Re: BUG: sched:sched_migrate_task event tracing returns incorrectevents

From: Ronny TschÃter
Date: Mon Jun 14 2010 - 03:45:14 EST

Am 10.06.2010 17:11, schrieb Peter Zijlstra:
On Thu, 2010-06-10 at 16:56 +0200, Ronny TschÃter wrote:

I want to know a little bit more about the execution of my program.
Especially I'm interested in cpu migrations of my process. Therefore I
use the Linux performance counter infrastructure, set appropriate
perf_event attributes and setup the sched:sched_migrate_task event with
a system call:
attr.size = sizeof( struct perf_event_attr );
attr.config = event_id; // id of sched:sched_migrate_task event
attr.freq = 0;
attr.sample_freq = 1;
fd = syscall( __NR_perf_counter_open,&attr, getpid(), -1, -1, 0 );
Finally to read the events I map the kernel event buffer via mmap and
process every event in this buffer. However I run into two problems:

(1) I need root privileges to open the counter successfully. Without
root privileges syscall returns -1. I think that is not OK because users
should be allowed to create per task counters.
It is, because you specify PERF_SAMPLE_RAW, which is considered a data
leak for unpriv users.

(2) The buffer contains events that aren't caused by my process (i.e.
the COMM field does not contain the name of my process and the PID field
is not equal to the pid of my process).
I think that could happen if the migration happens from interrupt
context while your task is running. Also wakeups your process does can
result in migrations of the woken task from the context of your task.
What I need is a counter that trace every scheduling event (at least every cpu migration event) of a given process. Is there such a counter in the perf_events or does all counters come with this weakness? At the moment I open one counter per cpu, trace all scheduling events on this cpu and filter the events manually. But this approach is not feasible for MPI programs, because the number of required counters will increase enormously ( #processes * #cpus counters are needed ).

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at