[PATCH] Avoid process cputimer state oscillation

[Olivier Langlois <olivier@trillion01.com>]

When a periodic process timer is fired, a signal is generated.
Rearming the timer, if necessary, will be performed in a second step
when the signal will be delivered.

Hence, checking the list of process timers list to decide to stop to cpu
timer right after generating the signal is premature and is leading the
cputimer to oscillate between the on/off states when a single process timer
is present.

The following posix-cpu-timers module intrumentation

In void thread_group_cputimer(struct task_s
		raw_spin_lock_irqsave(&cputimer->lock, flags);
 		cputimer->running = 1;
 		update_gt_cputime(&cputimer->cputime, &sum);
+		printk(KERN_DEBUG "cputimer started\n");
 	} else
 		raw_spin_lock_irqsave(&cputimer->lock, flags);
 	*times = cputimer->cputime;
and in static void stop_process_timers(struct s

 	raw_spin_lock_irqsave(&cputimer->lock, flags);
 	cputimer->running = 0;
+	printk(KERN_DEBUG "cputimer stopped\n");
 	raw_spin_unlock_irqrestore(&cputimer->lock, flags);
 }

with the small program:

\#include <signal.h>
\#include <time.h>
\#include <stddef.h>
\#include <stdio.h>
\#include <string.h>
\#include <fcntl.h>
\#include <unistd.h>

static void chew_cpu()
{
	static volatile char buf[4096];
	for (size_t i = 0; i < 100; ++i)
		for (size_t j = 0; j < sizeof buf; ++j)
			buf[j] = 0xaa;
	int nullfd = open("/dev/null", O_WRONLY);
	for (size_t i = 0; i < 100; ++ i)
		for (size_t j = 0; j < sizeof buf; ++j)
			buf[j] = 0xbb;
	write(nullfd, (char*)buf, sizeof buf);
	close(nullfd);
}

timer_t t;
volatile int sig_cnt;

static void
sig_handler (int sig, siginfo_t *info, void *ctx)
{
	if (sig_cnt >= 5) {
		struct itimerspec it = {};
		timer_settime(t, 0, &it, NULL);
	}
	++sig_cnt;
}

int main( int argc, char *argv[] )
{
	clockid_t my_process_clock;
	int e;

	struct sigaction sa = { .sa_sigaction = sig_handler,
				.sa_flags = SA_SIGINFO };
	sigemptyset(&sa.sa_mask);
	sigaction(SIGRTMIN, &sa, NULL);

	struct sigevent ev;
	memset( &ev, 0, sizeof (ev));
	ev.sigev_notify = SIGEV_SIGNAL;
	ev.sigev_signo  = SIGRTMIN;
	ev.sigev_value.sival_ptr = &ev;

	e = clock_getcpuclockid( 0, &my_process_clock);
	if (e) {
		printf("clock_getcpuclockid: (%d) %s\n", e, strerror(e));
		return 1;
	}
	if (timer_create(my_process_clock, &ev, &t) != 0) {
		printf("timer_create: %m\n");
		return 1;
	}

	struct itimerspec it;
	it.it_value.tv_sec    = 0;
	it.it_value.tv_nsec   = 100000;
	it.it_interval.tv_sec = 0;
	it.it_interval.tv_nsec = 100000;

	timer_settime(t,0,&it,NULL);

	while (sig_cnt < 5)
		chew_cpu();

	// process timer should stop
	struct timespec req = { .tv_sec = 0, .tv_nsec = 400000 };
	nanosleep(&req,NULL);

	// restart timer.
	sig_cnt = 0;
        timer_settime(t,0,&it,NULL);
	while (sig_cnt < 5)
		chew_cpu();

	timer_delete(t);

	return 0;
}

demonstrate the issue. Here are the results before and after applying the patch:

Before:

[  181.904571] cputimer started
[  181.905013] cputimer stopped
[  181.905027] cputimer started
[  181.906009] cputimer stopped
[  181.906016] cputimer started
[  181.907008] cputimer stopped
[  181.907014] cputimer started
[  181.908008] cputimer stopped
[  181.908013] cputimer started
[  181.909008] cputimer stopped
[  181.909020] cputimer started
[  181.910007] cputimer stopped
[  181.910050] cputimer started
[  181.911011] cputimer stopped
[  181.913619] cputimer started
[  181.914007] cputimer stopped
[  181.914015] cputimer started
[  181.915006] cputimer stopped
[  181.915011] cputimer started
[  181.916006] cputimer stopped
[  181.916010] cputimer started
[  181.917008] cputimer stopped
[  181.917058] cputimer started
[  181.918009] cputimer stopped
[  181.918029] cputimer started
[  181.919006] cputimer stopped
[  181.919010] cputimer started
[  181.920006] cputimer stopped

After:

[  859.722119] cputimer started
[  859.730004] cputimer stopped
[  859.731354] cputimer started
[  859.739004] cputimer stopped

Signed-off-by: Olivier Langlois <olivier@trillion01.com>
---
 kernel/posix-cpu-timers.c | 34 +++++++++++++++-------------------
 1 file changed, 15 insertions(+), 19 deletions(-)

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index c7f31aa..972f5cb 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -1049,8 +1049,6 @@ static void check_process_timers(struct task_struct *tsk,
 	sig->cputime_expires.prof_exp = expires_to_cputime(prof_expires);
 	sig->cputime_expires.virt_exp = expires_to_cputime(virt_expires);
 	sig->cputime_expires.sched_exp = sched_expires;
-	if (task_cputime_zero(&sig->cputime_expires))
-		stop_process_timers(sig);
 }
 
 /*
@@ -1158,34 +1156,32 @@ static inline int task_cputime_expired(const struct task_cputime *sample,
  */
 static inline int fastpath_timer_check(struct task_struct *tsk)
 {
-	struct signal_struct *sig;
-	cputime_t utime, stime;
+	struct signal_struct *sig = tsk->signal;
 
-	task_cputime(tsk, &utime, &stime);
+	if (sig->cputimer.running) {
+		if (likely(!task_cputime_zero(&sig->cputime_expires))) {
+			struct task_cputime group_sample;
+
+			raw_spin_lock(&sig->cputimer.lock);
+			group_sample = sig->cputimer.cputime;
+			raw_spin_unlock(&sig->cputimer.lock);
+
+			if (task_cputime_expired(&group_sample, &sig->cputime_expires))
+				return 1;
+		} else
+			stop_process_timers(sig);
+	}
 
 	if (!task_cputime_zero(&tsk->cputime_expires)) {
 		struct task_cputime task_sample = {
-			.utime = utime,
-			.stime = stime,
 			.sum_exec_runtime = tsk->se.sum_exec_runtime
 		};
+		task_cputime(tsk, &task_sample.utime, &task_sample.stime);
 
 		if (task_cputime_expired(&task_sample, &tsk->cputime_expires))
 			return 1;
 	}
 
-	sig = tsk->signal;
-	if (sig->cputimer.running) {
-		struct task_cputime group_sample;
-
-		raw_spin_lock(&sig->cputimer.lock);
-		group_sample = sig->cputimer.cputime;
-		raw_spin_unlock(&sig->cputimer.lock);
-
-		if (task_cputime_expired(&group_sample, &sig->cputime_expires))
-			return 1;
-	}
-
 	return 0;
 }
 
-- 
1.8.4.2