Re: [Qemu-devel] [Patch v2] migration/postcopy: make PostcopyDiscardState a static variable

[Dr. David Alan Gilbert]

* Wei Yang (address@hidden) wrote:
> In postcopy-ram.c, we provide three functions to discard certain
> RAMBlock range:
> 
>   * postcopy_discard_send_init()
>   * postcopy_discard_send_range()
>   * postcopy_discard_send_finish()
> 
> Currently, we allocate/deallocate PostcopyDiscardState for each RAMBlock
> on sending discard information to destination. This is not necessary and
> the same data area could be reused for each RAMBlock.
> 
> This patch defines PostcopyDiscardState a static variable. By doing so:
> 
>   1) avoid memory allocation and deallocation to the system
>   2) avoid potential failure of memory allocation
>   3) hide some details for their users
> 
> Signed-off-by: Wei Yang <address@hidden>


Reviewed-by: Dr. David Alan Gilbert <address@hidden>

> ---
> v2:
>   * make it a static variable, suggested by Dave
> ---
>  migration/postcopy-ram.c | 70 +++++++++++++++++-----------------------
>  migration/postcopy-ram.h | 13 +++-----
>  migration/ram.c          | 30 +++++++----------
>  3 files changed, 46 insertions(+), 67 deletions(-)
> 
> diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> index 7b3e198538..cf2400b47e 100644
> --- a/migration/postcopy-ram.c
> +++ b/migration/postcopy-ram.c
> @@ -1375,22 +1375,16 @@ void 
> postcopy_fault_thread_notify(MigrationIncomingState *mis)
>   *   asking to discard individual ranges.
>   *
>   * @ms: The current migration state.
> - * @offset: the bitmap offset of the named RAMBlock in the migration
> - *   bitmap.
> + * @offset: the bitmap offset of the named RAMBlock in the migration bitmap.
>   * @name: RAMBlock that discards will operate on.
> - *
> - * returns: a new PDS.
>   */
> -PostcopyDiscardState *postcopy_discard_send_init(MigrationState *ms,
> -                                                 const char *name)
> +static PostcopyDiscardState pds = {0};
> +void postcopy_discard_send_init(MigrationState *ms, const char *name)
>  {
> -    PostcopyDiscardState *res = g_malloc0(sizeof(PostcopyDiscardState));
> -
> -    if (res) {
> -        res->ramblock_name = name;
> -    }
> -
> -    return res;
> +    pds.ramblock_name = name;
> +    pds.cur_entry = 0;
> +    pds.nsentwords = 0;
> +    pds.nsentcmds = 0;
>  }
>  
>  /**
> @@ -1399,30 +1393,29 @@ PostcopyDiscardState 
> *postcopy_discard_send_init(MigrationState *ms,
>   *   be sent later.
>   *
>   * @ms: Current migration state.
> - * @pds: Structure initialised by postcopy_discard_send_init().
>   * @start,@length: a range of pages in the migration bitmap in the
>   *   RAM block passed to postcopy_discard_send_init() (length=1 is one page)
>   */
> -void postcopy_discard_send_range(MigrationState *ms, PostcopyDiscardState 
> *pds,
> -                                unsigned long start, unsigned long length)
> +void postcopy_discard_send_range(MigrationState *ms, unsigned long start,
> +                                 unsigned long length)
>  {
>      size_t tp_size = qemu_target_page_size();
>      /* Convert to byte offsets within the RAM block */
> -    pds->start_list[pds->cur_entry] = start  * tp_size;
> -    pds->length_list[pds->cur_entry] = length * tp_size;
> -    trace_postcopy_discard_send_range(pds->ramblock_name, start, length);
> -    pds->cur_entry++;
> -    pds->nsentwords++;
> +    pds.start_list[pds.cur_entry] = start  * tp_size;
> +    pds.length_list[pds.cur_entry] = length * tp_size;
> +    trace_postcopy_discard_send_range(pds.ramblock_name, start, length);
> +    pds.cur_entry++;
> +    pds.nsentwords++;
>  
> -    if (pds->cur_entry == MAX_DISCARDS_PER_COMMAND) {
> +    if (pds.cur_entry == MAX_DISCARDS_PER_COMMAND) {
>          /* Full set, ship it! */
>          qemu_savevm_send_postcopy_ram_discard(ms->to_dst_file,
> -                                              pds->ramblock_name,
> -                                              pds->cur_entry,
> -                                              pds->start_list,
> -                                              pds->length_list);
> -        pds->nsentcmds++;
> -        pds->cur_entry = 0;
> +                                              pds.ramblock_name,
> +                                              pds.cur_entry,
> +                                              pds.start_list,
> +                                              pds.length_list);
> +        pds.nsentcmds++;
> +        pds.cur_entry = 0;
>      }
>  }
>  
> @@ -1431,24 +1424,21 @@ void postcopy_discard_send_range(MigrationState *ms, 
> PostcopyDiscardState *pds,
>   * bitmap code. Sends any outstanding discard messages, frees the PDS
>   *
>   * @ms: Current migration state.
> - * @pds: Structure initialised by postcopy_discard_send_init().
>   */
> -void postcopy_discard_send_finish(MigrationState *ms, PostcopyDiscardState 
> *pds)
> +void postcopy_discard_send_finish(MigrationState *ms)
>  {
>      /* Anything unsent? */
> -    if (pds->cur_entry) {
> +    if (pds.cur_entry) {
>          qemu_savevm_send_postcopy_ram_discard(ms->to_dst_file,
> -                                              pds->ramblock_name,
> -                                              pds->cur_entry,
> -                                              pds->start_list,
> -                                              pds->length_list);
> -        pds->nsentcmds++;
> +                                              pds.ramblock_name,
> +                                              pds.cur_entry,
> +                                              pds.start_list,
> +                                              pds.length_list);
> +        pds.nsentcmds++;
>      }
>  
> -    trace_postcopy_discard_send_finish(pds->ramblock_name, pds->nsentwords,
> -                                       pds->nsentcmds);
> -
> -    g_free(pds);
> +    trace_postcopy_discard_send_finish(pds.ramblock_name, pds.nsentwords,
> +                                       pds.nsentcmds);
>  }
>  
>  /*
> diff --git a/migration/postcopy-ram.h b/migration/postcopy-ram.h
> index e3a5cfd2d8..e3dde32155 100644
> --- a/migration/postcopy-ram.h
> +++ b/migration/postcopy-ram.h
> @@ -43,10 +43,8 @@ int postcopy_ram_prepare_discard(MigrationIncomingState 
> *mis);
>  
>  /*
>   * Called at the start of each RAMBlock by the bitmap code.
> - * Returns a new PDS
>   */
> -PostcopyDiscardState *postcopy_discard_send_init(MigrationState *ms,
> -                                                 const char *name);
> +void postcopy_discard_send_init(MigrationState *ms, const char *name);
>  
>  /*
>   * Called by the bitmap code for each chunk to discard.
> @@ -55,15 +53,14 @@ PostcopyDiscardState 
> *postcopy_discard_send_init(MigrationState *ms,
>   * @start,@length: a range of pages in the migration bitmap in the
>   *  RAM block passed to postcopy_discard_send_init() (length=1 is one page)
>   */
> -void postcopy_discard_send_range(MigrationState *ms, PostcopyDiscardState 
> *pds,
> -                                 unsigned long start, unsigned long length);
> +void postcopy_discard_send_range(MigrationState *ms, unsigned long start,
> +                                 unsigned long length);
>  
>  /*
>   * Called at the end of each RAMBlock by the bitmap code.
> - * Sends any outstanding discard messages, frees the PDS.
> + * Sends any outstanding discard messages.
>   */
> -void postcopy_discard_send_finish(MigrationState *ms,
> -                                  PostcopyDiscardState *pds);
> +void postcopy_discard_send_finish(MigrationState *ms);
>  
>  /*
>   * Place a page (from) at (host) efficiently
> diff --git a/migration/ram.c b/migration/ram.c
> index ecd10baa43..7b7155a368 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -2761,12 +2761,9 @@ void 
> ram_postcopy_migrated_memory_release(MigrationState *ms)
>   *       with the dirtymap; so a '1' means it's either dirty or unsent.
>   *
>   * @ms: current migration state
> - * @pds: state for postcopy
>   * @block: RAMBlock to discard
>   */
> -static int postcopy_send_discard_bm_ram(MigrationState *ms,
> -                                        PostcopyDiscardState *pds,
> -                                        RAMBlock *block)
> +static int postcopy_send_discard_bm_ram(MigrationState *ms, RAMBlock *block)
>  {
>      unsigned long end = block->used_length >> TARGET_PAGE_BITS;
>      unsigned long current;
> @@ -2787,7 +2784,7 @@ static int postcopy_send_discard_bm_ram(MigrationState 
> *ms,
>          } else {
>              discard_length = zero - one;
>          }
> -        postcopy_discard_send_range(ms, pds, one, discard_length);
> +        postcopy_discard_send_range(ms, one, discard_length);
>          current = one + discard_length;
>      }
>  
> @@ -2813,16 +2810,15 @@ static int 
> postcopy_each_ram_send_discard(MigrationState *ms)
>      int ret;
>  
>      RAMBLOCK_FOREACH_NOT_IGNORED(block) {
> -        PostcopyDiscardState *pds =
> -            postcopy_discard_send_init(ms, block->idstr);
> +        postcopy_discard_send_init(ms, block->idstr);
>  
>          /*
>           * Postcopy sends chunks of bitmap over the wire, but it
>           * just needs indexes at this point, avoids it having
>           * target page specific code.
>           */
> -        ret = postcopy_send_discard_bm_ram(ms, pds, block);
> -        postcopy_discard_send_finish(ms, pds);
> +        ret = postcopy_send_discard_bm_ram(ms, block);
> +        postcopy_discard_send_finish(ms);
>          if (ret) {
>              return ret;
>          }
> @@ -2845,11 +2841,9 @@ static int 
> postcopy_each_ram_send_discard(MigrationState *ms)
>   * @unsent_pass: if true we need to canonicalize partially unsent host pages
>   *               otherwise we need to canonicalize partially dirty host pages
>   * @block: block that contains the page we want to canonicalize
> - * @pds: state for postcopy
>   */
>  static void postcopy_chunk_hostpages_pass(MigrationState *ms, bool 
> unsent_pass,
> -                                          RAMBlock *block,
> -                                          PostcopyDiscardState *pds)
> +                                          RAMBlock *block)
>  {
>      RAMState *rs = ram_state;
>      unsigned long *bitmap = block->bmap;
> @@ -2910,8 +2904,7 @@ static void 
> postcopy_chunk_hostpages_pass(MigrationState *ms, bool unsent_pass,
>                   *     (any partially sent pages were already discarded
>                   *     by the previous unsent_pass)
>                   */
> -                postcopy_discard_send_range(ms, pds, fixup_start_addr,
> -                                            host_ratio);
> +                postcopy_discard_send_range(ms, fixup_start_addr, 
> host_ratio);
>              }
>  
>              /* Clean up the bitmap */
> @@ -2954,18 +2947,17 @@ static void 
> postcopy_chunk_hostpages_pass(MigrationState *ms, bool unsent_pass,
>   */
>  static int postcopy_chunk_hostpages(MigrationState *ms, RAMBlock *block)
>  {
> -    PostcopyDiscardState *pds =
> -        postcopy_discard_send_init(ms, block->idstr);
> +    postcopy_discard_send_init(ms, block->idstr);
>  
>      /* First pass: Discard all partially sent host pages */
> -    postcopy_chunk_hostpages_pass(ms, true, block, pds);
> +    postcopy_chunk_hostpages_pass(ms, true, block);
>      /*
>       * Second pass: Ensure that all partially dirty host pages are made
>       * fully dirty.
>       */
> -    postcopy_chunk_hostpages_pass(ms, false, block, pds);
> +    postcopy_chunk_hostpages_pass(ms, false, block);
>  
> -    postcopy_discard_send_finish(ms, pds);
> +    postcopy_discard_send_finish(ms);
>      return 0;
>  }
>  
> -- 
> 2.17.1
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK