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Subject[REGRESSION 2.6.30][PATCH 0/1] sched: defer idle accounting till after load update period
Date
The following patch fixes a load avg calculation bug. Details are included with
the patch. Essentially, a task that often runs for less than 10 ticks at a time
is likely to be left out of the load avg calculation. A test case is provided
below. If you run the test case on a near zero-load system you will find top
report 90% cpu usage while the load avg stays at or near 0.00. With the patch,
the load avg is calculated correctly to be at least 0.90.

This is a regression since 2.6.30. The offending commit is:
dce48a84adf1806676319f6f480e30a6daa012f9.

--

#include <asm/param.h>
#include <sys/time.h>
#include <time.h>

int main() {
struct timespec ts;
ts.tv_sec = 0;
ts.tv_nsec = 1000000000 / HZ;
/*
* Run gettimeofday in a tight loop 9 ticks, then sleep for 1 tick
*/
while (1) {
struct timeval tv;
do {
gettimeofday(&tv, NULL);
} while ((tv.tv_usec * HZ / 1000000) % 10 != 0);
nanosleep(&ts, NULL);
}
return 0;
}

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