Re: [Qemu-devel] [PATCH v3] migration: always initial ram_counters for a new migration

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

* Ivan Ren (renyime@gmail.com) wrote:
> From: Ivan Ren <ivanren@tencent.com>
> 
> This patch fix a multifd migration bug in migration speed calculation, this
> problem can be reproduced as follows:
> 1. start a vm and give a heavy memory write stress to prevent the vm be
>    successfully migrated to destination
> 2. begin a migration with multifd
> 3. migrate for a long time [actually, this can be measured by transferred bytes]
> 4. migrate cancel
> 5. begin a new migration with multifd, the migration will directly run into
>    migration_completion phase
> 
> Reason as follows:
> 
> Migration update bandwidth and s->threshold_size in function
> migration_update_counters after BUFFER_DELAY time:
> 
>     current_bytes = migration_total_bytes(s);
>     transferred = current_bytes - s->iteration_initial_bytes;
>     time_spent = current_time - s->iteration_start_time;
>     bandwidth = (double)transferred / time_spent;
>     s->threshold_size = bandwidth * s->parameters.downtime_limit;
> 
> In multifd migration, migration_total_bytes function return
> qemu_ftell(s->to_dst_file) + ram_counters.multifd_bytes.
> s->iteration_initial_bytes will be initialized to 0 at every new migration,
> but ram_counters is a global variable, and history migration data will be
> accumulated. So if the ram_counters.multifd_bytes is big enough, it may lead
> pending_size >= s->threshold_size become false in migration_iteration_run
> after the first migration_update_counters.
> 
> Signed-off-by: Ivan Ren <ivanren@tencent.com>
> Reviewed-by: Juan Quintela <quintela@redhat.com>
> Suggested-by: Wei Yang <richardw.yang@linux.intel.com>

Thank you,

Queued

> ---
> v2->v3:
> - fix the bug of update_iteration_initial_status function prototype
> 
> v1->v2:
> - Add interface update_iteration_initial_status to update statistic fields
>   at the same time to avoid info mismatch lead wrong calculation result.
> 
>  migration/migration.c | 25 +++++++++++++++++++------
>  migration/savevm.c    |  1 +
>  2 files changed, 20 insertions(+), 6 deletions(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index 8a607fe1e2..bea9b1d796 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -1908,6 +1908,11 @@ static bool migrate_prepare(MigrationState *s, bool blk, bool blk_inc,
>      }
>  
>      migrate_init(s);
> +    /*
> +     * set ram_counters memory to zero for a
> +     * new migration
> +     */
> +    memset(&ram_counters, 0, sizeof(ram_counters));
>  
>      return true;
>  }
> @@ -3025,6 +3030,17 @@ static void migration_calculate_complete(MigrationState *s)
>      }
>  }
>  
> +static void update_iteration_initial_status(MigrationState *s)
> +{
> +    /*
> +     * Update these three fields at the same time to avoid mismatch info lead
> +     * wrong speed calculation.
> +     */
> +    s->iteration_start_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
> +    s->iteration_initial_bytes = migration_total_bytes(s);
> +    s->iteration_initial_pages = ram_get_total_transferred_pages();
> +}
> +
>  static void migration_update_counters(MigrationState *s,
>                                        int64_t current_time)
>  {
> @@ -3060,9 +3076,7 @@ static void migration_update_counters(MigrationState *s,
>  
>      qemu_file_reset_rate_limit(s->to_dst_file);
>  
> -    s->iteration_start_time = current_time;
> -    s->iteration_initial_bytes = current_bytes;
> -    s->iteration_initial_pages = ram_get_total_transferred_pages();
> +    update_iteration_initial_status(s);
>  
>      trace_migrate_transferred(transferred, time_spent,
>                                bandwidth, s->threshold_size);
> @@ -3186,7 +3200,7 @@ static void *migration_thread(void *opaque)
>      rcu_register_thread();
>  
>      object_ref(OBJECT(s));
> -    s->iteration_start_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
> +    update_iteration_initial_status(s);
>  
>      qemu_savevm_state_header(s->to_dst_file);
>  
> @@ -3251,8 +3265,7 @@ static void *migration_thread(void *opaque)
>               * the local variables. This is important to avoid
>               * breaking transferred_bytes and bandwidth calculation
>               */
> -            s->iteration_start_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
> -            s->iteration_initial_bytes = 0;
> +            update_iteration_initial_status(s);
>          }
>  
>          current_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
> diff --git a/migration/savevm.c b/migration/savevm.c
> index 79ed44d475..480c511b19 100644
> --- a/migration/savevm.c
> +++ b/migration/savevm.c
> @@ -1424,6 +1424,7 @@ static int qemu_savevm_state(QEMUFile *f, Error **errp)
>      }
>  
>      migrate_init(ms);
> +    memset(&ram_counters, 0, sizeof(ram_counters));
>      ms->to_dst_file = f;
>  
>      qemu_mutex_unlock_iothread();
> -- 
> 2.17.2 (Apple Git-113)
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK