Re: [Fwd: Re: [patch] Real-Time Preemption, -RT-2.6.9-mm1-V0.4]

[Ingo Molnar <mingo@elte.hu>]

* Florian Schmidt <mista.tapas@gmx.net> wrote:

> > do you compile jackd from sources? If yes then could you try the patch
> > below? With this added, the kernel will produce a stackdump whenever
> > jackd does an 'illegal' sleep.
> > 
> > Also, could you do a small modification to kernel/sched.c and remove
> > this line:
> > 
> > 		send_sig(SIGUSR1, current, 1);
> > 
> > just to make it easier to get Jack up and running. (by default an
> > atomicity violation triggers a signal to make it easier to debug it in
> > userspace, but i suspect there will be alot of such violations so jackd
> > would stop all the time.)
> 
> [snip]
> 
> will do so. btw: i think i'm a bit confused right now. What debugging
> features should i have enabled for this test?

this particular one (atomicity-checking) is always-enabled if you have
the -RT patch applied (it's a really cheap check).

for the 'application-triggered tracing' facility we talked about earlier
is only active if LATENCY_TRACING is enabled. In that case to turn the 
tracer on, call:

	gettimeofday(0,1);

and to turn the tracer off and save the current trace into 
/proc/latency_trace, call:

	gettimeofday(0,0);

or apply the patch below - i've added the tracer bits too. I've added a
simple limit: all delays above 2 msec will be saved - you might want to
do a maximum search there or something. And dont forget to:

	echo 2 > /proc/sys/kernel/trace_enabled

to activate the jackd-triggered kernel tracer.

	Ingo

--- jack-audio-connection-kit-0.99.0/drivers/alsa/alsa_driver.c.orig
+++ jack-audio-connection-kit-0.99.0/drivers/alsa/alsa_driver.c
@@ -1161,6 +1161,7 @@ alsa_driver_wait (alsa_driver_t *driver,
 		unsigned int p_timed_out, c_timed_out;
 		unsigned int ci = 0;
 		unsigned int nfds;
+		int ret;
 
 		nfds = 0;
 
@@ -1194,7 +1195,20 @@ alsa_driver_wait (alsa_driver_t *driver,
 
 		poll_enter = jack_get_microseconds ();
 
-		if (poll (driver->pfd, nfds, driver->poll_timeout) < 0) {
+		gettimeofday((void *)1,(void *)0); // atomic off
+
+		gettimeofday((void *)0,(void *)1); // start tracing
+
+		ret = poll (driver->pfd, nfds, driver->poll_timeout);
+
+		poll_ret = jack_get_microseconds ();
+
+		if (poll_ret - poll_enter > 2000)
+			gettimeofday((void *)0,(void *)0); // save trace
+
+		gettimeofday((void *)1,(void *)1); // atomic on
+
+		if (ret < 0) {
 
 			if (errno == EINTR) {
 				printf ("poll interrupt\n");
@@ -1214,8 +1228,6 @@ alsa_driver_wait (alsa_driver_t *driver,
 			
 		}
 
-		poll_ret = jack_get_microseconds ();
-
 		if (extra_fd < 0) {
 			if (driver->poll_next && poll_ret > driver->poll_next) {
 				*delayed_usecs = poll_ret - driver->poll_next;