Re: [RFC PATCH v2 1/6] migration/multifd: Remove channels_ready semaphore

[Fabiano Rosas <farosas@suse.de>]

Juan Quintela <quintela@redhat.com> writes:

> Peter Xu <peterx@redhat.com> wrote:
>> On Thu, Oct 19, 2023 at 08:41:26PM +0200, Juan Quintela wrote:
>>> We can changing pending_job to a bool if you preffer.  I think that we
>>> have nailed all the off_by_one errors by now (famous last words).
>>
>> Would it work to make pending_job a bool, even with SYNC?  It seems to me
>> multifd_send_sync_main() now can boost pending_job even if pending_job==1.
>
> Then a int is ok, I think.
>
>> That's also the place where I really think confusing too; where it looks
>> like the migration thread can modify a pending job's flag as long as it is
>> fast enough before the send thread put that onto the wire.
>
> It never does.
>
>     for (i = next_channel;; i = (i + 1) % migrate_multifd_channels()) {
>         qemu_mutex_lock(&p->mutex);
>         ...
>         if (!p->pending_job) {
>             p->pending_job++;
>             next_channel = (i + 1) % migrate_multifd_channels();
>             break;
>         }
>         qemu_mutex_unlock(&p->mutex);
>     }

We copy the flags into the packet header at multifd_send_fill_packet()
before unlocking. I think that is even independent of the pending_jobs
scheme.

> If pending_job == 0 -> owner of the channel is migration_thread and it
> can use it.
>
> If pending_job > 0 -> owner of the channel is the channel thread and
> migration_thread can't use it.

Do you really mean "migration_thread" here or just multifd_send_pages()?
Because multifd_send_sync_main() doesn't care about this ownership
rule. Does that mean that code is incorrect?

> I think that this is easy to understand.  You are right that it is not
> _explained_.  And clearly, if you have to ask, it is not obvious O:-)

It is explained at multifd.h. But it doesn't really matter because code
can drift and documentation doesn't assure correctness. That's why we
have to ask about seemingly obvious stuff.

> Yes, it was obvious to me, that is the reason why I wrote it on the
> 1st place.  Notice also that it is a common idiom in multithreaded
> apps.  That allows it to do stuff without having to have a mutex
> locked, so other threads can "look" into the state.
>> Then it's
>> unpredictable whether the SYNC flag will be sent with current packet (where
>> due to pending_jobs==1 already, can contain valid pages), or will be only
>> set for the next one (where there will have 0 real page).
>
> I have to think about this one.
> Decrease pending_jobs there if we are doing multiple jobs?
> But we still have the issue of the semaphore.
>
>> IMHO it'll be good to separate the sync task, then we can change
>> pending_jobs to bool.  Something like:
>>
>>   bool pending_send_page;
>>   bool pending_send_sync;
>
> current code:
>
>         qemu_mutex_lock(&p->mutex);
>         qemu_mutex_lock(&p->mutex);
>
>         if (p->pending_job) {
>             uint64_t packet_num = p->packet_num;
>             uint32_t flags;
>             p->normal_num = 0;
>
>             if (use_zero_copy_send) {
>                 p->iovs_num = 0;
>             } else {
>                 p->iovs_num = 1;
>             }
>
>             for (int i = 0; i < p->pages->num; i++) {
>                 p->normal[p->normal_num] = p->pages->offset[i];
>                 p->normal_num++;
>             }
>
>             if (p->normal_num) {
>                 ret = multifd_send_state->ops->send_prepare(p, &local_err);
>                 if (ret != 0) {
>                     qemu_mutex_unlock(&p->mutex);
>                     break;
>                 }
>             }
>             multifd_send_fill_packet(p);
>             flags = p->flags;
>             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,
>                                p->next_packet_size);
>
>             if (use_zero_copy_send) {
>                 /* Send header first, without zerocopy */
>                 ret = qio_channel_write_all(p->c, (void *)p->packet,
>                                             p->packet_len, &local_err);
>                 if (ret != 0) {
>                     break;
>                 }
>             } else {
>                 /* Send header using the same writev call */
>                 p->iov[0].iov_len = p->packet_len;
>                 p->iov[0].iov_base = p->packet;
>             }
>
>             ret = qio_channel_writev_full_all(p->c, p->iov, p->iovs_num, NULL,
>                                               0, p->write_flags, &local_err);
>             if (ret != 0) {
>                 break;
>             }
>
>             stat64_add(&mig_stats.multifd_bytes,
>                        p->next_packet_size + p->packet_len);
>             stat64_add(&mig_stats.transferred,
>                        p->next_packet_size + p->packet_len);
>             p->next_packet_size = 0;
>             qemu_mutex_lock(&p->mutex);
>             p->pending_job--;
>             qemu_mutex_unlock(&p->mutex);
>
>             if (flags & MULTIFD_FLAG_SYNC) {
>                 qemu_sem_post(&p->sem_sync);
>             }
>         } else {
>             qemu_mutex_unlock(&p->mutex);
>             /* sometimes there are spurious wakeups */
>         }
>
> Your suggested change:
>
>        qemu_mutex_lock(&p->mutex);
>
>        if (p->pending_job_page) {

It's a semantic issue really, but I'd rather we avoid locking ourselves
more into the "pages" idea for multifd threads. The data being sent by
the multifd thread should be opaque.