Re: [Qemu-devel] [PATCH v2 3/5] migration/rdma: Allow cancelling while waiting for wrid

[Dr. David Alan Gilbert]

* Peter Xu (address@hidden) wrote:
> On Thu, Jul 13, 2017 at 12:56:47PM +0100, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <address@hidden>
> > 
> > When waiting for a WRID, if the other side dies we end up waiting
> > for ever with no way to cancel the migration.
> > Cure this by poll()ing the fd first with a timeout and checking
> > error flags and migration state.
> > 
> > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> > ---
> >  migration/rdma.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++------
> >  1 file changed, 46 insertions(+), 6 deletions(-)
> > 
> > diff --git a/migration/rdma.c b/migration/rdma.c
> > index 6111e10c70..30f5542b49 100644
> > --- a/migration/rdma.c
> > +++ b/migration/rdma.c
> > @@ -1466,6 +1466,50 @@ static uint64_t qemu_rdma_poll(RDMAContext *rdma, 
> > uint64_t *wr_id_out,
> >      return  0;
> >  }
> >  
> > +/* Wait for activity on the completion channel.
> > + * Returns 0 on success, none-0 on error.
> > + */
> > +static int qemu_rdma_wait_comp_channel(RDMAContext *rdma)
> > +{
> > +    /*
> > +     * Coroutine doesn't start until migration_fd_process_incoming()
> > +     * so don't yield unless we know we're running inside of a coroutine.
> > +     */
> > +    if (rdma->migration_started_on_destination) {
> > +        yield_until_fd_readable(rdma->comp_channel->fd);
> > +    } else {
> > +        /* This is the source side, we're in a separate thread
> > +         * or destination prior to migration_fd_process_incoming()
> > +         * we can't yield; so we have to poll the fd.
> > +         * But we need to be able to handle 'cancel' or an error
> > +         * without hanging forever.
> > +         */
> > +        while (!rdma->error_state  && !rdma->received_error) {
> > +            GPollFD pfds[1];
> > +            pfds[0].fd = rdma->comp_channel->fd;
> > +            pfds[0].events = G_IO_IN | G_IO_HUP | G_IO_ERR;
> > +            /* 0.1s timeout, should be fine for a 'cancel' */
> > +            switch (qemu_poll_ns(pfds, 1, 100 * 1000 * 1000)) {
> > +            case 1: /* fd active */
> > +                return 0;
> > +
> > +            case 0: /* Timeout, go around again */
> > +                break;
> > +
> > +            default: /* Error of some type */
> > +                return -1;
> > +            }
> > +
> > +            if (migrate_get_current()->state == 
> > MIGRATION_STATUS_CANCELLING) {
> > +                /* Bail out and let the cancellation happen */
> > +                return -EPIPE;
> > +            }
> > +        }
> > +    }
> > +
> > +    return rdma->error_state || rdma->received_error;
> 
> Just to note that this operation will return either 0 or 1, but not
> anything <0 (most RDMA codes are using <0 as error, and error_state
> should be <= 0 always iiuc).
> 
> But as the comment for this function, it's fine.

It's interesting in that 'error_state' is a -ve error code, where as
received_error is just a boolean; I've changed this to:

    if (rdma->received->error) {
        return -EPIPE;
    }
    return rdma->error_state;

to make it a bit more consistent.

> > +}
> > +
> >  /*
> >   * Block until the next work request has completed.
> >   *
> > @@ -1513,12 +1557,8 @@ static int qemu_rdma_block_for_wrid(RDMAContext 
> > *rdma, int wrid_requested,
> >      }
> >  
> >      while (1) {
> > -        /*
> > -         * Coroutine doesn't start until migration_fd_process_incoming()
> > -         * so don't yield unless we know we're running inside of a 
> > coroutine.
> > -         */
> > -        if (rdma->migration_started_on_destination) {
> > -            yield_until_fd_readable(rdma->comp_channel->fd);
> > +        if (qemu_rdma_wait_comp_channel(rdma)) {
> 
> Do we want something like:
> 
>   ret = -EIO;
> 
> Here? Or capture the return code of qemu_rdma_wait_comp_channel() (but
> then we need to make sure its return code is <0)?
> 
> I guess "ret" is still zero, and it means this function will return
> with zero as well even timed out?

But rdma->error_state is set if wait_comp_channel fails;
(actually it wasn't in one case, where the poll fails, I've just fixed
that and will repost).

Dave

> Thanks,
> 
> -- 
> Peter Xu
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK