Re: LAST_CHECKPOINT and save/restore/migrate compatibility (was Re: [Xen-devel] [PATCH 3 of 4] libxc: provide notification of final checkpoint to restore end)

[Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>]

On Fri, 2011-04-01 at 12:02 +0100, George Dunlap wrote:
> On Mon, Sep 6, 2010 at 11:03 AM, Ian Campbell <ian.campbell@xxxxxxxxxx> wrote:
> > # HG changeset patch
> > # User Ian Campbell <ian.campbell@xxxxxxxxxx>
> > # Date 1283766891 -3600
> > # Node ID bdf8ce09160d715451e1204babe5f80886ea6183
> > # Parent  5f96f36feebdb87eaadbbcab0399f32eda86f735
> > libxc: provide notification of final checkpoint to restore end
> >
> > When the restore code sees this notification it will restore the
> > currently in-progress checkpoint when it completes.
> >
> > This allows the restore end to finish up without waiting for a
> > spurious timeout on the receive fd and thereby avoids unnecessary
> > error logging in the case of a successful migration or restore.
> >
> > In the normal migration or restore case the first checkpoint is always
> > the last. For a rolling checkpoint (such as Remus) the notification is
> > currently unused but could be used in the future for example to
> > provide a controlled failover for reasons other than error
> 
> Unfortunatley, this introduces a backwards-compatibility problem,
> since older versions of the tools never send LAST_CHECKPOINT.

Out of interest what actually happens? (I think you mentioned something
about failing when restoring from a file?)

My intention at the time was that in the absence of a LAST_CHECKPOINT
things would continue to behave as before e.g. you would see the
spurious timeout on the receive fd but still restore correctly.

Perhaps the associated changes to the O_NONBLOCK'ness of the fd at
various stages is what broken this for the non-Remus migrations?

> Would it make sense to invert the logic on this -- i.e., default to
> only one checkpoint, and send "MORE_CHECKPOINTS" if it expects this
> *not* to be the last (either by expecting the checkpoint() function to
> do it, or by sending it from xc_domain_save)?

I'd quite like to understand what I broke and investigate the
trivialness (or otherwise) of a fix, but this change would probably
independently make sense too.

> Now that 4.1 is out, we have to consider version compatibilities.  But
> we should be able to make it so that there would only be if:
> 1) You're running REMUS

Is it the case that Remus only cares about checkpointing between like
versions of the toolstack?

> 2) One of your toolstacks is 4.1.0, and one is not.
> 
> That seems like it shouldn't be too bad.
> 
> Thoughts?
> 
>  -George
> 
> >
> > Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> > Acked-by: Brendan Cully <brendan@xxxxxxxxx>
> >
> > diff -r 5f96f36feebd -r bdf8ce09160d tools/libxc/xc_domain_restore.c
> > --- a/tools/libxc/xc_domain_restore.c   Mon Sep 06 10:54:51 2010 +0100
> > +++ b/tools/libxc/xc_domain_restore.c   Mon Sep 06 10:54:51 2010 +0100
> > @@ -42,6 +42,7 @@ struct restore_ctx {
> >     xen_pfn_t *p2m; /* A table mapping each PFN to its new MFN. */
> >     xen_pfn_t *p2m_batch; /* A table of P2M mappings in the current region. 
> >  */
> >     int completed; /* Set when a consistent image is available */
> > +    int last_checkpoint; /* Set when we should commit to the current 
> > checkpoint when it completes. */
> >     struct domain_info_context dinfo;
> >  };
> >
> > @@ -765,6 +766,11 @@ static int pagebuf_get_one(xc_interface
> >         // DPRINTF("console pfn location: %llx\n", buf->console_pfn);
> >         return pagebuf_get_one(xch, ctx, buf, fd, dom);
> >
> > +    case XC_SAVE_ID_LAST_CHECKPOINT:
> > +        ctx->last_checkpoint = 1;
> > +        // DPRINTF("last checkpoint indication received");
> > +        return pagebuf_get_one(xch, ctx, buf, fd, dom);
> > +
> >     default:
> >         if ( (count > MAX_BATCH_SIZE) || (count < 0) ) {
> >             ERROR("Max batch size exceeded (%d). Giving up.", count);
> > @@ -1296,10 +1302,23 @@ int xc_domain_restore(xc_interface *xch,
> >             goto out;
> >         }
> >         ctx->completed = 1;
> > -        /* shift into nonblocking mode for the remainder */
> > -        if ( (flags = fcntl(io_fd, F_GETFL,0)) < 0 )
> > -            flags = 0;
> > -        fcntl(io_fd, F_SETFL, flags | O_NONBLOCK);
> > +
> > +        /*
> > +         * If more checkpoints are expected then shift into
> > +         * nonblocking mode for the remainder.
> > +         */
> > +        if ( !ctx->last_checkpoint )
> > +        {
> > +            if ( (flags = fcntl(io_fd, F_GETFL,0)) < 0 )
> > +                flags = 0;
> > +            fcntl(io_fd, F_SETFL, flags | O_NONBLOCK);
> > +        }
> > +    }
> > +
> > +    if ( ctx->last_checkpoint )
> > +    {
> > +        // DPRINTF("Last checkpoint, finishing\n");
> > +        goto finish;
> >     }
> >
> >     // DPRINTF("Buffered checkpoint\n");
> > diff -r 5f96f36feebd -r bdf8ce09160d tools/libxc/xc_domain_save.c
> > --- a/tools/libxc/xc_domain_save.c      Mon Sep 06 10:54:51 2010 +0100
> > +++ b/tools/libxc/xc_domain_save.c      Mon Sep 06 10:54:51 2010 +0100
> > @@ -1616,6 +1616,20 @@ int xc_domain_save(xc_interface *xch, in
> >         }
> >     }
> >
> > +    if ( !callbacks->checkpoint )
> > +    {
> > +        /*
> > +         * If this is not a checkpointed save then this must be the first 
> > and
> > +         * last checkpoint.
> > +         */
> > +        i = XC_SAVE_ID_LAST_CHECKPOINT;
> > +        if ( wrexact(io_fd, &i, sizeof(int)) )
> > +        {
> > +            PERROR("Error when writing last checkpoint chunk");
> > +            goto out;
> > +        }
> > +    }
> > +
> >     /* Zero terminate */
> >     i = 0;
> >     if ( wrexact(io_fd, &i, sizeof(int)) )
> > diff -r 5f96f36feebd -r bdf8ce09160d tools/libxc/xg_save_restore.h
> > --- a/tools/libxc/xg_save_restore.h     Mon Sep 06 10:54:51 2010 +0100
> > +++ b/tools/libxc/xg_save_restore.h     Mon Sep 06 10:54:51 2010 +0100
> > @@ -131,6 +131,7 @@
> >  #define XC_SAVE_ID_TMEM_EXTRA         -6
> >  #define XC_SAVE_ID_TSC_INFO           -7
> >  #define XC_SAVE_ID_HVM_CONSOLE_PFN    -8 /* (HVM-only) */
> > +#define XC_SAVE_ID_LAST_CHECKPOINT    -9 /* Commit to restoring after 
> > completion of current iteration. */
> >
> >  /*
> >  ** We process save/restore/migrate in batches of pages; the below
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel
> >



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