Messages in this thread | | | Date | Thu, 16 Jun 2011 10:46:26 -0400 | Subject | scheduler / perf stat question about CPU-migrations | From | "Theodore Ts'o" <> |
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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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