Re: [Qemu-devel] [PATCH v4 0/3] High downtime with 95+ throttle pct

[Paolo Bonzini]

On 15/08/19 11:13, Yury Kotov wrote:
> Ping ping

Hi,

sorry for the delay, I was waiting for the 4.1 release.

I would like to make a small change so that preemption of QEMU does not
result in overly long sleeps.  The following patch on top of yours computes
the throttle-end time just once.  Of course you can still be unlucky if
you are preempted at the wrong time, but the window becomes much smaller.

diff --git a/cpus.c b/cpus.c
index d091dbd..d7e2145 100644
--- a/cpus.c
+++ b/cpus.c
@@ -781,7 +781,7 @@ static void cpu_throttle_thread(CPUState *cpu, 
run_on_cpu_data opaque)
 {
     double pct;
     double throttle_ratio;
-    int64_t sleeptime_ns;
+    int64_t sleeptime_ns, end;
 
     if (!cpu_throttle_get_percentage()) {
         return;
@@ -792,18 +792,17 @@ static void cpu_throttle_thread(CPUState *cpu, 
run_on_cpu_data opaque)
     /* 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();
+    end = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) + sleeptime_ns;
+    while (sleeptime_ns > 0 && !cpu->stop) {
+        if (sleeptime_ns > SCALE_MS) {
+            qemu_cond_timedwait(cpu->halt_cond, &qemu_global_mutex,
+                                sleeptime_ns / SCALE_MS);
+        } else {
+            qemu_mutex_unlock_iothread();
+            g_usleep(sleeptime_ns / SCALE_US);
+            qemu_mutex_lock_iothread();
+        }
+        sleeptime_ns = end - qemu_clock_get_ns(QEMU_CLOCK_REALTIME);
     }
     atomic_set(&cpu->throttle_thread_scheduled, 0);
 }