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Message-ID: <20061201080826.GA17504@elte.hu>
Date: Fri, 1 Dec 2006 09:08:27 +0100
From: Ingo Molnar <mingo@...e.hu>
To: Fernando Lopez-Lezcano <nando@...ma.Stanford.EDU>
Cc: "Linux-Kernel," <linux-kernel@...r.kernel.org>
Subject: Re: 2.6.19-rt1: max latencies with jackd
* Fernando Lopez-Lezcano <nando@...ma.Stanford.EDU> wrote:
> Hi Ingo, I finally have a rebuilt kernel with latency tracing enabled,
> a jackd with the proper prctl incantations built in and I'm getting
> some (hopefully) meaningful data.
ok. I'm first trying to understand what is happening. For example lets
take the longest latency from the 'idle' collection:
latency: 801 us, #470/470, CPU#0 | (M:rt VP:0, KP:0, SP:1 HP:1 #P:2)
tracing starts here:
japa-27320 0D... 18us : user_trace_start (sys_prctl)
then some 400 usecs later japa-27320 is scheduled away:
softirq--5 0D..2 445us : thread_return <japa-27320> (161 199)
it has been scheduled away because softirq--5 [softirq-timer/0] has
SCHED_FIFO 99, which preempts japa-27320 which has SCHED_FIFO 61.
200 usecs later softirq--5 has finished processing and goes to
posix_cp-3, which too is SCHED_FIFO 99:
posix_cp-3 0D..2 607us : thread_return <softirq--5> (199 199)
and then 60 usecs later we go back to japa-27320:
japa-27320 0D..2 663us : thread_return <posix_cp-3> (199 161)
which, 130 usecs later, stops tracing:
japa-27320 0.... 793us : user_trace_stop (sys_prctl)
this particular latency is caused by softirq--5 and posix_cpu_thread
having SCHED_FIFO 99. Why do they have that high priority?
also, note that there are 465 trace entries in your log, and the
overhead of pmtimer based trace entries is ~1.5 usecs. So there's an
artificial +600 usecs latency in this critical path, purely caused by
tracing.
could you try the patch below? It changes trace entries to be measured
via get_cycles() again [which should be must faster than pmtimer on your
CPU], but keeps the latency tracing timestamps (the timestamp of the
start/end of critical sections) on pmtimer again. This should give us
accurate latency measurements, but fast trace entries.
you can still revert to the conservative timestamping behavior via:
echo 0 > /proc/sys/kernel/trace_use_raw_cycles
note: you can probably get a higher quality of 'overview trace' via:
echo 0 > /proc/sys/kernel/mcount_enabled
echo 1 > /proc/sys/kernel/stackframe_tracing
the latter should turn such entries:
trace-it-1967 1...1 7us : trace_special_sym (c0390143 0 0)
into full symbolic backtraces:
trace-it-2110 0...1 6us : schedule()<-do_nanosleep()<-hrtimer_nanosleep()<-sys_nanosleep()
trace-it-2110 0...1 6us : syscall_call()<-(-1)()<-(0)()<-(0)()
Ingo
Index: linux/kernel/latency_trace.c
===================================================================
--- linux.orig/kernel/latency_trace.c
+++ linux/kernel/latency_trace.c
@@ -34,8 +34,36 @@
# define RAW_SPIN_LOCK_UNLOCKED SPIN_LOCK_UNLOCKED
#endif
+int trace_use_raw_cycles = 1;
+
+/*
+ * Convert raw cycles to usecs.
+ * Note: this is not the 'clocksource cycles' value, it's the raw
+ * cycle counter cycles. We use GTOD to timestamp latency start/end
+ * points, but the trace entries inbetween are timestamped with
+ * get_cycles().
+ */
+static unsigned long notrace cycles_to_us(cycles_t delta)
+{
+ if (!trace_use_raw_cycles)
+ return cycles_to_usecs(delta);
+#ifdef CONFIG_X86
+ do_div(delta, cpu_khz/1000+1);
+#elif defined(CONFIG_PPC)
+ delta = mulhwu(tb_to_us, delta);
+#elif defined(CONFIG_ARM)
+ delta = mach_cycles_to_usecs(delta);
+#else
+ #error Implement cycles_to_usecs.
+#endif
+
+ return (unsigned long) delta;
+}
+
static notrace inline cycles_t now(void)
{
+ if (trace_use_raw_cycles)
+ return get_cycles();
return get_monotonic_cycles();
}
@@ -1294,8 +1322,8 @@ static int notrace l_show_fn(struct seq_
{
unsigned long abs_usecs, rel_usecs;
- abs_usecs = cycles_to_usecs(entry->timestamp - entry0->timestamp);
- rel_usecs = cycles_to_usecs(next_entry->timestamp - entry->timestamp);
+ abs_usecs = cycles_to_us(entry->timestamp - entry0->timestamp);
+ rel_usecs = cycles_to_us(next_entry->timestamp - entry->timestamp);
if (trace_verbose) {
seq_printf(m, "%16s %5d %d %d %08x %08lx [%016Lx] %ld.%03ldms (+%ld.%03ldms): ",
@@ -1325,8 +1353,8 @@ static int notrace l_show_special(struct
{
unsigned long abs_usecs, rel_usecs;
- abs_usecs = cycles_to_usecs(entry->timestamp - entry0->timestamp);
- rel_usecs = cycles_to_usecs(next_entry->timestamp - entry->timestamp);
+ abs_usecs = cycles_to_us(entry->timestamp - entry0->timestamp);
+ rel_usecs = cycles_to_us(next_entry->timestamp - entry->timestamp);
print_generic(m, entry);
print_timestamp(m, abs_usecs, rel_usecs);
@@ -1368,8 +1396,8 @@ l_show_special_pid(struct seq_file *m, u
pid = entry->u.special.v1;
- abs_usecs = cycles_to_usecs(entry->timestamp - entry0->timestamp);
- rel_usecs = cycles_to_usecs(next_entry->timestamp - entry->timestamp);
+ abs_usecs = cycles_to_us(entry->timestamp - entry0->timestamp);
+ rel_usecs = cycles_to_us(next_entry->timestamp - entry->timestamp);
print_generic(m, entry);
print_timestamp(m, abs_usecs, rel_usecs);
@@ -1391,8 +1419,8 @@ l_show_special_sym(struct seq_file *m, u
{
unsigned long abs_usecs, rel_usecs;
- abs_usecs = cycles_to_usecs(entry->timestamp - entry0->timestamp);
- rel_usecs = cycles_to_usecs(next_entry->timestamp - entry->timestamp);
+ abs_usecs = cycles_to_us(entry->timestamp - entry0->timestamp);
+ rel_usecs = cycles_to_us(next_entry->timestamp - entry->timestamp);
print_generic(m, entry);
print_timestamp(m, abs_usecs, rel_usecs);
@@ -1422,8 +1450,8 @@ static int notrace l_show_cmdline(struct
if (!trace_verbose)
return 0;
- abs_usecs = cycles_to_usecs(entry->timestamp - entry0->timestamp);
- rel_usecs = cycles_to_usecs(next_entry->timestamp - entry->timestamp);
+ abs_usecs = cycles_to_us(entry->timestamp - entry0->timestamp);
+ rel_usecs = cycles_to_us(next_entry->timestamp - entry->timestamp);
seq_printf(m,
"[ => %16s ] %ld.%03ldms (+%ld.%03ldms)\n",
@@ -1448,8 +1476,8 @@ static int notrace l_show_syscall(struct
unsigned long abs_usecs, rel_usecs;
unsigned long nr;
- abs_usecs = cycles_to_usecs(entry->timestamp - entry0->timestamp);
- rel_usecs = cycles_to_usecs(next_entry->timestamp - entry->timestamp);
+ abs_usecs = cycles_to_us(entry->timestamp - entry0->timestamp);
+ rel_usecs = cycles_to_us(next_entry->timestamp - entry->timestamp);
print_generic(m, entry);
print_timestamp_short(m, abs_usecs, rel_usecs);
@@ -1487,8 +1515,8 @@ static int notrace l_show_sysret(struct
{
unsigned long abs_usecs, rel_usecs;
- abs_usecs = cycles_to_usecs(entry->timestamp - entry0->timestamp);
- rel_usecs = cycles_to_usecs(next_entry->timestamp - entry->timestamp);
+ abs_usecs = cycles_to_us(entry->timestamp - entry0->timestamp);
+ rel_usecs = cycles_to_us(next_entry->timestamp - entry->timestamp);
print_generic(m, entry);
print_timestamp_short(m, abs_usecs, rel_usecs);
@@ -1676,7 +1704,7 @@ check_critical_timing(int cpu, struct cp
* as long as possible:
*/
T0 = tr->preempt_timestamp;
- T1 = now();
+ T1 = get_monotonic_cycles();
delta = T1-T0;
local_save_flags(flags);
@@ -1690,7 +1718,7 @@ check_critical_timing(int cpu, struct cp
* might change it (it can only get larger so the latency
* is fair to be reported):
*/
- T2 = now();
+ T2 = get_monotonic_cycles();
/* check for buggy clocks, handling wrap for 32-bit clocks */
if (TYPE_EQUAL(cycles_t, unsigned long)) {
@@ -1748,7 +1776,7 @@ check_critical_timing(int cpu, struct cp
printk(" => ended at timestamp %lu: ", t1);
print_symbol("<%s>\n", tr->critical_end);
dump_stack();
- t1 = cycles_to_usecs(now());
+ t1 = cycles_to_usecs(get_monotonic_cycles());
printk(" => dump-end timestamp %lu\n\n", t1);
#endif
@@ -1758,7 +1786,7 @@ check_critical_timing(int cpu, struct cp
out:
tr->critical_sequence = max_sequence;
- tr->preempt_timestamp = now();
+ tr->preempt_timestamp = get_monotonic_cycles();
tr->early_warning = 0;
reset_trace_idx(cpu, tr);
_trace_cmdline(cpu, tr);
@@ -1807,7 +1835,7 @@ __start_critical_timing(unsigned long ei
atomic_inc(&tr->disabled);
tr->critical_sequence = max_sequence;
- tr->preempt_timestamp = now();
+ tr->preempt_timestamp = get_monotonic_cycles();
tr->critical_start = eip;
atomic_set(&tr->overrun, 0);
reset_trace_idx(cpu, tr);
@@ -2082,7 +2110,7 @@ check_wakeup_timing(struct cpu_trace *tr
goto out;
T0 = tr->preempt_timestamp;
- T1 = now();
+ T1 = get_monotonic_cycles();
/*
* Any wraparound or time warp and we are out:
*/
@@ -2184,7 +2212,7 @@ void __trace_start_sched_wakeup(struct t
// if (!atomic_read(&tr->disabled)) {
atomic_inc(&tr->disabled);
tr->critical_sequence = max_sequence;
- tr->preempt_timestamp = now();
+ tr->preempt_timestamp = get_monotonic_cycles();
tr->latency_type = WAKEUP_LATENCY;
tr->critical_start = CALLER_ADDR0;
atomic_set(&tr->overrun, 0);
@@ -2295,7 +2323,7 @@ long user_trace_start(void)
reset_trace_idx(cpu, tr);
tr->critical_sequence = max_sequence;
- tr->preempt_timestamp = now();
+ tr->preempt_timestamp = get_monotonic_cycles();
tr->critical_start = CALLER_ADDR0;
atomic_set(&tr->overrun, 0);
_trace_cmdline(cpu, tr);
@@ -2347,7 +2375,7 @@ long user_trace_stop(void)
unsigned long long tmp0;
T0 = tr->preempt_timestamp;
- T1 = now();
+ T1 = get_monotonic_cycles();
tmp0 = preempt_max_latency;
if (T1 < T0)
T0 = T1;
@@ -2416,7 +2444,7 @@ static void print_entry(struct trace_ent
unsigned long abs_usecs;
int hardirq, softirq;
- abs_usecs = cycles_to_usecs(entry->timestamp - entry0->timestamp);
+ abs_usecs = cycles_to_us(entry->timestamp - entry0->timestamp);
printk("%-5d ", entry->pid);
Index: linux/kernel/sysctl.c
===================================================================
--- linux.orig/kernel/sysctl.c
+++ linux/kernel/sysctl.c
@@ -470,6 +470,14 @@ static ctl_table kern_table[] = {
.mode = 0644,
.proc_handler = &proc_dointvec,
},
+ {
+ .ctl_name = KERN_PANIC,
+ .procname = "trace_use_raw_cycles",
+ .data = &trace_use_raw_cycles,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = &proc_dointvec,
+ },
#endif
#ifdef CONFIG_GENERIC_HARDIRQS
{
Index: linux/include/linux/sched.h
===================================================================
--- linux.orig/include/linux/sched.h
+++ linux/include/linux/sched.h
@@ -239,6 +239,7 @@ extern cpumask_t nohz_cpu_mask;
extern void show_state(void);
extern void show_regs(struct pt_regs *);
+extern void irq_show_regs_callback(int cpu, struct pt_regs *regs);
/*
* TASK is a pointer to the task whose backtrace we want to see (or NULL for current
@@ -323,7 +324,7 @@ extern int debug_direct_keyboard;
extern int mcount_enabled, trace_enabled, trace_user_triggered,
trace_user_trigger_irq, trace_freerunning, trace_verbose,
trace_print_at_crash, trace_all_cpus, print_functions,
- syscall_tracing, stackframe_tracing;
+ syscall_tracing, stackframe_tracing, trace_use_raw_cycles;
extern void notrace trace_special(unsigned long v1, unsigned long v2, unsigned long v3);
extern void notrace trace_special_pid(int pid, unsigned long v1, unsigned long v2);
extern void notrace trace_special_u64(unsigned long long v1, unsigned long v2);
-
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