Re: [PATCH v2 06/17] migration: Add save_live_complete_precopy_{begin,end} handlers

[Peter Xu]

On Thu, Sep 19, 2024 at 09:47:53PM +0200, Maciej S. Szmigiero wrote:
> On 9.09.2024 21:08, Peter Xu wrote:
> > On Mon, Sep 09, 2024 at 08:32:45PM +0200, Maciej S. Szmigiero wrote:
> > > On 9.09.2024 19:59, Peter Xu wrote:
> > > > On Thu, Sep 05, 2024 at 04:45:48PM +0300, Avihai Horon wrote:
> > > > > 
> > > > > On 27/08/2024 20:54, Maciej S. Szmigiero wrote:
> > > > > > External email: Use caution opening links or attachments
> > > > > > 
> > > > > > 
> > > > > > From: "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
> > > > > > 
> > > > > > These SaveVMHandlers help device provide its own asynchronous
> > > > > > transmission of the remaining data at the end of a precopy phase.
> > > > > > 
> > > > > > In this use case the save_live_complete_precopy_begin handler might
> > > > > > be used to mark the stream boundary before proceeding with 
> > > > > > asynchronous
> > > > > > transmission of the remaining data while the
> > > > > > save_live_complete_precopy_end handler might be used to mark the
> > > > > > stream boundary after performing the asynchronous transmission.
> > > > > > 
> > > > > > Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
> > > > > > ---
> > > > > >     include/migration/register.h | 36 
> > > > > > ++++++++++++++++++++++++++++++++++++
> > > > > >     migration/savevm.c           | 35 
> > > > > > +++++++++++++++++++++++++++++++++++
> > > > > >     2 files changed, 71 insertions(+)
> > > > > > 
> > > > > > diff --git a/include/migration/register.h 
> > > > > > b/include/migration/register.h
> > > > > > index f60e797894e5..9de123252edf 100644
> > > > > > --- a/include/migration/register.h
> > > > > > +++ b/include/migration/register.h
> > > > > > @@ -103,6 +103,42 @@ typedef struct SaveVMHandlers {
> > > > > >          */
> > > > > >         int (*save_live_complete_precopy)(QEMUFile *f, void 
> > > > > > *opaque);
> > > > > > 
> > > > > > +    /**
> > > > > > +     * @save_live_complete_precopy_begin
> > > > > > +     *
> > > > > > +     * Called at the end of a precopy phase, before all
> > > > > > +     * @save_live_complete_precopy handlers and before launching
> > > > > > +     * all @save_live_complete_precopy_thread threads.
> > > > > > +     * The handler might, for example, mark the stream boundary 
> > > > > > before
> > > > > > +     * proceeding with asynchronous transmission of the remaining 
> > > > > > data via
> > > > > > +     * @save_live_complete_precopy_thread.
> > > > > > +     * When postcopy is enabled, devices that support postcopy 
> > > > > > will skip this step.
> > > > > > +     *
> > > > > > +     * @f: QEMUFile where the handler can synchronously send data 
> > > > > > before returning
> > > > > > +     * @idstr: this device section idstr
> > > > > > +     * @instance_id: this device section instance_id
> > > > > > +     * @opaque: data pointer passed to register_savevm_live()
> > > > > > +     *
> > > > > > +     * Returns zero to indicate success and negative for error
> > > > > > +     */
> > > > > > +    int (*save_live_complete_precopy_begin)(QEMUFile *f,
> > > > > > +                                            char *idstr, uint32_t 
> > > > > > instance_id,
> > > > > > +                                            void *opaque);
> > > > > > +    /**
> > > > > > +     * @save_live_complete_precopy_end
> > > > > > +     *
> > > > > > +     * Called at the end of a precopy phase, after 
> > > > > > @save_live_complete_precopy
> > > > > > +     * handlers and after all @save_live_complete_precopy_thread 
> > > > > > threads have
> > > > > > +     * finished. When postcopy is enabled, devices that support 
> > > > > > postcopy will
> > > > > > +     * skip this step.
> > > > > > +     *
> > > > > > +     * @f: QEMUFile where the handler can synchronously send data 
> > > > > > before returning
> > > > > > +     * @opaque: data pointer passed to register_savevm_live()
> > > > > > +     *
> > > > > > +     * Returns zero to indicate success and negative for error
> > > > > > +     */
> > > > > > +    int (*save_live_complete_precopy_end)(QEMUFile *f, void 
> > > > > > *opaque);
> > > > > 
> > > > > Is this handler necessary now that migration core is responsible for 
> > > > > the
> > > > > threads and joins them? I don't see VFIO implementing it later on.
> > > > 
> > > > Right, I spot the same thing.
> > > > 
> > > > This series added three hooks: begin, end, precopy_thread.
> > > > 
> > > > What I think is it only needs one, which is precopy_async.  My vague 
> > > > memory
> > > > was that was what we used to discuss too, so that when migration precopy
> > > > flushes the final round of iterable data, it does:
> > > > 
> > > >     (1) loop over all complete_precopy_async() and enqueue the tasks if
> > > >         existed into the migration worker pool.  Then,
> > > > 
> > > >     (2) loop over all complete_precopy() like before.
> > > > 
> > > > Optionally, we can enforce one vmstate handler only provides either
> > > > complete_precopy_async() or complete_precopy().  In this case VFIO can
> > > > update the two hooks during setup() by detecting multifd && !mapped_ram 
> > > > &&
> > > > nocomp.
> > > > 
> > > 
> > > The "_begin" hook is still necessary to mark the end of the device state
> > > sent via the main migration stream (during the phase VM is still running)
> > > since we can't start loading the multifd sent device state until all of
> > > that earlier data finishes loading first.
> > 
> > Ah I remembered some more now, thanks.
> > 
> > If vfio can send data during iterations this new hook will also not be
> > needed, right?
> > 
> > I remember you mentioned you'd have a look and see the challenges there, is
> > there any conclusion yet on whether we can use multifd even during that?
> 
> Yeah, I looked at that and it wasn't a straightforward thing to introduce.
> 
> I am worried that with all the things that already piled up (including the
> new thread pool implementation) we risk missing QEMU 9.2 too if this is
> included.

Not explicitly required, but IMHO it'll be nice to provide a paragraph in
the new version when repost explaining the challenges of using it during
iterations.  It'll be not only for me but for whoever may want to extend
your solution to iterations.

I asked this question again mostly because I found that when with iteration
support the design looks simpler in begin(), so that the extra sync is not
needed.  But I confess you know better than me, so whatever you think best
is ok here.

> 
> > It's also a pity that we introduce this hook only because we want a
> > boundary between "iterable stage" and "final stage".  IIUC if we have any
> > kind of message telling dest before hand that "we're going to the last
> > stage" then this hook can be avoided.  Now it's at least inefficient
> > because we need to trigger begin() per-device, even if I think it's more of
> > a global request saying that "we need to load all main stream data first
> > before moving on".
> 
> It should be pretty easy to remove that begin() hook once it is no longer
> needed - after all, it's only necessary for the sender.

Do you mean you have plan to remove the begin() hook even without making
interate() work too?  That's definitely nice if so.

> 
> > > 
> > > We shouldn't send that boundary marker in .save_live_complete_precopy
> > > either since it would meant unnecessary waiting for other devices
> > > (not necessary VFIO ones) .save_live_complete_precopy bulk data.
> > > 
> > > And VFIO SaveVMHandlers are shared for all VFIO devices (and const) so
> > > we can't really change them at runtime.
> > 
> > In all cases, please consider dropping end() if it's never used; IMO it's
> > fine if there is only begin(), and we shouldn't keep hooks that are never
> > used.
> 
> Okay, will remove the end() hook then.
> 
> > Thanks,
> > 
> 
> Thanks,
> Maciej
> 

-- 
Peter Xu