13 @@ static void *multifd_send_thread(void *opaque)
> }
> }
> multifd_send_fill_packet(p);
> - p->flags = 0;
> p->num_packets++;
> p->total_normal_pages += p->normal_num;
> p->pages->num = 0;
> p->pages->block = NULL;
> qemu_mutex_unlock(&p->mutex);
>
> - trace_multifd_send(p->id, packet_num, p->normal_num, flags,
> + trace_multifd_send(p->id, packet_num, p->normal_num, p->flags,
> p->next_packet_size);
>
> if (use_zero_copy_send) {
> @@ -715,7 +718,7 @@ static void *multifd_send_thread(void *opaque)
> p->pending_job--;
> qemu_mutex_unlock(&p->mutex);
>
> - if (flags & MULTIFD_FLAG_SYNC) {
> + if (p->flags & MULTIFD_FLAG_SYNC) {
> qemu_sem_post(&p->sem_sync);
> }
> qemu_sem_post(&multifd_send_state->channels_ready);
IIUC it uses p->sync_needed to keep the sync info, instead of the previous flags
local var, and thus it can set p->flags = 0 earlier. Seems to not change any
behavior AFAICS.
> @@ -1090,7 +1093,7 @@ static void *multifd_recv_thread(void *opaque)
> rcu_register_thread();
>
> while (true) {
> - uint32_t flags;
> + bool sync_needed = false;
>
> if (p->quit) {
> break;
> @@ -1112,11 +1115,11 @@ static void *multifd_recv_thread(void *opaque)
> break;
> }
>
> - flags = p->flags;
> + trace_multifd_recv(p->id, p->packet_num, p->normal_num, p->flags,
> + p->next_packet_size);
> + sync_needed = p->flags & MULTIFD_FLAG_SYNC;
> /* recv methods don't know how to handle the SYNC flag */
> p->flags &= ~MULTIFD_FLAG_SYNC;
> - trace_multifd_recv(p->id, p->packet_num, p->normal_num, flags,
> - p->next_packet_size);
> p->num_packets++;
> p->total_normal_pages += p->normal_num;
> qemu_mutex_unlock(&p->mutex);
> @@ -1128,7 +1131,7 @@ static void *multifd_recv_thread(void *opaque)
> }
> }
>
> - if (flags & MULTIFD_FLAG_SYNC) {
> + if (sync_needed) {
> qemu_sem_post(&multifd_recv_state->sem_sync);
> qemu_sem_wait(&p->sem_sync);
> }
Ok, IIUC this part should have the same behavior as before, but using a bool
instead of an u32.
Other than question [1], LGTM.
FWIW:
Reviewed-by: Leonardo Bras <leob...@redhat.com>
