On Thu, Mar 24, 2016 at 05:49:33PM +0000, Dr. David Alan Gilbert wrote: > * Michael S. Tsirkin (mst@xxxxxxxxxx) 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@xxxxxxxxxx] > > > > Sent: Thursday, March 24, 2016 11:57 PM > > > > To: Li, Liang Z > > > > Cc: Dr. David Alan Gilbert; Wei Yang; qemu-devel@xxxxxxxxxx; > > > > kvm@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxx; pbonzini@xxxxxxxxxx; > > > > rth@xxxxxxxxxxx; ehabkost@xxxxxxxxxx; amit.shah@xxxxxxxxxx; > > > > quintela@xxxxxxxxxx; mohan_parthasarathy@xxxxxxx; > > > > jitendra.kolhe@xxxxxxx; simhan@xxxxxxx; rkagan@xxxxxxxxxxxxx; > > > > riel@xxxxxxxxxx > > > > 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 That's a subset of course - after the first pass == after migration_bitmap_sync. But it's really nasty - for example, how do you know it's the response from this migration round and not a previous one? It is really better to just keep things orthogonal and not introduce arbitrary limitations. For example, with post-copy there's no first pass, and it can still benefit from this optimization. > > > > > > 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 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
