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Subject[PATCH v2 5/6] mips specific clock function to get precise timestamp
Date
From: Wu Zhangjin <wuzj@lemote.com>

by default, trace_clock_local calling sched_clock(jiffies-based) to get
timestamp, in x86, there is a tsc(64bit) based sched_clock, but in mips,
the 'tsc'(clock counter) is only 32bit long, which will easily rollover,
and there is no existing high precise sched_clock in mips, we need to
get one ourselves.

to avoid invading the whole linux-mips, i do not want to implement a
tsc-based native_sched_clock instead of sched_clock like x86 does.
because, there is a need to handling rollover of the only 32-bit long
'tsc' of mips, which will need extra overhead. in reality, i have tried
to do it, but made the kernel hangs when booting, I'm not sure why it
not work.

so, I just implement a native_sched_clock in arch/mips/kernel/ftrace.c,
but not override the original sched_clock(). to get high precise
timestamp, we implement a native_trace_clock_local, which will not call
original sched_clock again, but native_sched_clock().

and what about the trace_clock and trace_clock_global function, should
we also implement a mips-secific one? I'm not sure.

Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Wu Zhangjin <wuzj@lemote.com>
---
arch/mips/kernel/Makefile | 2 +
arch/mips/kernel/csrc-r4k.c | 2 +-
arch/mips/kernel/ftrace_clock.c | 71 +++++++++++++++++++++++++++++++++++++++
kernel/trace/trace_clock.c | 2 +-
4 files changed, 75 insertions(+), 2 deletions(-)
create mode 100644 arch/mips/kernel/ftrace_clock.c

diff --git a/arch/mips/kernel/Makefile b/arch/mips/kernel/Makefile
index 8dabcc6..44ec7e0 100644
--- a/arch/mips/kernel/Makefile
+++ b/arch/mips/kernel/Makefile
@@ -11,6 +11,7 @@ obj-y += cpu-probe.o branch.o entry.o genex.o irq.o process.o \
ifdef CONFIG_FUNCTION_TRACER
# Do not profile debug and lowlevel utilities
CFLAGS_REMOVE_ftrace.o = -pg
+CFLAGS_REMOVE_ftrace_clock.o = -pg
CFLAGS_REMOVE_early_printk.o = -pg
endif

@@ -32,6 +33,7 @@ obj-$(CONFIG_MODULES) += mips_ksyms.o module.o

obj-$(CONFIG_FUNCTION_TRACER) += mcount.o
obj-$(CONFIG_FUNCTION_TRACER) += ftrace.o
+obj-$(CONFIG_NOP_TRACER) += ftrace_clock.o

obj-$(CONFIG_CPU_LOONGSON2) += r4k_fpu.o r4k_switch.o
obj-$(CONFIG_CPU_MIPS32) += r4k_fpu.o r4k_switch.o
diff --git a/arch/mips/kernel/csrc-r4k.c b/arch/mips/kernel/csrc-r4k.c
index e95a3cd..3da1c7a 100644
--- a/arch/mips/kernel/csrc-r4k.c
+++ b/arch/mips/kernel/csrc-r4k.c
@@ -10,7 +10,7 @@

#include <asm/time.h>

-static cycle_t c0_hpt_read(struct clocksource *cs)
+static cycle_t notrace c0_hpt_read(struct clocksource *cs)
{
return read_c0_count();
}
diff --git a/arch/mips/kernel/ftrace_clock.c b/arch/mips/kernel/ftrace_clock.c
new file mode 100644
index 0000000..8ad896e
--- /dev/null
+++ b/arch/mips/kernel/ftrace_clock.c
@@ -0,0 +1,71 @@
+/*
+ * This file is subject to the terms and conditions of the GNU General Public
+ * License. See the file "COPYING" in the main directory of this archive for
+ * more details.
+ *
+ * Copyright (C) 2009 DSLab, Lanzhou University, China
+ * Author: Wu Zhangjin <wuzj@lemote.com>
+ */
+
+#include <linux/types.h>
+#include <linux/sched.h>
+#include <linux/jiffies.h>
+#include <linux/clocksource.h>
+
+/*
+ * mips-specific high precise sched_clock() implementation,
+ *
+ * currently, this is only needed in ftrace, so not override the original
+ * sched_clock().
+ */
+
+unsigned long long native_sched_clock(void)
+{
+ u64 current_cycles;
+ static unsigned long old_jiffies;
+ static u64 time, old_cycles;
+
+ preempt_disable_notrace();
+ /* update timestamp to avoid missing the timer interrupt */
+ if (time_before(jiffies, old_jiffies)) {
+ old_jiffies = jiffies;
+ time = sched_clock();
+ old_cycles = clock->cycle_last;
+ }
+ current_cycles = clock->read(clock);
+
+ time = (time + cyc2ns(clock, (current_cycles - old_cycles)
+ & clock->mask));
+
+ old_cycles = current_cycles;
+ preempt_enable_no_resched_notrace();
+
+ return time;
+}
+
+/*
+ * native_trace_clock_local(): the simplest and least coherent tracing clock.
+ *
+ * Useful for tracing that does not cross to other CPUs nor
+ * does it go through idle events.
+ */
+u64 native_trace_clock_local(void)
+{
+ unsigned long flags;
+ u64 clock;
+
+ /*
+ * herein, we use the above native_sched_clock() to get high precise
+ * timestamp, because the original sched_clock in mips is jiffies based,
+ * which not have enough precision.
+ */
+ raw_local_irq_save(flags);
+ clock = native_sched_clock();
+ raw_local_irq_restore(flags);
+
+ return clock;
+}
+
+u64 trace_clock_local(void)
+ __attribute__((alias("native_trace_clock_local")));
+
diff --git a/kernel/trace/trace_clock.c b/kernel/trace/trace_clock.c
index b588fd8..78c98c8 100644
--- a/kernel/trace/trace_clock.c
+++ b/kernel/trace/trace_clock.c
@@ -26,7 +26,7 @@
* Useful for tracing that does not cross to other CPUs nor
* does it go through idle events.
*/
-u64 notrace trace_clock_local(void)
+u64 __attribute__((weak)) notrace trace_clock_local(void)
{
unsigned long flags;
u64 clock;
--
1.6.0.4


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