[Qemu-devel] [RFC PATCH 2/2] cpus: Fix throttling during vm_stop

[Yury Kotov]

Throttling thread sleeps in VCPU thread. For high throttle percentage
this sleep is more than 10ms. E.g. for 60% - 15ms, for 99% - 990ms.
vm_stop() kicks all VCPUs and waits for them. It's called at the end of
migration and because of the long sleep the migration downtime might be
more than 100ms even for downtime-limit 1ms.
Use qemu_cond_timedwait for high percentage to wake up during vm_stop.

Signed-off-by: Yury Kotov <address@hidden>
---
 cpus.c | 27 +++++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/cpus.c b/cpus.c
index ffc57119ca..3c069cdc33 100644
--- a/cpus.c
+++ b/cpus.c
@@ -74,6 +74,8 @@
 
 #endif /* CONFIG_LINUX */
 
+static QemuMutex qemu_global_mutex;
+
 int64_t max_delay;
 int64_t max_advance;
 
@@ -776,7 +778,7 @@ static void cpu_throttle_thread(CPUState *cpu, 
run_on_cpu_data opaque)
 {
     double pct;
     double throttle_ratio;
-    long sleeptime_ns;
+    int64_t sleeptime_ns;
 
     if (!cpu_throttle_get_percentage()) {
         return;
@@ -784,11 +786,22 @@ static void cpu_throttle_thread(CPUState *cpu, 
run_on_cpu_data opaque)
 
     pct = (double)cpu_throttle_get_percentage()/100;
     throttle_ratio = pct / (1 - pct);
-    sleeptime_ns = (long)(throttle_ratio * CPU_THROTTLE_TIMESLICE_NS);
-
-    qemu_mutex_unlock_iothread();
-    g_usleep(sleeptime_ns / 1000); /* Convert ns to us for usleep call */
-    qemu_mutex_lock_iothread();
+    /* Add 1ns to fix double's rounding error (like 0.9999999...) */
+    sleeptime_ns = (int64_t)(throttle_ratio * CPU_THROTTLE_TIMESLICE_NS + 1);
+
+    while (sleeptime_ns >= SCALE_MS && !cpu->stop) {
+        int64_t start, end;
+        start = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
+        qemu_cond_timedwait(cpu->halt_cond, &qemu_global_mutex,
+                            sleeptime_ns / SCALE_MS);
+        end = qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
+        sleeptime_ns -= end - start;
+    }
+    if (sleeptime_ns >= SCALE_US && !cpu->stop) {
+        qemu_mutex_unlock_iothread();
+        g_usleep(sleeptime_ns / SCALE_US);
+        qemu_mutex_lock_iothread();
+    }
     atomic_set(&cpu->throttle_thread_scheduled, 0);
 }
 
@@ -1166,8 +1179,6 @@ static void qemu_init_sigbus(void)
 }
 #endif /* !CONFIG_LINUX */
 
-static QemuMutex qemu_global_mutex;
-
 static QemuThread io_thread;
 
 /* cpu creation */
-- 
2.22.0