Re: [PATCH v7 06/12] multifd: Make flags field thread local

["Leonardo Brás" <leobras@redhat.com>]

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 <quintela@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 <leobras@redhat.com>