linux-kernel - [PATCH resend] Reading POSIX CPU timer from outside the process.

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Date:	Tue, 28 Dec 2010 11:55:42 +0100
From:	Dario Faggioli <raistlin@...ux.it>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	torbenh <torbenh@....de>, oleg <oleg@...hat.com>,
	john.stultz@...aro.org, roland@...hat.com,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Stanislaw Gruszka <sgruszka@...hat.com>,
	Dhaval Giani <dhaval.giani@...il.com>,
	Randy Dunlap <rdunlap@...otime.net>
Subject: [PATCH resend] Reading POSIX CPU timer from outside the process.

Trying to call clock_getcpuclockid (and then clock_gettime) on a thread
from outside the process that spawned it results in this:
  ### Testing tid 24207: CPU-time clock for PID 24207 is 1.132371729 seconds
  ### Testing tid 24209: clock_getcpuclockid: Success

OTOH, if full-fledged processes are involved, the behaviour is this:
  ### Testing tid 24218: CPU-time clock for PID 24218 is 0.001059305 seconds
  ### Testing tid 24220: CPU-time clock for PID 24220 is 1.044057391 seconds

Test programs available here: http://gitorious.org/clockid

All that because clock_getcpuclockid forbids accessing thread
specific CPU-time clocks from outside the thread group, but this is not
requested (e.g., by POSIX), or at least no indication that it should fail
can be found in `man clock_getcpuclockid' and alike.
Moreover, having such capability could be useful, if you want to monitor
the execution of a bunch of thread from some kind of "manager" which might
not be part of the same process. A typical example is the JACK graph-manager.

Therefore, this commit makes clock_getcpuclockid behave as follows:
 - if it's called on a tid which is also a PID (i.e., the thread is
   a thread group leader), it returns the CLOCK_PROCESS_CPUTIME_ID of
   the process;
 - if it's called on a tid of a non-group leader, it returns the
   CLOCK_THREAD_CPUTIME_ID of such specific thread.

This enables the following behaviour, for the threaded and process-based
scenarios, respectively:

  ### Testing tid 24704: CPU-time clock for PID 24704 is 1.049570008 seconds
  ### Testing tid 24706: CPU-time clock for PID 24706 is 1.028650801 seconds

  ### Testing tid 24715: CPU-time clock for PID 24715 is 0.000957685 seconds
  ### Testing tid 24717: CPU-time clock for PID 24717 is 1.045351509 seconds

Signed-off-by: Dario Faggioli <raistlin@...ux.it>
---
 kernel/posix-cpu-timers.c |   13 +++++--------
 1 files changed, 5 insertions(+), 8 deletions(-)

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 05bb717..ddbbabc 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -39,10 +39,9 @@ static int check_clock(const clockid_t which_clock)
 
 	rcu_read_lock();
 	p = find_task_by_vpid(pid);
-	if (!p || !(CPUCLOCK_PERTHREAD(which_clock) ?
-		   same_thread_group(p, current) : has_group_leader_pid(p))) {
+	if (!p || (CPUCLOCK_PERTHREAD(which_clock) &&
+		   same_thread_group(p, current) && !has_group_leader_pid(p)))
 		error = -EINVAL;
-	}
 	rcu_read_unlock();
 
 	return error;
@@ -349,11 +348,9 @@ int posix_cpu_clock_get(const clockid_t which_clock, struct timespec *tp)
 		rcu_read_lock();
 		p = find_task_by_vpid(pid);
 		if (p) {
-			if (CPUCLOCK_PERTHREAD(which_clock)) {
-				if (same_thread_group(p, current)) {
-					error = cpu_clock_sample(which_clock,
-								 p, &rtn);
-				}
+			if (CPUCLOCK_PERTHREAD(which_clock) ||
+			    !thread_group_leader(p)) {
+				error = cpu_clock_sample(which_clock, p, &rtn);
 			} else {
 				read_lock(&tasklist_lock);
 				if (thread_group_leader(p) && p->sighand) {
-- 
1.7.2.3


-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
----------------------------------------------------------------------
Dario Faggioli, ReTiS Lab, Scuola Superiore Sant'Anna, Pisa  (Italy)

http://retis.sssup.it/people/faggioli -- dario.faggioli@...ber.org

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