Re: [Qemu-devel] [QEMU 7/7] migration: skip free pages during live migration

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jun 13, 2016 at 06:16:49PM +0800, Liang Li wrote:
> After sending out the request for free pages, live migration
> process will start without waiting for the free page bitmap is
> ready. If the free page bitmap is not ready when doing the 1st
> migration_bitmap_sync() after ram_save_setup(), the free page
> bitmap will be ignored, this means the free pages will not be
> filtered out in this case.
> The current implementation can not work with post copy, if post
> copy is enabled, we simply ignore the free pages. Will make it
> work later.
> 
> Signed-off-by: Liang Li <liang.z.li@intel.com>

Tying migration to balloon in this way seems rather ugly.
So with request ID, the logic would basically be

	- add memory listener with high priority
	- before sync bitmap, increment request id
	- when we get response, if it has latest request id,
	  clear qemu migration bitmap
	  otherwise, ignore


> ---
>  migration/ram.c | 93 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 93 insertions(+)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 844ea46..5f1c3ff 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -43,6 +43,8 @@
>  #include "trace.h"
>  #include "exec/ram_addr.h"
>  #include "qemu/rcu_queue.h"
> +#include "sysemu/balloon.h"
> +#include "sysemu/kvm.h"
>  
>  #ifdef DEBUG_MIGRATION_RAM
>  #define DPRINTF(fmt, ...) \
> @@ -228,6 +230,7 @@ static QemuMutex migration_bitmap_mutex;
>  static uint64_t migration_dirty_pages;
>  static uint32_t last_version;
>  static bool ram_bulk_stage;
> +static bool ignore_freepage_rsp;
>  
>  /* used by the search for pages to send */
>  struct PageSearchStatus {
> @@ -244,6 +247,7 @@ static struct BitmapRcu {
>      struct rcu_head rcu;
>      /* Main migration bitmap */
>      unsigned long *bmap;
> +    unsigned long *free_page_bmap;
>      /* bitmap of pages that haven't been sent even once
>       * only maintained and used in postcopy at the moment
>       * where it's used to send the dirtymap at the start
> @@ -639,6 +643,7 @@ static void migration_bitmap_sync(void)
>      rcu_read_unlock();
>      qemu_mutex_unlock(&migration_bitmap_mutex);
>  
> +    ignore_freepage_rsp = true;
>      trace_migration_bitmap_sync_end(migration_dirty_pages
>                                      - num_dirty_pages_init);
>      num_dirty_pages_period += migration_dirty_pages - num_dirty_pages_init;
> @@ -1417,6 +1422,7 @@ static void migration_bitmap_free(struct BitmapRcu *bmap)
>  {
>      g_free(bmap->bmap);
>      g_free(bmap->unsentmap);
> +    g_free(bmap->free_page_bmap);
>      g_free(bmap);
>  }
>  
> @@ -1487,6 +1493,85 @@ void migration_bitmap_extend(ram_addr_t old, ram_addr_t new)
>      }
>  }
>  
> +static void filter_out_guest_free_page(unsigned long *free_page_bmap,
> +                                       long nbits)
> +{
> +    long i, page_count = 0, len;
> +    unsigned long *bitmap;
> +
> +    tighten_guest_free_page_bmap(free_page_bmap);
> +    qemu_mutex_lock(&migration_bitmap_mutex);
> +    bitmap = atomic_rcu_read(&migration_bitmap_rcu)->bmap;
> +    slow_bitmap_complement(bitmap, free_page_bmap, nbits);
> +
> +    len = (last_ram_offset() >> TARGET_PAGE_BITS) / BITS_PER_LONG;
> +    for (i = 0; i < len; i++) {
> +        page_count += hweight_long(bitmap[i]);
> +    }
> +
> +    migration_dirty_pages = page_count;
> +    qemu_mutex_unlock(&migration_bitmap_mutex);
> +}
> +
> +static void ram_request_free_page(unsigned long *bmap, unsigned long max_pfn)
> +{
> +    BalloonReqStatus status;
> +
> +    status = balloon_get_free_pages(bmap, max_pfn);
> +    switch (status) {
> +    case REQ_DONE:
> +        ignore_freepage_rsp = false;
> +        break;
> +    case REQ_ERROR:
> +        error_report("Errro happend when request free page");
> +        break;
> +    default:
> +        error_report("unexpected response status: %d", status);
> +        break;
> +    }
> +}
> +
> +static void ram_handle_free_page(void)
> +{
> +    unsigned long nbits;
> +    RAMBlock *pc_ram_block;
> +    BalloonReqStatus status;
> +
> +    status = balloon_get_free_pages(migration_bitmap_rcu->free_page_bmap,
> +                                    get_guest_max_pfn());
> +    switch (status) {
> +    case REQ_DONE:
> +        rcu_read_lock();
> +        pc_ram_block = QLIST_FIRST_RCU(&ram_list.blocks);
> +        nbits = pc_ram_block->used_length >> TARGET_PAGE_BITS;
> +        filter_out_guest_free_page(migration_bitmap_rcu->free_page_bmap, nbits);
> +        rcu_read_unlock();
> +
> +        qemu_mutex_lock_iothread();
> +        migration_bitmap_sync();
> +        qemu_mutex_unlock_iothread();
> +        /*
> +         * bulk stage assumes in (migration_bitmap_find_and_reset_dirty) that
> +         * every page is dirty, that's no longer ture at this point.
> +         */
> +        ram_bulk_stage = false;
> +        last_seen_block = NULL;
> +        last_sent_block = NULL;
> +        last_offset = 0;
> +        break;
> +    case REQ_ERROR:
> +        ignore_freepage_rsp = true;
> +        error_report("failed to get free page");
> +        break;
> +    case REQ_INVALID_PARAM:
> +        ignore_freepage_rsp = true;
> +        error_report("buffer overflow");
> +        break;
> +    default:
> +        break;
> +    }
> +}
> +
>  /*
>   * 'expected' is the value you expect the bitmap mostly to be full
>   * of; it won't bother printing lines that are all this value.
> @@ -1950,6 +2035,11 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
>      qemu_mutex_unlock_ramlist();
>      qemu_mutex_unlock_iothread();
>  
> +    if (balloon_free_pages_support() && !migrate_postcopy_ram()) {
> +        unsigned long max_pfn = get_guest_max_pfn();
> +        migration_bitmap_rcu->free_page_bmap = bitmap_new(max_pfn);
> +        ram_request_free_page(migration_bitmap_rcu->free_page_bmap, max_pfn);
> +    }
>      qemu_put_be64(f, ram_bytes_total() | RAM_SAVE_FLAG_MEM_SIZE);
>  
>      QLIST_FOREACH_RCU(block, &ram_list.blocks, next) {
> @@ -1990,6 +2080,9 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
>      while ((ret = qemu_file_rate_limit(f)) == 0) {
>          int pages;
>  
> +        if (!ignore_freepage_rsp) {
> +            ram_handle_free_page();
> +        }
>          pages = ram_find_and_save_block(f, false, &bytes_transferred);
>          /* no more pages to sent */
>          if (pages == 0) {
> -- 
> 1.9.1