Re: [Qemu-devel] [RFC PATCH] migration/postcopy: enable compress with postcopy

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

* Wei Yang (richardw.yang@linux.intel.com) wrote:
> This patch enable compress with postcopy.
> 
> This is a RFC and based on some unmerged patch
> 
>   "migration: extract ram_load_precopy"
>   "migration/postcopy: skip compression when postcopy is active"
> 
> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
> ---
>  migration/postcopy-ram.c |  3 +--
>  migration/ram.c          | 35 +++++++++++++++++++++--------------
>  2 files changed, 22 insertions(+), 16 deletions(-)
> 
> diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> index a7e7ec9c22..70b6beb5a9 100644
> --- a/migration/postcopy-ram.c
> +++ b/migration/postcopy-ram.c
> @@ -1252,8 +1252,7 @@ int postcopy_place_page_zero(MigrationIncomingState *mis, void *host,
>              }
>              memset(mis->postcopy_tmp_zero_page, '\0', mis->largest_page_size);
>          }
> -        return postcopy_place_page(mis, host, mis->postcopy_tmp_zero_page,
> -                                   rb);
> +        return postcopy_place_page(mis, host, mis->postcopy_tmp_zero_page, rb);

Please keep these type of cleanups separate.

>      }
>  }
>  
> diff --git a/migration/ram.c b/migration/ram.c
> index a0d3bc60b2..c1d6eadf38 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -2384,16 +2384,6 @@ static bool save_page_use_compression(RAMState *rs)
>          return false;
>      }
>  
> -    /*
> -     * The decompression threads asynchronously write into RAM
> -     * rather than use the atomic copies needed to avoid
> -     * userfaulting.  It should be possible to fix the decompression
> -     * threads for compatibility in future.
> -     */
> -    if (migration_in_postcopy()) {
> -        return false;
> -    }
> -
>      /*
>       * If xbzrle is on, stop using the data compression after first
>       * round of migration even if compression is enabled. In theory,
> @@ -3433,6 +3423,11 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
>          }
>          i++;
>      }
> +
> +    if (migrate_postcopy_ram()) {
> +        flush_compressed_data(rs);
> +    }
> +
>      rcu_read_unlock();
>  
>      /*
> @@ -4019,6 +4014,7 @@ static int ram_load_postcopy(QEMUFile *f)
>          void *place_source = NULL;
>          RAMBlock *block = NULL;
>          uint8_t ch;
> +        int len;
>  
>          addr = qemu_get_be64(f);
>  
> @@ -4036,7 +4032,8 @@ static int ram_load_postcopy(QEMUFile *f)
>  
>          trace_ram_load_postcopy_loop((uint64_t)addr, flags);
>          place_needed = false;
> -        if (flags & (RAM_SAVE_FLAG_ZERO | RAM_SAVE_FLAG_PAGE)) {
> +        if (flags & (RAM_SAVE_FLAG_ZERO | RAM_SAVE_FLAG_PAGE |
> +                     RAM_SAVE_FLAG_COMPRESS_PAGE)) {
>              block = ram_block_from_stream(f, flags);
>  
>              host = host_from_ram_block_offset(block, addr);
> @@ -4109,6 +4106,17 @@ static int ram_load_postcopy(QEMUFile *f)
>                                           TARGET_PAGE_SIZE);
>              }
>              break;
> +        case RAM_SAVE_FLAG_COMPRESS_PAGE:
> +            all_zero = false;
> +            len = qemu_get_be32(f);
> +            if (len < 0 || len > compressBound(TARGET_PAGE_SIZE)) {
> +                error_report("Invalid compressed data length: %d", len);
> +                ret = -EINVAL;
> +                break;
> +            }
> +            decompress_data_with_multi_threads(f, page_buffer, len);
> +            ret |= wait_for_decompress_done();

I think this might work for a 4k page host; but I'm not sure it's
safe on hugepages or ARM/Power where they have bigger pages.
ram_load_postcopy relies on all of the pages within a single hostpage
arriving before the last subpage and that's what then triggers the call
to postcopy_place_page;  that relies on some ordering - but I don't
think that the multiple compress threads on the source have any ordering
between the threads - or am I missing something about how the multiple
threads are organised?

Dave

> +            break;
>          case RAM_SAVE_FLAG_EOS:
>              /* normal exit */
>              multifd_recv_sync_main();
> @@ -4130,8 +4138,7 @@ static int ram_load_postcopy(QEMUFile *f)
>              void *place_dest = host + TARGET_PAGE_SIZE - block->page_size;
>  
>              if (all_zero) {
> -                ret = postcopy_place_page_zero(mis, place_dest,
> -                                               block);
> +                ret = postcopy_place_page_zero(mis, place_dest, block);
>              } else {
>                  ret = postcopy_place_page(mis, place_dest,
>                                            place_source, block);
> @@ -4372,6 +4379,7 @@ static int ram_load_precopy(QEMUFile *f)
>          }
>      }
>  
> +    ret |= wait_for_decompress_done();
>      return ret;
>  }
>  
> @@ -4405,7 +4413,6 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
>          ret = ram_load_precopy(f);
>      }
>  
> -    ret |= wait_for_decompress_done();
>      rcu_read_unlock();
>      trace_ram_load_complete(ret, seq_iter);
>  
> -- 
> 2.17.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK