Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles

Steven Rostedt
2008-01-16T23:34:11Z

Thanks John for doing this!

(comments imbedded)

On Wed, 16 Jan 2008, john stultz wrote:

>
> On Wed, 2008-01-16 at 14:36 -0800, john stultz wrote:
>
> Completely un-tested, but it builds, so I figured I'd send it out for
> review.

heh, ok, I'll take it and run it.

>
> I'm not super sure the update or the read doesn't need something
> additional to force a memory access, but as I didn't see anything
> special in Mathieu's implementation, I'm going to guess this is ok.
>
> Mathieu, Let me know if this isn't what you're suggesting.
>
> Signed-off-by: John Stultz <johnstul@us.ibm.com>
>
> Index: monotonic-cleanup/include/linux/clocksource.h
> ===================================================================
> --- monotonic-cleanup.orig/include/linux/clocksource.h 2008-01-16 12:22:04.000000000 -0800
> +++ monotonic-cleanup/include/linux/clocksource.h 2008-01-16 14:41:31.000000000 -0800
> @@ -87,9 +87,17 @@
> * more than one cache line.
> */
> struct {
> - cycle_t cycle_last, cycle_accumulated, cycle_raw;
> - } ____cacheline_aligned_in_smp;
> + cycle_t cycle_last, cycle_accumulated;
>
> + /* base structure provides lock-free read
> + * access to a virtualized 64bit counter
> + * Uses RCU-like update.
> + */
> + struct {
> + cycle_t cycle_base_last, cycle_base;
> + } base[2];
> + int base_num;
> + } ____cacheline_aligned_in_smp;
> u64 xtime_nsec;
> s64 error;
>
> @@ -175,19 +183,21 @@
> }
>
> /**
> - * clocksource_get_cycles: - Access the clocksource's accumulated cycle value
> + * clocksource_get_basecycles: - get the clocksource's accumulated cycle value
> * @cs: pointer to clocksource being read
> * @now: current cycle value
> *
> * Uses the clocksource to return the current cycle_t value.
> * NOTE!!!: This is different from clocksource_read, because it
> - * returns the accumulated cycle value! Must hold xtime lock!
> + * returns a 64bit wide accumulated value.
> */
> static inline cycle_t
> -clocksource_get_cycles(struct clocksource *cs, cycle_t now)
> +clocksource_get_basecycles(struct clocksource *cs, cycle_t now)
> {
> - cycle_t offset = (now - cs->cycle_last) & cs->mask;
> - offset += cs->cycle_accumulated;
> + int num = cs->base_num;
> + cycle_t offset = (now - cs->base[num].cycle_base_last);
> + offset &= cs->mask;
> + offset += cs->base[num].cycle_base;
> return offset;
> }
>
> @@ -197,14 +207,25 @@
> * @now: current cycle value
> *
> * Used to avoids clocksource hardware overflow by periodically
> - * accumulating the current cycle delta. Must hold xtime write lock!
> + * accumulating the current cycle delta. Uses RCU-like update, but
> + * ***still requires the xtime_lock is held for writing!***
> */
> static inline void clocksource_accumulate(struct clocksource *cs, cycle_t now)
> {
> - cycle_t offset = (now - cs->cycle_last) & cs->mask;
> + /* First update the monotonic base portion.
> + * The dual array update method allows for lock-free reading.
> + */
> + int num = !cs->base_num;
> + cycle_t offset = (now - cs->base[!num].cycle_base_last);
> + offset &= cs->mask;
> + cs->base[num].cycle_base = cs->base[!num].cycle_base + offset;
> + cs->base[num].cycle_base_last = now;

I would think that we would need some sort of barrier here. Otherwise,
base_num could be updated before all the cycle_base. I'd expect a smp_wmb
is needed.

> + cs->base_num = num;
> +
> + /* Now update the cycle_accumulated portion */
> + offset = (now - cs->cycle_last) & cs->mask;
> cs->cycle_last = now;
> cs->cycle_accumulated += offset;
> - cs->cycle_raw += offset;
> }
>
> /**
> Index: monotonic-cleanup/kernel/time/timekeeping.c
> ===================================================================
> --- monotonic-cleanup.orig/kernel/time/timekeeping.c 2008-01-16 12:21:46.000000000 -0800
> +++ monotonic-cleanup/kernel/time/timekeeping.c 2008-01-16 14:15:31.000000000 -0800
> @@ -71,10 +71,12 @@
> */
> static inline s64 __get_nsec_offset(void)
> {
> - cycle_t cycle_delta;
> + cycle_t now, cycle_delta;
> s64 ns_offset;
>
> - cycle_delta = clocksource_get_cycles(clock, clocksource_read(clock));
> + now = clocksource_read(clock);
> + cycle_delta = (now - clock->cycle_last) & clock->mask;
> + cycle_delta += clock->cycle_accumulated;

Is the above just to decouple the two methods?

> ns_offset = cyc2ns(clock, cycle_delta);
>
> return ns_offset;
> @@ -105,35 +107,7 @@
>
> cycle_t notrace get_monotonic_cycles(void)
> {
> - cycle_t cycle_now, cycle_delta, cycle_raw, cycle_last;
> -
> - do {
> - /*
> - * cycle_raw and cycle_last can change on
> - * another CPU and we need the delta calculation
> - * of cycle_now and cycle_last happen atomic, as well
> - * as the adding to cycle_raw. We don't need to grab
> - * any locks, we just keep trying until get all the
> - * calculations together in one state.
> - *
> - * In fact, we __cant__ grab any locks. This
> - * function is called from the latency_tracer which can
> - * be called anywhere. To grab any locks (including
> - * seq_locks) we risk putting ourselves into a deadlock.
> - */
> - cycle_raw = clock->cycle_raw;
> - cycle_last = clock->cycle_last;
> -
> - /* read clocksource: */
> - cycle_now = clocksource_read(clock);
> -
> - /* calculate the delta since the last update_wall_time: */
> - cycle_delta = (cycle_now - cycle_last) & clock->mask;
> -
> - } while (cycle_raw != clock->cycle_raw ||
> - cycle_last != clock->cycle_last);
> -
> - return cycle_raw + cycle_delta;
> + return clocksource_get_basecycles(clock, clocksource_read(clock));

Nice ;-)

> }
>
> unsigned long notrace cycles_to_usecs(cycle_t cycles)

-- Steve