program execution time in ARM Cortex-A8 processor

https://stackoverflow.com/questions/14158475

13-01-2022
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Question

I'm using ARM Cortex-A8 and trying to read the value from CCNT time counter through the assembly code. I am following this post How to measure program execution time in ARM Cortex-A8 processor? . In accordance with it, before I can read the value from timer, I have to enable counter, enable a 64-bit divider and clear overflows. These operations are performed by writing inside the appropriate registers (for instance, PMCR (Performance Monitro Control Register)). So, I am printed counter values in a loop to keep track how overflow occurs and I have this behavior:

1           (starts to incrementing after it was reset to zero)
4650
4858
4943
5023
...
...        (incrementing...)
...
4293939054
4293939128       (overflow happens)
1602570          
1602703
1602788
...
...
4293522911
4293522987
4293523062
4293523137
1186243
1186367
1186453
1186536
1186612
1186686
...
4293536300
4293536377
4293536456
4293536533
4293536612
1199090
1199209
1199295
1199373
1199453
1199530
….
and so forth.

Accordingly, I have a set of questions:

a) Which or the said above registers are used by the Linux kernel ? (how reliable is the information for further kernel versions). How safe can be the change of their values?

b) what is the accurate value of CCNT frequency and how to get it? Unfortunately, I can’t find the value in processor spec. However, dmesg says that

[ 0.000000] OMAP clocksource: GPTIMER2 at 24000000 Hz
[ 0.000000] sched_clock: 32 bits at 24MHz, resolution 41ns, wraps every 178956ms
[ 0.132855] Switching to clocksource gp timer

But identifying it manually, against the clock_gettime, gives me 7 MHz. So, why it is not 24 MHz as expected?

c) According to my first output, why after the overflow it starts not with zero, but from about 1 mil ?

d) Why without 64 divider am I getting wrong results? The value starts to jump this way:

I'd appreciate any help. Thanks

Solution

a) PMU regs may be used by the perf subsystem of the Linux kernel (accessed through the perf userspace tool).

b) CCNT frequency is a Cortex-A9 CPU cycle counter, or cycles/64 if you enable the divider. So 7MHz with the divider would be an average CPU clock of around 450 MHz. This is separate from the 24 MHz system clock.

c) maybe your process got scheduled out. This is a low level cycle counter for the whole CPU, not just your process. It will keep running when in the kernel or in another process. On the other hand if your process migrates to another CPU you will then access that CPU's cycle counter (which might not even have the same divider setting). If you want a consistent count you should be pinning your process to one CPU.

d) similar answer to (c), you may be seeing the effect of process scheduling and migration.

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