Re: [PATCH] sched: Support current clocksource handling in fallback sched_clock().

Paul Mundt
2009-05-28T09:19:36Z

On Wed, May 27, 2009 at 09:25:25AM -0700, Daniel Walker wrote:
> On Wed, 2009-05-27 at 09:15 +0900, Paul Mundt wrote:
>
> > unsigned long long __attribute__((weak)) sched_clock(void)
> > {
> > + /*
> > + * Use the current clocksource when it becomes available later in
> > + * the boot process, and ensure that it is usable for sched_clock().
> > + */
> > + if (clock && (clock->flags & CLOCK_SOURCE_USE_FOR_SCHED_CLOCK))
> > + return cyc2ns(clock, clocksource_read(clock));
> > +
> > + /* Otherwise just fall back on jiffies */
> > return (unsigned long long)(jiffies - INITIAL_JIFFIES)
>
> Do we really need all this complexity in a fast path? the jiffies
> clocksource is static and always ready. Could we instead remove the
> usage of "clock" and create a new pointer called "sched_clocksource" and
> initialize it to the jiffies clock, and allow the clocksource management
> code to update that new pointer based on the
> CLOCK_SOURCE_USE_FOR_SCHED_CLOCK flag when new clocksources are
> registered/removed/marked unstable etc..
>
> that would eliminate all the code in sched_clock except one line,
>
> unsigned long long __attribute__((weak)) sched_clock(void)
> {
> return cyc2ns(sched_clocksource, clocksource_read(sched_clocksource));
> }
>
Ok, there were some ordering problems with the early platform code, but
I've played with this a bit more and got it to the point where this now
also works. I can live with this over the v3 version if people prefer
this approach instead.

--

include/linux/clocksource.h | 4 +++-
kernel/sched_clock.c | 4 ++--
kernel/time/clocksource.c | 4 ++++
kernel/time/jiffies.c | 2 +-
4 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index c56457c..2109940 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -202,7 +202,8 @@ struct clocksource {
#endif
};

-extern struct clocksource *clock; /* current clocksource */
+extern struct clocksource *clock; /* current clocksource */
+extern struct clocksource *sched_clocksource; /* sched_clock() clocksource */

/*
* Clock source flags bits::
@@ -212,6 +213,7 @@ extern struct clocksource *clock; /* current clocksource */

#define CLOCK_SOURCE_WATCHDOG 0x10
#define CLOCK_SOURCE_VALID_FOR_HRES 0x20
+#define CLOCK_SOURCE_USE_FOR_SCHED_CLOCK 0x40

/* simplify initialization of mask field */
#define CLOCKSOURCE_MASK(bits) (cycle_t)((bits) < 64 ? ((1ULL<<(bits))-1) : -1)
diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c
index e1d16c9..c06c285 100644
--- a/kernel/sched_clock.c
+++ b/kernel/sched_clock.c
@@ -30,6 +30,7 @@
#include <linux/percpu.h>
#include <linux/ktime.h>
#include <linux/sched.h>
+#include <linux/clocksource.h>

/*
* Scheduler clock - returns current time in nanosec units.
@@ -38,8 +39,7 @@
*/
unsigned long long __attribute__((weak)) sched_clock(void)
{
- return (unsigned long long)(jiffies - INITIAL_JIFFIES)
- * (NSEC_PER_SEC / HZ);
+ return cyc2ns(sched_clocksource, clocksource_read(sched_clocksource));
}

static __read_mostly int sched_clock_running;
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 80189f6..d148a75 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -109,6 +109,7 @@ EXPORT_SYMBOL(timecounter_cyc2time);

/* XXX - Would like a better way for initializing curr_clocksource */
extern struct clocksource clocksource_jiffies;
+struct clocksource *sched_clocksource = &clocksource_jiffies;

/*[Clocksource internal variables]---------
* curr_clocksource:
@@ -362,6 +363,9 @@ static struct clocksource *select_clocksource(void)
if (next = curr_clocksource)
return NULL;

+ if (next->flags & CLOCK_SOURCE_USE_FOR_SCHED_CLOCK)
+ sched_clocksource = next;
+
return next;
}

diff --git a/kernel/time/jiffies.c b/kernel/time/jiffies.c
index c3f6c30..727d881 100644
--- a/kernel/time/jiffies.c
+++ b/kernel/time/jiffies.c
@@ -52,7 +52,7 @@

static cycle_t jiffies_read(struct clocksource *cs)
{
- return (cycle_t) jiffies;
+ return (cycle_t) (jiffies - INITIAL_JIFFIES);
}

struct clocksource clocksource_jiffies = {