cpus: reset throttle_thread_scheduled after sleep

Currently, the throttle_thread_scheduled flag is reset back to 0 before
sleeping (as part of the throttling logic). Given that throttle_timer
(well, any timer) may tick with a slight delay, it so happens that under
heavy throttling (ie. close or on CPU_THROTTLE_PCT_MAX) the tick may
schedule a further cpu_throttle_thread() work item after the flag reset,
but before the previous sleep completed. This results on the vCPU thread
sleeping continuously for potentially several seconds in a row.

The chances of that happening can be drastically minimised by resetting
the flag after the sleep.
Signed-off-by: NFelipe Franciosi <felipe@nutanix.com>
Signed-off-by: NMalcolm Crossley <malcolm@nutanix.com>
Message-Id: <1495229390-18909-1-git-send-email-felipe@nutanix.com>
Acked-by: NJason J. Herne <jjherne@linux.vnet.ibm.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

cpus.c

@@ -677,9 +677,9 @@ static void cpu_throttle_thread(CPUState *cpu, run_on_cpu_data opaque)
     sleeptime_ns = (long)(throttle_ratio * CPU_THROTTLE_TIMESLICE_NS);
 
     qemu_mutex_unlock_iothread();
-    atomic_set(&cpu->throttle_thread_scheduled, 0);
     g_usleep(sleeptime_ns / 1000); /* Convert ns to us for usleep call */
     qemu_mutex_lock_iothread();
+    atomic_set(&cpu->throttle_thread_scheduled, 0);
 }
 
 static void cpu_throttle_timer_tick(void *opaque)