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

[Wolfgang Mauerer <wolfgang.mauerer@domain.hid>]

Philippe Gerum wrote:
> 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.

so something along the lines of

xntimer_enqueue_aperiodic(...)
{
...
	large_latency_hook_whatever(args);
...
}

where large_latency_hook_whatever() is #ifdef'd to nop if not
compile-time configured would be acceptable I suppose?

> - 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.

oh, yeah. procfs would be a replacement, but using debugfs via ipipe
is much nicer since this is definitely debugging stuff.

> - 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.

Sounds reasonable.

Would it make sense to augment ipipe_trace_latency() with an additional
type argument to distinguish different latency sources, or is this
overkill?

Best, Wolfgang

> 
> 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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> 
>