Re: [patch] voluntary-preempt-2.6.9-rc1-bk4-R1

[Ingo Molnar <mingo@elte.hu>]

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* Thomas Charbonnel <thomas@undata.org> wrote:

> I still get > 170 us latency from rtl8139 :
> http://www.undata.org/~thomas/R1_rtl8139.trace

this is a single-packet latency, we wont get much lower than this with
the current techniques. Disabling ip_conntrack and tracing ought to
lower the real latency somewhat.

> And again this one :
> preemption latency trace v1.0.5 on 2.6.9-rc1-VP-R1
> --------------------------------------------------
>  latency: 597 us, entries: 12 (12)
>     -----------------
>     | task: swapper/0, uid:0 nice:0 policy:0 rt_prio:0
>     -----------------
>  => started at: smp_apic_timer_interrupt+0x32/0xd0
>  => ended at:   smp_apic_timer_interrupt+0x86/0xd0
> =======>
> 00010000 0.000ms (+0.000ms): smp_apic_timer_interrupt (apic_timer_interrupt)
> 00010000 0.000ms (+0.000ms): profile_tick (smp_apic_timer_interrupt)
> 00010000 0.000ms (+0.000ms): profile_hook (profile_tick)
> 00010001 0.000ms (+0.595ms): notifier_call_chain (profile_hook)
> 00010000 0.595ms (+0.000ms): do_nmi (mcount)
> 00020000 0.596ms (+0.000ms): profile_tick (nmi_watchdog_tick)
> 00020000 0.596ms (+0.000ms): profile_hook (profile_tick)
> 00020001 0.597ms (+0.000ms): notifier_call_chain (profile_hook)
> 00020000 0.597ms (+689953.444ms): profile_hit (nmi_watchdog_tick)
> 00010001 689954.042ms (+1.141ms): update_process_times (do_timer)
> 00000001 0.597ms (+0.000ms): sub_preempt_count (smp_apic_timer_interrupt)
> 00000001 0.598ms (+0.000ms): update_max_trace (check_preempt_timing)

this is a pretty weird one. First it shows an apparently non-monotonic
RDTSC: the jump forward and backward in time around profile_hit. I
suspect the real RDTSC value was lower than the previous one and caused
an underflow. What is your cpu_khz in /proc/cpuinfo?

the other weird one is the +0.595 usec entry at notifier_call_chain(). 
That code is just a couple of instructions, so no real for any overhead 
there.

could you try the attached robust-get-cycles.patch ontop of your current
tree and see whether it impacts these weirdnesses? The patch makes sure
that the cycle counter is sane: two subsequent readings of it were
monotonic and less than 1000 cycles apart.

this patch probably wont remove the +0.595 msec latency though - the
RDTSC value jumped forward there permanently. Maybe the RDTSC value is
somehow corrupted by NMIs - could you turn off the NMI watchdog to
check?

	Ingo

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--- linux/kernel/latency.c.orig2	
+++ linux/kernel/latency.c	
@@ -66,6 +66,18 @@ static unsigned long notrace cycles_to_u
 	return (unsigned long) delta;
 }
 
+static cycles_t notrace robust_get_cycles(void)
+{
+	cycles_t t0 = get_cycles(), t1;
+
+	for (;;) {
+		t1 = get_cycles();
+		if (t1 - t0 < 1000)
+			return t1;
+		t0 = t1;
+	}
+}
+
 #ifdef CONFIG_LATENCY_TRACE
 
 unsigned int trace_enabled = 1;
@@ -89,7 +101,7 @@ ____trace(struct cpu_trace *tr, unsigned
 		entry = tr->trace + tr->trace_idx;
 		entry->eip = eip;
 		entry->parent_eip = parent_eip;
-		entry->timestamp = get_cycles();
+		entry->timestamp = robust_get_cycles();
 		entry->preempt_count = preempt_count();
 	}
 	tr->trace_idx++;
@@ -295,7 +307,7 @@ check_preempt_timing(struct cpu_trace *t
 		return;
 #endif
 	atomic_inc(&tr->disabled);
-	latency = cycles_to_usecs(get_cycles() - tr->preempt_timestamp);
+	latency = cycles_to_usecs(robust_get_cycles() - tr->preempt_timestamp);
 
 	if (preempt_thresh) {
 		if (latency < preempt_thresh)
@@ -337,7 +349,7 @@ check_preempt_timing(struct cpu_trace *t
 out:
 #ifdef CONFIG_LATENCY_TRACE
 	tr->trace_idx = 0;
-	tr->preempt_timestamp = get_cycles();
+	tr->preempt_timestamp = robust_get_cycles();
 #endif
 	tr->critical_start = parent_eip;
 	__trace(eip, parent_eip);
@@ -376,7 +388,7 @@ void notrace add_preempt_count(int val)
 		struct cpu_trace *tr = &__get_cpu_var(trace);
 
 		local_irq_save(flags);
-		tr->preempt_timestamp = get_cycles();
+		tr->preempt_timestamp = robust_get_cycles();
 		tr->critical_start = eip;
 #ifdef CONFIG_LATENCY_TRACE
 		tr->trace_idx = 0;