Messages in this thread Patch in this message | | | Date | Wed, 27 May 2009 08:13:13 +0900 | From | Paul Mundt <> | Subject | Re: [PATCH] sched: Support current clocksource handling in fallback sched_clock(). |
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On Wed, May 27, 2009 at 08:08:55AM +0900, Paul Mundt wrote: > On Tue, May 26, 2009 at 10:17:02PM +0200, Thomas Gleixner wrote: > > On Tue, 26 May 2009, Peter Zijlstra wrote: > > > On Tue, 2009-05-26 at 16:31 +0200, Linus Walleij wrote: > > > > The definition of "rating" from the kerneldoc does not > > > > seem to imply that, it's a subjective measure AFAICT. > > > > Right, there is no rating threshold defined, which allows to deduce > > that. The TSC on x86 which might be unreliable, but usable as > > sched_clock has an initial rating of 300 which can be changed later > > on to 0 when the TSC is unusable as a time of day source. In that > > case clock is replaced by HPET which has a rating > 100 but is > > definitely not a good choice for sched_clock > > > > > > Else you might want an additional criteria, like > > > > cyc2ns(1) (much less than) jiffies_to_usecs(1)*1000 > > > > (however you do that the best way) > > > > so you don't pick something > > > > that isn't substantially faster than the jiffy counter atleast? > > > > What we can do is add another flag to the clocksource e.g. > > CLOCK_SOURCE_USE_FOR_SCHED_CLOCK and check this instead of the > > rating. > > > Ok, so based on this and John's locking concerns, how about something > like this? It doesn't handle the wrapping cases, but I wonder if we > really want to add that amount of logic to sched_clock() in the first > place. Clocksources that wrap frequently could either leave the flag > unset, or do something similar to the TSC code where the cyc2ns shift is > used. If this is something we want to handle generically, then I'll have > a go at generalizing the TSC cyc2ns scaling bits for the next spin. > Lets try that again..
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include/linux/clocksource.h | 2 ++ kernel/sched_clock.c | 22 ++++++++++++++++++++++ kernel/time/clocksource.c | 2 +- 3 files changed, 25 insertions(+), 1 deletion(-)
diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h index c56457c..cfd873e 100644 --- a/include/linux/clocksource.h +++ b/include/linux/clocksource.h @@ -203,6 +203,7 @@ struct clocksource { }; extern struct clocksource *clock; /* current clocksource */ +extern spinlock_t clocksource_lock; /* * 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..c7027cd 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. @@ -38,6 +39,27 @@ */ unsigned long long __attribute__((weak)) sched_clock(void) { + /* + * Use the current clocksource when it becomes available later in + * the boot process. As this needs to be fast, we only make a + * single pass at grabbing the spinlock. If the clock is changing + * out from underneath us, fall back on jiffies and try it again + * the next time around. + */ + if (clock && _raw_spin_trylock(&clocksource_lock)) { + /* + * Only use clocksources suitable for sched_clock() + */ + if (clock->flags & CLOCK_SOURCE_USE_FOR_SCHED_CLOCK) { + cycle_t now = cyc2ns(clock, clocksource_read(clock)); + _raw_spin_unlock(&clocksource_lock); + return now; + } + + _raw_spin_unlock(&clocksource_lock); + } + + /* If all else fails, fall back on jiffies */ return (unsigned long long)(jiffies - INITIAL_JIFFIES) * (NSEC_PER_SEC / HZ); } diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c index 80189f6..437a6cf 100644 --- a/kernel/time/clocksource.c +++ b/kernel/time/clocksource.c @@ -127,7 +127,7 @@ static struct clocksource *curr_clocksource = &clocksource_jiffies; static struct clocksource *next_clocksource; static struct clocksource *clocksource_override; static LIST_HEAD(clocksource_list); -static DEFINE_SPINLOCK(clocksource_lock); +DEFINE_SPINLOCK(clocksource_lock); static char override_name[32]; static int finished_booting;
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