> > > > On Thu, Mar 24, 2016 at 03:53:25PM +0000, Li, Liang Z wrote:
> > > > > > > > > Not very complex, we can implement like this:
> > > > > > > > >
> > > > > > > > > 1. Set all the bits in the migration_bitmap_rcu->bmap to 1 2.
> > > > > > > > > Clear all the bits in ram_list.
> > > > > > > > > dirty_memory[DIRTY_MEMORY_MIGRATION]
> > > > > > > > > 3. Send the get_free_page_bitmap request 4. Start to
> > > > > > > > > send pages to destination and check if the free_page_bitmap
> is ready
> > > > > > > > > if (is_ready) {
> > > > > > > > > filter out the free pages from migration_bitmap_rcu-
> >bmap;
> > > > > > > > > migration_bitmap_sync();
> > > > > > > > > }
> > > > > > > > > continue until live migration complete.
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > Is that right?
> > > > > > > >
> > > > > > > > The order I'm trying to understand is something like:
> > > > > > > >
> > > > > > > > a) Send the get_free_page_bitmap request
> > > > > > > > b) Start sending pages
> > > > > > > > c) Reach the end of memory
> > > > > > > > [ is_ready is false - guest hasn't made free map yet ]
> > > > > > > > d) normal migration_bitmap_sync() at end of first pass
> > > > > > > > e) Carry on sending dirty pages
> > > > > > > > f) is_ready is true
> > > > > > > > f.1) filter out free pages?
> > > > > > > > f.2) migration_bitmap_sync()
> > > > > > > >
> > > > > > > > It's f.1 I'm worried about. If the guest started
> > > > > > > > generating the free bitmap before (d), then a page marked
> > > > > > > > as 'free' in f.1 might have become dirty before (d) and so
> > > > > > > > (f.2) doesn't set the dirty again, and so we can't filter out pages
> in f.1.
> > > > > > > >
> > > > > > >
> > > > > > > As you described, the order is incorrect.
> > > > > > >
> > > > > > > Liang
> > > > > >
> > > > > >
> > > > > > So to make it safe, what is required is to make sure no free
> > > > > > list us outstanding before calling migration_bitmap_sync.
> > > > > >
> > > > > > If one is outstanding, filter out pages before calling
> > > > migration_bitmap_sync.
> > > > > >
> > > > > > Of course, if we just do it like we normally do with
> > > > > > migration, then by the time we call migration_bitmap_sync
> > > > > > dirty bitmap is completely empty, so there won't be anything to
> filter out.
> > > > > >
> > > > > > One way to address this is call migration_bitmap_sync in the
> > > > > > IO handler, while VCPU is stopped, then make sure to filter
> > > > > > out pages before the next migration_bitmap_sync.
> > > > > >
> > > > > > Another is to start filtering out pages upon IO handler, but
> > > > > > make sure to flush the queue before calling migration_bitmap_sync.
> > > > > >
> > > > >
> > > > > It's really complex, maybe we should switch to a simple start,
> > > > > just skip the free page in the ram bulk stage and make it
> asynchronous?
> > > > >
> > > > > Liang
> > > >
> > > > You mean like your patches do? No, blocking bulk migration until
> > > > guest response is basically a non-starter.
> > > >
> > >
> > > No, don't wait anymore. Like below (copy from previous thread)
> > > --------------------------------------------------------------
> > > 1. Set all the bits in the migration_bitmap_rcu->bmap to 1 2. Clear
> > > all the bits in ram_list.dirty_memory[DIRTY_MEMORY_MIGRATION]
> > > 3. Send the get_free_page_bitmap request 4. Start to send pages to
> > > destination and check if the free_page_bitmap is ready
> > > if (is_ready) {
> > > filter out the free pages from migration_bitmap_rcu->bmap;
> > > migration_bitmap_sync();
> > > }
> > > continue until live migration complete.
> > > ---------------------------------------------------------------
> > > Can this work?
> > >
> > > Liang
> >
> > Not if you get the ready bit asynchronously like you wrote here since
> > is_ready can get set while you called migration_bitmap_sync.
> >
> > As I said previously,
> > to make this work you need to filter out synchronously while VCPU is
> > stopped and while free pages from list are not being used.
> >
> > Alternatively prevent getting free page list and filtering them out
> > from guest from racing with migration_bitmap_sync.
> >
> > For example, flush the VQ after migration_bitmap_sync.
> > So:
> >
> > lock
> > migration_bitmap_sync();
> > while (elem = virtqueue_pop) {
> > virtqueue_push(elem)
> > g_free(elem)
> > }
> > unlock
> >
> >
> > while in handle_output
> >
> > lock
> > while (elem = virtqueue_pop) {
> > list = get_free_list(elem)
> > filter_out_free(list)
> > virtqueue_push(elem)
> > free(elem)
> > }
> > unlock
> >
> >
> > lock prevents migration_bitmap_sync from racing against handle_output
>
> I think the easier way is just to ignore the guests free list response if it comes
> back after the first pass.
>
> Dave
Yes, this is what my pseudo code tried to do. I should make it more clear.
Liang
>
> >
> >
> > This way you can actually use ioeventfd for this VQ so VCPU won't be
> > blocked.
> >
> > I do not think this is so complex, and this way you can add requests
> > for guest free bitmap at an arbitary interval either in host or in
> > guest.
> >
> > For example, add a value that says how often should guest update the
> > bitmap, set it to 0 to disable updates after migration done.
> >
> > Or, make guest resubmit a new one when we consume the old one, run
> > handle_output about through a periodic timer on host.
> >
> >
> > > > --
> > > > MST
> --
> Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
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