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Subjectscheduler / perf stat question about CPU-migrations
From
Can someone tell me how I'm being confused?

I ran the following command as root:

perf stat schedtool -a 1 -e e2fsck -ft /dev/funarg/kbuild

This runs e2fsck under perf, with the cpu affinity nailed to a single
CPU. I therefore expected the CPU-migrations field reported by perf to
be 0. That was not what I found, though:

Performance counter stats for 'schedtool -a 1 -e e2fsck -ft /dev/funarg/kbuild':

1169.715766 task-clock-msecs # 0.180 CPUs
9212 context-switches # 0.008 M/sec
307 CPU-migrations # 0.000 M/sec
1875 page-faults # 0.002 M/sec
2737168498 cycles # 2340.029 M/sec
3125632038 instructions # 1.142 IPC
688556730 branches # 588.653 M/sec
7263580 branch-misses # 1.055 %
15222417 cache-references # 13.014 M/sec
1488633 cache-misses # 1.273 M/sec

6.481483548 seconds time elapsed

How could this be? The CPU-migrations event counter only gets
incremented if a task changes CPU's, as seen in kernel/sched.c:

if (task_cpu(p) != new_cpu) {
p->se.nr_migrations++;
perf_sw_event(PERF_COUNT_SW_CPU_MIGRATIONS, 1, 1, NULL, 0);
}

So it should be 0, not 307, right? What am I missing?

- Ted


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