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From: Ronny Tschüter on 10 Jun 2010 11:00
Hi,

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.type = PERF_TYPE_TRACEPOINT;
attr.size = sizeof( struct perf_event_attr );
attr.config = event_id; // id of sched:sched_migrate_task event
attr.sample_type = PERF_SAMPLE_TIME|PERF_SAMPLE_CPU|PERF_SAMPLE_RAW;
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.

(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).

Some information about my system:
Processor: Core 2 Duo E6750
Kernel information: Linux version 2.6.34-100610-1530 (gcc version 4.4.3
(Ubuntu 4.4.3-4ubuntu5) ) #1 SMP

This behaviour could also be observed in tests with
- Linux version 2.6.33.3 (gcc version 4.4.1 (Ubuntu 4.4.1-4ubuntu9) ) #1 SMP
- Linux version 2.6.32-22-generic (buildd(a)yellow) (gcc version 4.4.3
(Ubuntu 4.4.3-4ubuntu5) ) #36-Ubuntu SMP (Ubuntu 10.04)


Bye, Ronny Tschueter

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