3rd-party/linux-kernel-module-cheat
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readme: document rdtsc

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Ciro Santilli
2018-04-15 23:23:40 +01:00
parent 6eae8bb052
commit 24cdcdc756
3 changed files with 37 additions and 11 deletions
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README.adoc
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@@ -327,6 +327,7 @@ printk debug
....
head kernel_module/modulename.c
grep 'modulename\.' README.adoc
....
Many of the modules have userland test scripts / executables with the same name as the module, e.g. form inside the guest:
@@ -3357,7 +3358,7 @@ and also: `gem5-dist`: https://publish.illinois.edu/icsl-pdgem5/
Clock frequency: TODO how does it affect performance in benchmarks?
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./run -a arm -g -- --cpu-clock 10000000
./run -a aarch64 -g -- --cpu-clock 10000000
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Check with:
@@ -3369,7 +3370,7 @@ m5 resetstats && sleep 10 && m5 dumpstats
and then:
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grep numCycles out/aarch64/gem5/m5out/stats.txt
./gem5-stat -a aarch64
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TODO: why doesn't this exist:
@@ -4163,6 +4164,39 @@ chmoe +x /tmp/execfile
m5 execfile
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=== gem5 stats
Lets try to understand some stats better.
==== rdtsc
....
./build -kg && ./run -E '/rdtsc.out;m5 exit;' -g
./gem5-stat
....
gives cycle counts:
* `3828578153`
* `3830832635`
Which are pretty close, and serve as a nice sanity check that the cycle counter is coherent.
It is also nice to see that `rdtsc` is a bit smaller than the `stats.txt` value, since the latter also includes the exec syscall for `m5`.
See also:
* https://en.wikipedia.org/wiki/Time_Stamp_Counter
* https://stackoverflow.com/questions/9887839/clock-cycle-count-wth-gcc/9887979
===== pmccntr kernel module
Unfortunately-we didn't manage to find an ARM analogue: link:kernel_module/pmccntr.c[] is oopsing, and even it if weren't, it likely won't give the cycle count since boot since it needs to be activate before it starts counting anything:
* https://stackoverflow.com/questions/40454157/is-there-an-equivalent-instruction-to-rdtsc-in-arm
* https://stackoverflow.com/questions/31620375/arm-cortex-a7-returning-pmccntr-0-in-kernel-mode-and-illegal-instruction-in-u/31649809#31649809
* https://blog.regehr.org/archives/794
=== gem5 limitations
* networking not working. We currently just disable it from `inittab` by default to prevent waiting at startup