[PATCH] cpu-throttle: Remove timer_mod() from cpu_throttle_set()

[Utkarsh Tripathi <utkarsh.tripathi@nutanix.com>]

During migrations, after each iteration, cpu_throttle_set() is called,
which irrespective of input, re-arms the timer according to value of
new_throttle_pct. This causes cpu_throttle_thread() to be delayed in
getting scheduled and consqeuntly lets guest run for more time than what
the throttle value should allow. This leads to spikes in guest throughput
at high cpu-throttle percentage whenever cpu_throttle_set() is called.

A solution would be not to modify the timer immediately in
cpu_throttle_set(), instead, only modify throttle_percentage so that the
throttle would automatically adjust to the required percentage when
cpu_throttle_timer_tick() is invoked.

Manually tested the patch using following configuration:

Guest:
Centos7 (3.10.0-123.el7.x86_64)
Total Memory - 64GB , CPUs - 16
Tool used - stress (1.0.4)
Workload - stress --vm 32 --vm-bytes 1G --vm-keep

Migration Parameters:
Network Bandwidth - 500MBPS
cpu-throttle-initial - 99

Results:
With timer_mod(): fails to converge, continues indefinitely
Without timer_mod(): converges in 249 sec

Signed-off-by: Utkarsh Tripathi <utkarsh.tripathi@nutanix.com>
---
 softmmu/cpu-throttle.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/softmmu/cpu-throttle.c b/softmmu/cpu-throttle.c
index 2ec4b8e..8c2144a 100644
--- a/softmmu/cpu-throttle.c
+++ b/softmmu/cpu-throttle.c
@@ -90,14 +90,21 @@ static void cpu_throttle_timer_tick(void *opaque)
 
 void cpu_throttle_set(int new_throttle_pct)
 {
+    /*
+     * boolean to store whether throttle is already active or not,
+     * before modifying throttle_percentage
+     */
+    bool throttle_active = cpu_throttle_active();
+
     /* Ensure throttle percentage is within valid range */
     new_throttle_pct = MIN(new_throttle_pct, CPU_THROTTLE_PCT_MAX);
     new_throttle_pct = MAX(new_throttle_pct, CPU_THROTTLE_PCT_MIN);
 
     qatomic_set(&throttle_percentage, new_throttle_pct);
 
-    timer_mod(throttle_timer, qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL_RT) +
-                                       CPU_THROTTLE_TIMESLICE_NS);
+    if (!throttle_active) {
+        cpu_throttle_timer_tick(NULL);
+    }
 }
 
 void cpu_throttle_stop(void)
-- 
1.8.3.1