[PATCH v3 3/4] migration: Support periodic ramblock dirty sync

[yong.huang@smartx.com]

From: Hyman Huang <yong.huang@smartx.com>

When VM is configured with huge memory, the current throttle logic
doesn't look like to scale, because migration_trigger_throttle()
is only called for each iteration, so it won't be invoked for a long
time if one iteration can take a long time.

The periodic dirty sync aims to fix the above issue by synchronizing
the ramblock from remote dirty bitmap and, when necessary, triggering
the CPU throttle multiple times during a long iteration.

This is a trade-off between synchronization overhead and CPU throttle
impact.

Signed-off-by: Hyman Huang <yong.huang@smartx.com>
---
 migration/cpu-throttle.c | 70 +++++++++++++++++++++++++++++++++++++++-
 migration/cpu-throttle.h | 14 ++++++++
 migration/migration.h    |  1 +
 migration/ram.c          |  9 ++++--
 migration/trace-events   |  1 +
 5 files changed, 92 insertions(+), 3 deletions(-)

diff --git a/migration/cpu-throttle.c b/migration/cpu-throttle.c
index fa47ee2e21..784b51ab35 100644
--- a/migration/cpu-throttle.c
+++ b/migration/cpu-throttle.c
@@ -28,16 +28,23 @@
 #include "qemu/main-loop.h"
 #include "sysemu/cpus.h"
 #include "cpu-throttle.h"
+#include "migration.h"
+#include "migration-stats.h"
+#include "options.h"
 #include "trace.h"
 
 /* vcpu throttling controls */
-static QEMUTimer *throttle_timer;
+static QEMUTimer *throttle_timer, *throttle_dirty_sync_timer;
 static unsigned int throttle_percentage;
+static bool throttle_dirty_sync_timer_active;
 
 #define CPU_THROTTLE_PCT_MIN 1
 #define CPU_THROTTLE_PCT_MAX 99
 #define CPU_THROTTLE_TIMESLICE_NS 10000000
 
+/* RAMBlock dirty sync trigger every five seconds */
+#define CPU_THROTTLE_DIRTY_SYNC_TIMESLICE_MS 5000
+
 static void cpu_throttle_thread(CPUState *cpu, run_on_cpu_data opaque)
 {
     double pct;
@@ -112,6 +119,7 @@ void cpu_throttle_set(int new_throttle_pct)
 void cpu_throttle_stop(void)
 {
     qatomic_set(&throttle_percentage, 0);
+    cpu_throttle_dirty_sync_timer(false);
 }
 
 bool cpu_throttle_active(void)
@@ -124,8 +132,68 @@ int cpu_throttle_get_percentage(void)
     return qatomic_read(&throttle_percentage);
 }
 
+void cpu_throttle_dirty_sync_timer_tick(void *opaque)
+{
+    static uint64_t prev_sync_cnt = 2;
+    uint64_t sync_cnt = stat64_get(&mig_stats.dirty_sync_count);
+
+    if (!migrate_auto_converge()) {
+        /* Stop the timer when auto converge is disabled */
+        return;
+    }
+
+    /*
+     * The first iteration copies all memory anyhow and has no
+     * effect on guest performance, therefore omit it to avoid
+     * paying extra for the sync penalty.
+     */
+    if (sync_cnt <= 1) {
+        goto end;
+    }
+
+    if (sync_cnt == prev_sync_cnt) {
+        trace_cpu_throttle_dirty_sync();
+        WITH_RCU_READ_LOCK_GUARD() {
+            migration_bitmap_sync_precopy(false);
+        }
+    }
+
+end:
+    prev_sync_cnt = stat64_get(&mig_stats.dirty_sync_count);
+
+    timer_mod(throttle_dirty_sync_timer,
+        qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL_RT) +
+            CPU_THROTTLE_DIRTY_SYNC_TIMESLICE_MS);
+}
+
+static bool cpu_throttle_dirty_sync_active(void)
+{
+    return qatomic_read(&throttle_dirty_sync_timer_active);
+}
+
+void cpu_throttle_dirty_sync_timer(bool enable)
+{
+    if (enable) {
+        assert(throttle_dirty_sync_timer);
+        if (!cpu_throttle_dirty_sync_active()) {
+            timer_mod(throttle_dirty_sync_timer,
+                qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL_RT) +
+                    CPU_THROTTLE_DIRTY_SYNC_TIMESLICE_MS);
+            qatomic_set(&throttle_dirty_sync_timer_active, 1);
+        }
+    } else {
+        if (throttle_dirty_sync_timer != NULL) {
+            timer_del(throttle_dirty_sync_timer);
+            qatomic_set(&throttle_dirty_sync_timer_active, 0);
+        }
+    }
+}
+
 void cpu_throttle_init(void)
 {
     throttle_timer = timer_new_ns(QEMU_CLOCK_VIRTUAL_RT,
                                   cpu_throttle_timer_tick, NULL);
+    throttle_dirty_sync_timer =
+        timer_new_ms(QEMU_CLOCK_VIRTUAL_RT,
+                     cpu_throttle_dirty_sync_timer_tick, NULL);
 }
diff --git a/migration/cpu-throttle.h b/migration/cpu-throttle.h
index d65bdef6d0..420702b8d3 100644
--- a/migration/cpu-throttle.h
+++ b/migration/cpu-throttle.h
@@ -65,4 +65,18 @@ bool cpu_throttle_active(void);
  */
 int cpu_throttle_get_percentage(void);
 
+/**
+ * cpu_throttle_dirty_sync_timer_tick:
+ *
+ * Dirty sync timer hook.
+ */
+void cpu_throttle_dirty_sync_timer_tick(void *opaque);
+
+/**
+ * cpu_throttle_dirty_sync_timer:
+ *
+ * Start or stop the dirty sync timer.
+ */
+void cpu_throttle_dirty_sync_timer(bool enable);
+
 #endif /* SYSEMU_CPU_THROTTLE_H */
diff --git a/migration/migration.h b/migration/migration.h
index 38aa1402d5..fbd0d19092 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -537,4 +537,5 @@ int migration_rp_wait(MigrationState *s);
  */
 void migration_rp_kick(MigrationState *s);
 
+void migration_bitmap_sync_precopy(bool last_stage);
 #endif
diff --git a/migration/ram.c b/migration/ram.c
index 9b5b350405..ac34e731e2 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1020,6 +1020,11 @@ static void migration_trigger_throttle(RAMState *rs)
         migration_transferred_bytes() - rs->bytes_xfer_prev;
     uint64_t bytes_dirty_period = rs->num_dirty_pages_period * TARGET_PAGE_SIZE;
     uint64_t bytes_dirty_threshold = bytes_xfer_period * threshold / 100;
+    bool auto_converge = migrate_auto_converge();
+
+    if (auto_converge) {
+        cpu_throttle_dirty_sync_timer(true);
+    }
 
     /*
      * The following detection logic can be refined later. For now:
@@ -1031,7 +1036,7 @@ static void migration_trigger_throttle(RAMState *rs)
     if ((bytes_dirty_period > bytes_dirty_threshold) &&
         (++rs->dirty_rate_high_cnt >= 2)) {
         rs->dirty_rate_high_cnt = 0;
-        if (migrate_auto_converge()) {
+        if (auto_converge) {
             trace_migration_throttle();
             mig_throttle_guest_down(bytes_dirty_period,
                                     bytes_dirty_threshold);
@@ -1088,7 +1093,7 @@ static void migration_bitmap_sync(RAMState *rs, bool last_stage)
     }
 }
 
-static void migration_bitmap_sync_precopy(bool last_stage)
+void migration_bitmap_sync_precopy(bool last_stage)
 {
     Error *local_err = NULL;
     assert(ram_state);
diff --git a/migration/trace-events b/migration/trace-events
index 9a19599804..0638183056 100644
--- a/migration/trace-events
+++ b/migration/trace-events
@@ -381,3 +381,4 @@ migration_pagecache_insert(void) "Error allocating page"
 
 # cpu-throttle.c
 cpu_throttle_set(int new_throttle_pct)  "set guest CPU throttled by %d%%"
+cpu_throttle_dirty_sync(void) ""
-- 
2.27.0