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

Paul Mundt
2009-05-28T13:20:51Z

On Thu, May 28, 2009 at 02:59:30PM +0200, Peter Zijlstra wrote:
> On Thu, 2009-05-28 at 21:42 +0900, Paul Mundt wrote:
>
> > unsigned long long __attribute__((weak)) sched_clock(void)
> > {
> > - return (unsigned long long)(jiffies - INITIAL_JIFFIES)
> > - * (NSEC_PER_SEC / HZ);
> > + struct clocksource *clock = ACCESS_ONCE(sched_clocksource);
> > +
> > + return cyc2ns(clock, clocksource_read(clock));
> > }
> >
>
> > @@ -440,7 +444,17 @@ void clocksource_unregister(struct clocksource *cs)
> > list_del(&cs->list);
> > if (clocksource_override == cs)
> > clocksource_override = NULL;
> > +
> > next_clocksource = select_clocksource();
> > +
> > + /*
> > + * If select_clocksource() fails to find another suitable
> > + * clocksource for sched_clocksource and we are unregistering
> > + * it, switch back to jiffies.
> > + */
> > + if (sched_clocksource == cs)
> > + sched_clocksource = &clocksource_jiffies;
> > +
> > spin_unlock_irqrestore(&clocksource_lock, flags);
> > }
>
>
> CPU0 CPU1
>
> clock = ACCESS_ONCE(sched_clocksource);
>
> unload module
> clocksource_unregister()
> sched_clocksource = jiffies
> unmap data/text
>
> cyc2ns(clock, clocksource_read(clock)) <--- fireworks
>

Right, lets try that again..

--

include/linux/clocksource.h | 4 +++-
kernel/sched_clock.c | 13 +++++++++++--
kernel/time/clocksource.c | 19 +++++++++++++++++++
kernel/time/jiffies.c | 2 +-
4 files changed, 34 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..b51d48d 100644
--- a/kernel/sched_clock.c
+++ b/kernel/sched_clock.c
@@ -30,6 +30,8 @@
#include <linux/percpu.h>
#include <linux/ktime.h>
#include <linux/sched.h>
+#include <linux/clocksource.h>
+#include <linux/rcupdate.h>

/*
* Scheduler clock - returns current time in nanosec units.
@@ -38,8 +40,15 @@
*/
unsigned long long __attribute__((weak)) sched_clock(void)
{
- return (unsigned long long)(jiffies - INITIAL_JIFFIES)
- * (NSEC_PER_SEC / HZ);
+ unsigned long long time;
+ struct clocksource *clock;
+
+ rcu_read_lock();
+ clock = rcu_dereference(sched_clocksource);
+ time = cyc2ns(clock, clocksource_read(clock));
+ rcu_read_unlock();
+
+ return time;
}

static __read_mostly int sched_clock_running;
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 80189f6..3795954 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -25,6 +25,7 @@
*/

#include <linux/clocksource.h>
+#include <linux/rcupdate.h>
#include <linux/sysdev.h>
#include <linux/init.h>
#include <linux/module.h>
@@ -109,6 +110,7 @@ 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 +364,9 @@ 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;
}

@@ -440,7 +445,21 @@ void clocksource_unregister(struct clocksource *cs)
list_del(&cs->list);
if (clocksource_override == cs)
clocksource_override = NULL;
+
next_clocksource = select_clocksource();
+
+ /*
+ * If select_clocksource() fails to find another suitable
+ * clocksource for sched_clocksource and we are unregistering
+ * it, switch back to jiffies.
+ */
+ if (sched_clocksource == cs) {
+ rcu_assign_pointer(sched_clocksource, &clocksource_jiffies);
+ spin_unlock_irqrestore(&clocksource_lock, flags);
+ synchronize_rcu();
+ return;
+ }
+
spin_unlock_irqrestore(&clocksource_lock, flags);
}

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 = {