Re: [PATCH v4 1/3] perf: Use monotonic clock as a source for timestamps

On Mon, 2015-01-05 at 13:00 +0000, Peter Zijlstra wrote:

On Thu, Nov 06, 2014 at 04:51:56PM +0000, Pawel Moll wrote:

quoted

 Documentation/kernel-parameters.txt |  9 +++++++++
 kernel/events/core.c                | 37 +++++++++++++++++++++++++++++++++++++
 2 files changed, 46 insertions(+)

quoted

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 4c81a86..8ead8d8 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt

quoted

@@ -2763,6 +2764,14 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			allocator.  This parameter is primarily	for debugging
 			and performance comparison.
 
+	perf_use_local_clock
+			[PERF]
+			Use local_clock() as a source for perf timestamps
+			generation. This was be the default behaviour and
+			this parameter can be used to maintain backward
+			compatibility or on older hardware with expensive
+			monotonic clock source.
+
 	pf.		[PARIDE]
 			See Documentation/blockdev/paride.txt.

So I'm always terminally confused on the naming of kernel parameters,
sometimes things are modules (even when they're not actually =m capable)
and get a module::foo naming or so and sometimes they're not.

I guess you mean module.foo?

So we want to use the module naming thing or not?

Honestly, I don't mind either way. For one thing ftrace doesn't bother
and just uses __setup() as well.

quoted

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 2b02c9f..5d0aa03 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c

quoted

@@ -322,8 +323,41 @@ extern __weak const char *perf_pmu_name(void)
 	return "pmu";
 }
 
+static bool perf_use_local_clock;
+static int __init perf_use_local_clock_setup(char *__unused)
+{
+	perf_use_local_clock = true;
+	return 1;
+}
+__setup("perf_use_local_clock", perf_use_local_clock_setup);

quoted

 static inline u64 perf_clock(void)
 {
+	if (likely(!perf_use_local_clock))
+		return ktime_get_mono_fast_ns();
+
 	return local_clock();
 }

Since this all is boot time, should we not use things like static_key
and avoid the 'pointless' conditional at runtime?

Right. it's good to learn new stuff - I didn't know there was
architecture-agnostic support for jump labels. Definitely worth the
effort, will give it a try and spin the patch.

Pawel