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

[Juan Quintela]

Leonardo Brás <leobras@redhat.com> wrote:
> 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. 

It is common idiom in QEMU.  The principal reason is that if you don't
want anyone to use "struct MultiFDSendParams" but MultiFDSendParams, the
best way to achieve that is to do it this way.

>> @@ -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.

It has changed in the last two weeks or so in upstream (it has been on
this patchset for several months.)


>
>> 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
> ?

It is a bitmap field, and if there is anything on the future, we need to
set it.  I agree that when there is only one flag, it seems "weird".

> [1] Couldn't it be done without the |= , since it's already being set to zero
> before? (becoming "p->flags = MULTIFD_FLAG_SYNC" )

As said, easier to modify later, and also easier if we want to setup a
flag by default.

I agree that it is a matter of style/taste.

>>              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.

The protection of the global flags was being wrong.  That is the reason
that I decided to change it to the sync_needed.

The problem was that at some point we were still sending a packet (that
shouldn't have the SYNC flag enabled), but we received a
multifd_main_sync() and it got enabled anyways.  The easier way that I
found te fix it was this way.

Problem was difficult to detect, that is the reason that I change it
this way.

>> -        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.

I changed it to make sure that we only checked the flags at the
beggining of the function, with the lock taken.

>
> FWIW:
> Reviewed-by: Leonardo Bras <leobras@redhat.com>

Thanks, Juan.