Re: [Qemu-devel] [PATCH v5 13/17] migration: Create thread infrastructure for multifd recv side

[Peter Xu <peterx@redhat.com>]

On Tue, Aug 08, 2017 at 01:41:13PM +0200, Juan Quintela wrote:
> Peter Xu <peterx@redhat.com> wrote:
> > On Mon, Jul 17, 2017 at 03:42:34PM +0200, Juan Quintela wrote:
> 
> >> +static void multifd_recv_page(uint8_t *address, uint16_t fd_num)
> >> +{
> >> +    int thread_count;
> >> +    MultiFDRecvParams *p;
> >> +    static multifd_pages_t pages;
> >> +    static bool once;
> >> +
> >> +    if (!once) {
> >> +        multifd_init_group(&pages);
> >> +        once = true;
> >> +    }
> >> +
> >> +    pages.iov[pages.num].iov_base = address;
> >> +    pages.iov[pages.num].iov_len = TARGET_PAGE_SIZE;
> >> +    pages.num++;
> >> +
> >> +    if (fd_num == UINT16_MAX) {
> >
> > (so this check is slightly mistery as well if we don't define
> >  something... O:-)
> 
> It means that we continue sending pages on the same "group".  Will add a
> comment.
> 
> >
> >> +        return;
> >> +    }
> >> +
> >> +    thread_count = migrate_multifd_threads();
> >> +    assert(fd_num < thread_count);
> >> +    p = multifd_recv_state->params[fd_num];
> >> +
> >> +    qemu_sem_wait(&p->ready);
> >
> > Shall we check for p->pages.num == 0 before wait? What if the
> > corresponding thread is already finished its old work and ready?
> 
> this is a semaphore, not a condition variable.  We only use it with
> values 0 and 1.  We only wait if the other thread hasn't done the post,
> if it has done the post, the wait don't have to wait. (no, I didn't
> invented the semaphore names).

Yeah I think you are right. :)  Thanks,

-- 
Peter Xu