Re: [Xenomai-core] [PATCH, RFC] Instrumentation to detect delayed timers

[Philippe Gerum <rpm@xenomai.org>]

On Tue, 2010-06-08 at 16:21 +0200, Wolfgang Mauerer wrote:
> Hi,
> 
> the attached patch augments the timer expiration handler with some checks
> for excessive timer latencies. Depending on the configuration options, it
> freezes the ipipe tracer and creates a list of delayed timers.
> 
> This code was useful for us in debugging some problems with faulty hardware.
> Would such a thing be suitable for mainline?

No, I don't think so, albeit the debug feature it brings in makes sense.

- the code, as is, adds significant cruft to the timer code, which is
per se a problem. Debug is ok, but I really would like that code to have
a much lower impact source-wise.

- dependency on debugfs is wrong; we still have to take care of legacy
2.4 kernels. I know, this is a real PITA, and I'm more than willing to
get rid of that "heritage" in 3.x, but for now, 2.x still requires to be
2.4-compatible.

- but, beyond this, this is something that should go into the I-pipe
tracer instead, exporting a new dedicated interface to log a high
latency event. There, you could use whatever fits to implement it, since
there is by definition no consideration for compat with legacy kernels.
I'm thinking of having ipipe_trace_latency(unsigned long delta, const
char *source). That would be reusable in other, non-Xenomai contexts as
well.

Additionally, the latency spots could appear as events in the tracer log
as well, which may help debugging them a lot, since you would know about
the actual context.

> 
> Regards, Wolfgang
> 
> ---
> commit 9c2a3908ac4aff21022a577552002fc2622d3670
> Author: Wolfgang Mauerer <wolfgang.mauerer@domain.hid>
> Date:   Mon Jun 7 15:34:39 2010 +0200
> 
>     This patch provides the possibility to check upon timer expiration
>     how much a timer has been delayed. When the delay surpasses a configurable
>     threshold, the ipipe tracer is frozen. Additionally, the patch allows
>     for the creation of a list of timers that have suffered from excessive
>     delays.
>     
>     The mechanism can be disabled at both, compile- and runtime. There is no
>     overhead in the former case.
>     
>     Signed-off-by: Wolfgang Mauerer <wolfgang.mauerer@domain.hid>
> 
> diff --git a/ksrc/nucleus/Kconfig b/ksrc/nucleus/Kconfig
> index 211a4ad..ffed3f8 100644
> --- a/ksrc/nucleus/Kconfig
> +++ b/ksrc/nucleus/Kconfig
> @@ -293,6 +293,24 @@ config XENO_OPT_DEBUG_TIMERS
>  	This option activates debugging output for critical
>  	timer-related operations performed by the Xenomai core.
>  
> +config XENO_OPT_DEBUG_TIMER_LATENCY
> +	bool "Log excessive timer latencies"
> +	depends on XENO_OPT_DEBUG && (XENO_OPT_STATS || IPIPE_TRACE)
> +	help
> +
> +	This option activates recording of excessive timer latencies.
> +	In contrast to the userland latency checker which is statistical
> +	in nature, this test checks every expiring timer and is thus
> +	more precise. However, it adds some runtime overhead when
> +	activated.
> +
> +	When the ipipe tracer is enabled, the trace is frozen
> +	when a latency maximum above the threshold set in
> +	the debugfs file timer_lattest/lat_threshold_us is detected.
> +	When Xenomai statistics collection is enabled, a list of
> +	excessive latencies together with the names of the timers
> +	is kept in the debugfs file timer_lattest/exc_timer_list.
> +
>  config XENO_OPT_DEBUG_SYNCH_RELAX
>  	bool "Detect mutexes held in relaxed sections"
>  	depends on XENO_OPT_PERVASIVE && XENO_OPT_DEBUG
> diff --git a/ksrc/nucleus/timer.c b/ksrc/nucleus/timer.c
> index d813c4f..01dad44 100644
> --- a/ksrc/nucleus/timer.c
> +++ b/ksrc/nucleus/timer.c
> @@ -36,11 +36,37 @@
>   *
>   *@{*/
>  
> +#include <linux/debugfs.h>
>  #include <nucleus/pod.h>
>  #include <nucleus/thread.h>
>  #include <nucleus/timer.h>
> +#include <nucleus/trace.h>
>  #include <asm/xenomai/bits/timer.h>
>  
> +#ifdef CONFIG_XENO_OPT_DEBUG_TIMER_LATENCY
> +#define NANO 1000000000ll
> +#define MU   1000000ll
> +#define NANO_PER_MU (NANO/MU)
> +
> +static u32 lat_thresh = 0; /* in \mu s, set to zero to disable */
> +
> +#ifdef CONFIG_XENO_OPT_STATS
> +#include <linux/seq_file.h>
> +
> +#define NUM_REC_LATENCIES	500
> +static int num_lat_exc_timers = -1;
> +static bool lat_rec_overflow = false; /* Set when more than NUM_REC_LATENCIES
> +				       * excessive latencies have been
> +				       * observed */
> +struct lat_rec {
> +	char name[XNOBJECT_NAME_LEN];
> +	xnsticks_t delta;
> +};
> +
> +static struct lat_rec delayed_timers[NUM_REC_LATENCIES];
> +#endif // CONFIG_XENO_OPT_STATS
> +#endif // XENO_OPT_DEBUG_TIMER_LATENCY
> +
>  static inline void xntimer_enqueue_aperiodic(xntimer_t *timer)
>  {
>  	xntimerq_t *q = &timer->sched->timerqueue;
> @@ -368,6 +394,32 @@ void xntimer_tick_aperiodic(void)
>  		if (delta > (xnsticks_t)nklatency)
>  			break;
>  
> +#ifdef CONFIG_XENO_OPT_DEBUG_TIMER_LATENCY
> +		/*
> +		 * Check for severly delayed timers, but omit the host tick
> +		 * timer as it can be delayed intentionally, thus causing
> +		 * spurious excessive latencies (see
> +		 * xntimer_next_local_shot()).
> +		 */
> +		if (lat_thresh && (timer != &sched->htimer)) {
> +			delta = xnarch_tsc_to_ns(-delta);
> +			if (delta > ((xnsticks_t)lat_thresh)*NANO_PER_MU) {
> +				xnarch_trace_user_freeze(delta, 1);
> +#ifdef CONFIG_XENO_OPT_STATS
> +				if (num_lat_exc_timers < NUM_REC_LATENCIES - 1) {
> +					num_lat_exc_timers++;
> +				} else {
> +					lat_rec_overflow = true;
> +					num_lat_exc_timers = 0;
> +				}
> +				xnobject_copy_name(delayed_timers[num_lat_exc_timers].name,
> +						   timer->name);
> +				delayed_timers[num_lat_exc_timers].delta = delta;
> +#endif // XENO_OPT_STATS
> +			}
> +		}
> +#endif // XENO_OPT_DEBUG_TIMER_LATENCY
> +
>  		trace_mark(xn_nucleus, timer_expire, "timer %p", timer);
>  
>  		xntimer_dequeue_aperiodic(timer);
> @@ -1147,10 +1199,70 @@ static int timer_read_proc(char *page,
>  	return len;
>  }
>  
> +#ifdef CONFIG_XENO_OPT_STATS
> +static int lat_exc_seq_show(struct seq_file *f, void *v)
> +{
> +	int count, max;
> +
> +	if (num_lat_exc_timers < 0) {
> +		seq_printf(f, "(No excessive timer latencies detected)\n");
> +		return 0;
> +	}
> +
> +	if (lat_rec_overflow)
> +		max = NUM_REC_LATENCIES;
> +	else
> +		max = num_lat_exc_timers;
> +
> +	for (count = 0; count < max; count++) {
> +		seq_printf(f, "Timer '%s', delay %lld ns\n",
> +			   delayed_timers[count].name,
> +			   delayed_timers[count].delta);
> +	}
> +
> +	if (lat_rec_overflow) {
> +		seq_printf(f, "WARNING: Latency tracking array overflow detected.\n");
> +		seq_printf(f, "         Not all excessive latencies are shown.\n");
> +
> +	}
> +	return 0;
> +}
> +
> +static int lat_exc_seq_open(struct inode *inode, struct file *file)
> +{
> +	return single_open(file, lat_exc_seq_show, NULL);
> +}
> +
> +static struct file_operations lat_exc_fops = {
> +        .owner          = THIS_MODULE,
> +        .open           = lat_exc_seq_open,
> +        .read           = seq_read,
> +        .llseek         = seq_lseek,
> +};
> +#endif // CONFIG_XENO_OPT_STATS
> +
>  void xntimer_init_proc(void)
>  {
>  	rthal_add_proc_leaf("timer", &timer_read_proc, NULL, NULL,
>  			    rthal_proc_root);
> +
> +#ifdef CONFIG_XENO_OPT_DEBUG_TIMER_LATENCY
> +	struct dentry *debugfs_dir;
> +
> +	debugfs_dir = debugfs_create_dir("timer_lattest", NULL);
> +
> +	if (debugfs_dir) {
> +		if (!debugfs_create_u32("lat_threshold_us", 0666,
> +					debugfs_dir, &lat_thresh))
> +			panic("Could not create lat_threshold_us");
> +
> +#ifdef CONFIG_XENO_OPT_STATS
> +		if (!debugfs_create_file("exc_timer_list", 0600,
> +					 debugfs_dir, NULL, &lat_exc_fops))
> +			panic("Could not create exc_timer_list");
> +#endif // XENO_OPT_STATS
> +	}
> +#endif // XENO_OPT_DEBUG_TIMER_LATENCY
>  }
>  
>  void xntimer_cleanup_proc(void)
> 
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-- 
Philippe.