[RFC patch 17/18] MIPS : Trace clock

[Mathieu Desnoyers]

MIPS get_cycles only returns a 32 bits TSC (see timex.h). The assumption there
is that the reschedule is done every 8 seconds or so. Given that tracing needs
to detect delays longer than 8 seconds, we need a full 64-bits TSC, which is
provided by trace-clock-32-to-64.

I leave the "depends on !CPU_R4400_WORKAROUNDS" in Kconfig because the solution
proposed by Ralf to deal with the R4400 bug is racy, so let's just not support
this broken architecture. :(

Note for Peter Zijlstra :
You should probably have a look at lockdep.c raw_spinlock_t lockdep_lock usage.
I suspect it may be used with preemption enabled in graph_lock(). (not sure
though, but it's worth double-checking.

This patch uses the same cache-line bouncing algorithm used for x86. This is a
best-effort to support architectures lacking synchronized TSC without adding a
lot of complexity too soon. This keeps room for improvement in a second phase.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
CC: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
CC: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
---
 arch/mips/Kconfig                   |    3 
 arch/mips/include/asm/timex.h       |   17 +++
 arch/mips/include/asm/trace-clock.h |   68 ++++++++++++++
 arch/mips/kernel/Makefile           |    2 
 arch/mips/kernel/trace-clock.c      |  172 ++++++++++++++++++++++++++++++++++++
 5 files changed, 261 insertions(+), 1 deletion(-)

Index: linux.trees.git/arch/mips/Kconfig
===================================================================
--- linux.trees.git.orig/arch/mips/Kconfig	2008-11-07 00:10:10.000000000 -0500
+++ linux.trees.git/arch/mips/Kconfig	2008-11-07 00:16:42.000000000 -0500
@@ -1614,6 +1614,9 @@ config CPU_R4400_WORKAROUNDS
 config HAVE_GET_CYCLES_32
 	def_bool y
 	depends on !CPU_R4400_WORKAROUNDS
+	select HAVE_TRACE_CLOCK
+	select HAVE_TRACE_CLOCK_32_TO_64
+	select HAVE_UNSYNCHRONIZED_TSC
 
 #
 # Use the generic interrupt handling code in kernel/irq/:
Index: linux.trees.git/arch/mips/include/asm/trace-clock.h
===================================================================
--- /dev/null	1970-01-01 00:00:00.000000000 +0000
+++ linux.trees.git/arch/mips/include/asm/trace-clock.h	2008-11-07 00:16:42.000000000 -0500
@@ -0,0 +1,68 @@
+/*
+ * Copyright (C) 2005,2008 Mathieu Desnoyers
+ *
+ * Trace clock MIPS definitions.
+ */
+
+#ifndef _ASM_MIPS_TRACE_CLOCK_H
+#define _ASM_MIPS_TRACE_CLOCK_H
+
+#include <linux/timex.h>
+#include <asm/processor.h>
+
+#define TRACE_CLOCK_MIN_PROBE_DURATION 200
+
+extern u64 trace_clock_read_synthetic_tsc(void);
+
+/*
+ * MIPS get_cycles only returns a 32 bits TSC (see timex.h). The assumption
+ * there is that the reschedule is done every 8 seconds or so. Given that
+ * tracing needs to detect delays longer than 8 seconds, we need a full 64-bits
+ * TSC, whic is provided by trace-clock-32-to-64.
+*/
+extern u64 trace_clock_async_tsc_read(void);
+
+static inline u32 trace_clock_read32(void)
+{
+	u32 cycles;
+
+	if (likely(tsc_is_sync()))
+		cycles = (u32)get_cycles(); /* only need the 32 LSB */
+	else
+		cycles = (u32)trace_clock_async_tsc_read();
+	return cycles;
+}
+
+static inline u64 trace_clock_read64(void)
+{
+	u64 cycles;
+
+	if (likely(tsc_is_sync()))
+		cycles = trace_clock_read_synthetic_tsc();
+	else
+		cycles = trace_clock_async_tsc_read();
+	return cycles;
+}
+
+static inline void trace_clock_add_timestamp(unsigned long ticks)
+{ }
+
+static inline unsigned int trace_clock_frequency(void)
+{
+	return mips_hpt_frequency;
+}
+
+static inline u32 trace_clock_freq_scale(void)
+{
+	return 1;
+}
+
+extern void get_trace_clock(void);
+extern void put_trace_clock(void);
+extern void get_synthetic_tsc(void);
+extern void put_synthetic_tsc(void);
+
+static inline void set_trace_clock_is_sync(int state)
+{
+}
+#endif /* _ASM_MIPS_TRACE_CLOCK_H */
Index: linux.trees.git/arch/mips/kernel/trace-clock.c
===================================================================
--- /dev/null	1970-01-01 00:00:00.000000000 +0000
+++ linux.trees.git/arch/mips/kernel/trace-clock.c	2008-11-07 00:16:42.000000000 -0500
@@ -0,0 +1,172 @@
+/*
+ * arch/mips/kernel/trace-clock.c
+ *
+ * Trace clock for mips.
+ *
+ * Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>, October 2008
+ */
+
+#include <linux/module.h>
+#include <linux/trace-clock.h>
+#include <linux/jiffies.h>
+#include <linux/mutex.h>
+#include <linux/timer.h>
+#include <linux/spinlock.h>
+
+static u64 trace_clock_last_tsc;
+static DEFINE_PER_CPU(struct timer_list, update_timer);
+static DEFINE_MUTEX(async_tsc_mutex);
+static int async_tsc_refcount;	/* Number of readers */
+static int async_tsc_enabled;	/* Async TSC enabled on all online CPUs */
+
+/*
+ * Support for architectures with non-sync TSCs.
+ * When the local TSC is discovered to lag behind the highest TSC counter, we
+ * increment the TSC count of an amount that should be, ideally, lower than the
+ * execution time of this routine, in cycles : this is the granularity we look
+ * for : we must be able to order the events.
+ */
+
+#if BITS_PER_LONG == 64
+notrace u64 trace_clock_async_tsc_read(void)
+{
+	u64 new_tsc, last_tsc;
+
+	WARN_ON(!async_tsc_refcount || !async_tsc_enabled);
+	new_tsc = trace_clock_read_synthetic_tsc();
+	do {
+		last_tsc = trace_clock_last_tsc;
+		if (new_tsc < last_tsc)
+			new_tsc = last_tsc + TRACE_CLOCK_MIN_PROBE_DURATION;
+		/*
+		 * If cmpxchg fails with a value higher than the new_tsc, don't
+		 * retry : the value has been incremented and the events
+		 * happened almost at the same time.
+		 * We must retry if cmpxchg fails with a lower value :
+		 * it means that we are the CPU with highest frequency and
+		 * therefore MUST update the value.
+		 */
+	} while (cmpxchg64(&trace_clock_last_tsc, last_tsc, new_tsc) < new_tsc);
+	return new_tsc;
+}
+EXPORT_SYMBOL_GPL(trace_clock_async_tsc_read);
+#else
+/*
+ * Emulate an atomic 64-bits update with a spinlock.
+ * Note : preempt_disable or irq save must be explicit with raw_spinlock_t.
+ * Given we use a spinlock for this time base, we should never be called from
+ * NMI context.
+ */
+static raw_spinlock_t trace_clock_lock =
+	(raw_spinlock_t)__RAW_SPIN_LOCK_UNLOCKED;
+
+static inline u64 trace_clock_cmpxchg64(u64 *ptr, u64 old, u64 new)
+{
+	u64 val;
+
+	val = *ptr;
+	if (likely(val == old))
+		*ptr = val = new;
+	return val;
+}
+
+notrace u64 trace_clock_async_tsc_read(void)
+{
+	u64 new_tsc, last_tsc;
+	unsigned long flags;
+
+	WARN_ON(!async_tsc_refcount || !async_tsc_enabled);
+	local_irq_save(flags);
+	__raw_spin_lock(&trace_clock_lock);
+	new_tsc = trace_clock_read_synthetic_tsc();
+	do {
+		last_tsc = trace_clock_last_tsc;
+		if (new_tsc < last_tsc)
+			new_tsc = last_tsc + TRACE_CLOCK_MIN_PROBE_DURATION;
+		/*
+		 * If cmpxchg fails with a value higher than the new_tsc, don't
+		 * retry : the value has been incremented and the events
+		 * happened almost at the same time.
+		 * We must retry if cmpxchg fails with a lower value :
+		 * it means that we are the CPU with highest frequency and
+		 * therefore MUST update the value.
+		 */
+	} while (trace_clock_cmpxchg64(&trace_clock_last_tsc, last_tsc,
+				       new_tsc) < new_tsc);
+	__raw_spin_unlock(&trace_clock_lock);
+	local_irq_restore(flags);
+	return new_tsc;
+}
+EXPORT_SYMBOL_GPL(trace_clock_async_tsc_read);
+#endif
+
+
+static void update_timer_ipi(void *info)
+{
+	(void)trace_clock_async_tsc_read();
+}
+
+/*
+ * update_timer_fct : - Timer function to resync the clocks
+ * @data: unused
+ *
+ * Fires every jiffy.
+ */
+static void update_timer_fct(unsigned long data)
+{
+	(void)trace_clock_async_tsc_read();
+
+	per_cpu(update_timer, smp_processor_id()).expires = jiffies + 1;
+	add_timer_on(&per_cpu(update_timer, smp_processor_id()),
+		     smp_processor_id());
+}
+
+static void enable_trace_clock(int cpu)
+{
+	init_timer(&per_cpu(update_timer, cpu));
+	per_cpu(update_timer, cpu).function = update_timer_fct;
+	per_cpu(update_timer, cpu).expires = jiffies + 1;
+	smp_call_function_single(cpu, update_timer_ipi, NULL, 1);
+	add_timer_on(&per_cpu(update_timer, cpu), cpu);
+}
+
+static void disable_trace_clock(int cpu)
+{
+	del_timer_sync(&per_cpu(update_timer, cpu));
+}
+
+void get_trace_clock(void)
+{
+	int cpu;
+
+	mutex_lock(&async_tsc_mutex);
+	if (async_tsc_refcount++ || tsc_is_sync())
+		goto end;
+
+	async_tsc_enabled = 1;
+	for_each_online_cpu(cpu)
+		enable_trace_clock(cpu);
+end:
+	mutex_unlock(&async_tsc_mutex);
+	get_synthetic_tsc();
+}
+EXPORT_SYMBOL_GPL(get_trace_clock);
+
+void put_trace_clock(void)
+{
+	int cpu;
+
+	put_synthetic_tsc();
+	mutex_lock(&async_tsc_mutex);
+	WARN_ON(async_tsc_refcount <= 0);
+	if (async_tsc_refcount != 1 || !async_tsc_enabled)
+		goto end;
+
+	for_each_online_cpu(cpu)
+		disable_trace_clock(cpu);
+	async_tsc_enabled = 0;
+end:
+	async_tsc_refcount--;
+	mutex_unlock(&async_tsc_mutex);
+}
+EXPORT_SYMBOL_GPL(put_trace_clock);
Index: linux.trees.git/arch/mips/include/asm/timex.h
===================================================================
--- linux.trees.git.orig/arch/mips/include/asm/timex.h	2008-11-07 00:16:17.000000000 -0500
+++ linux.trees.git/arch/mips/include/asm/timex.h	2008-11-07 00:16:42.000000000 -0500
@@ -42,7 +42,7 @@
 
 typedef unsigned int cycles_t;
 
-#ifdef HAVE_GET_CYCLES_32
+#ifdef CONFIG_HAVE_GET_CYCLES_32
 static inline cycles_t get_cycles(void)
 {
 	return read_c0_count();
@@ -91,6 +91,21 @@ static inline void write_tsc(u32 val1, u
 
 extern unsigned int mips_hpt_frequency;
 
+/*
+ * Currently unused, should update internal tsc-related timekeeping sources.
+ */
+static inline void mark_tsc_unstable(char *reason)
+{
+}
+
+/*
+ * Currently simply use the tsc_is_sync value.
+ */
+static inline int unsynchronized_tsc(void)
+{
+	return !tsc_is_sync();
+}
+
 #endif /* __KERNEL__ */
 
 #endif /*  _ASM_TIMEX_H */
Index: linux.trees.git/arch/mips/kernel/Makefile
===================================================================
--- linux.trees.git.orig/arch/mips/kernel/Makefile	2008-10-30 20:22:50.000000000 -0400
+++ linux.trees.git/arch/mips/kernel/Makefile	2008-11-07 00:16:42.000000000 -0500
@@ -85,6 +85,8 @@ obj-$(CONFIG_GPIO_TXX9)		+= gpio_txx9.o
 obj-$(CONFIG_KEXEC)		+= machine_kexec.o relocate_kernel.o
 obj-$(CONFIG_EARLY_PRINTK)	+= early_printk.o
 
+obj-$(CONFIG_HAVE_GET_CYCLES_32)	+= trace-clock.o
+
 CFLAGS_cpu-bugs64.o	= $(shell if $(CC) $(KBUILD_CFLAGS) -Wa,-mdaddi -c -o /dev/null -xc /dev/null >/dev/null 2>&1; then echo "-DHAVE_AS_SET_DADDI"; fi)
 
 obj-$(CONFIG_HAVE_STD_PC_SERIAL_PORT)	+= 8250-platform.o

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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