accuarcy of function perf_get_mcycle64()

Hello! I have some questions about function perf_get_mcycle64().
There exists some difference in the total cycles if I use perf_counters in contrast to not using them.
The following shows the result without perf_counter when running KWS model.
```
Running kws
.............
"Event","Tag","Ticks"
0,CONV_2D,18426
1,DEPTHWISE_CONV_2D,4340
2,CONV_2D,12430
3,DEPTHWISE_CONV_2D,4290
4,CONV_2D,11224
5,DEPTHWISE_CONV_2D,4094
6,CONV_2D,12518
7,DEPTHWISE_CONV_2D,4397
8,CONV_2D,11369
9,AVERAGE_POOL_2D,90
10,RESHAPE,2
11,FULLY_CONNECTED,18
12,SOFTMAX,12
Counter | Total | Starts | Average | Raw
---------+--------+--------+---------+--------------
0 | 0 | 0 | n/a | 0
1 | 0 | 0 | n/a | 0
2 | 0 | 0 | n/a | 0
3 | 0 | 0 | n/a | 0
4 | 0 | 0 | n/a | 0
5 | 0 | 0 | n/a | 0
6 | 0 | 0 | n/a | 0
7 | 0 | 0 | n/a | 0
85M ( 85231808 ) cycles total
```
However, if I add `perf_enable_counter` and `perf_disable_counter` in `conv.h` to use some perf_counters, the total cycles change.
```
Running kws
.............
"Event","Tag","Ticks"
0,CONV_2D,21499
1,DEPTHWISE_CONV_2D,4239
2,CONV_2D,9217
3,DEPTHWISE_CONV_2D,4193
4,CONV_2D,8028
5,DEPTHWISE_CONV_2D,3992
6,CONV_2D,8771
7,DEPTHWISE_CONV_2D,4709
8,CONV_2D,8350
9,AVERAGE_POOL_2D,90
10,RESHAPE,2
11,FULLY_CONNECTED,18
12,SOFTMAX,14
Counter | Total | Starts | Average | Raw
---------+--------+--------+---------+--------------
0 | 57M | 5 | 11M | 57177105
1 | 33M | 302720 | 108 | 32978671
2 | 0 | 0 | n/a | 0
3 | 0 | 0 | n/a | 0
4 | 0 | 0 | n/a | 0
5 | 0 | 0 | n/a | 0
6 | 0 | 0 | n/a | 0
7 | 0 | 0 | n/a | 0
75M ( 74897487 ) cycles total
```
It's strange that total cycles reduce a lot, I don't think it's due to error because the difference is so huge.
I know the total cycles are counted by using function `perf_get_mcycle64()`, which I think will not be affected by whether to use perf_counters.
Thanks in advance!