I'm planning to use QueryThreadCycleTime() to in order to calculate the percentage of CPU some specific thread has used during a given interval (between two samples).
The main issue is I need to know how many CPU cycles (in total) have elapsed between the two samples :
QueryThreadCycleTime(threadId, &threadCycleTime);
QueryTotalCPUCycleTime(&totalCPUTime); //does not exists
cpuUsage = (threadCycleTime - previousThreadCycleTime) / (totalCPUTime - previousCPUTime) * 100;
previousThreadCycleTime = threadCycleTime;
previousCPUTime= totalCPUTime;
One possibility would be to loop on all threads and sum up their cycle time (based on difference with previous sample), but the major issue is not all of can be opened (additionally, most systems have thousands of threads).
Another is to read RDTSC. Unfortunately this will only return the amount of CPU cycles for the current CPU, not all.
I don't want to use GetThreadTimes() because (based on my own experiments and what I read about it) it does not report any CPU usage unless threads as used at least one quantum (15 ms on my system). If a thread use CPU sporadically (eg: goes into wait state before quantum has elapsed) nothing will be reported.