Re: [RFC Design Doc]Speed up live migration by skipping free pages

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Michael S. Tsirkin (mst@redhat.com) wrote:
> On Thu, Mar 24, 2016 at 04:05:16PM +0000, Li, Liang Z wrote:
> > 
> > 
> > On %D, %SN wrote:
> > %Q
> > 
> > %C
> > 
> > Liang
> > 
> > 
> > > -----Original Message-----
> > > From: Michael S. Tsirkin [mailto:mst@redhat.com]
> > > Sent: Thursday, March 24, 2016 11:57 PM
> > > To: Li, Liang Z
> > > Cc: Dr. David Alan Gilbert; Wei Yang; qemu-devel@nongnu.org;
> > > kvm@vger.kernel.org; linux-kernel@vger.kenel.org; pbonzini@redhat.com;
> > > rth@twiddle.net; ehabkost@redhat.com; amit.shah@redhat.com;
> > > quintela@redhat.com; mohan_parthasarathy@hpe.com;
> > > jitendra.kolhe@hpe.com; simhan@hpe.com; rkagan@virtuozzo.com;
> > > riel@redhat.com
> > > Subject: Re: [RFC Design Doc]Speed up live migration by skipping free pages
> > > 
> > > 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

> 
> 
> 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@redhat.com / Manchester, UK