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

[Wei Yang <richardw.yang@linux.intel.com>]

On Tue, Jul 30, 2019 at 01:36:32PM +0800, Ivan Ren 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>
>---
> migration/migration.c | 15 ++++++++++++++-
> migration/savevm.c    |  1 +
> 2 files changed, 15 insertions(+), 1 deletion(-)
>
>diff --git a/migration/migration.c b/migration/migration.c
>index 8a607fe1e2..d35a6ae6f9 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;
> }
>@@ -3187,6 +3192,10 @@ static void *migration_thread(void *opaque)
> 
>     object_ref(OBJECT(s));
>     s->iteration_start_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
>+    /*
>+     * Update s->iteration_initial_bytes to match s->iteration_start_time.
>+     */
>+    s->iteration_initial_bytes = migration_total_bytes(s);

Is this one necessary? We have sent out nothing yet.

> 
>     qemu_savevm_state_header(s->to_dst_file);
> 
>@@ -3252,7 +3261,11 @@ static void *migration_thread(void *opaque)
>              * breaking transferred_bytes and bandwidth calculation
>              */
>             s->iteration_start_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
>-            s->iteration_initial_bytes = 0;
>+            /*
>+             * Update s->iteration_initial_bytes to current size to
>+             * avoid historical data lead wrong bandwidth.
>+             */
>+            s->iteration_initial_bytes = migration_total_bytes(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)
>

-- 
Wei Yang
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