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

Mathieu Desnoyers
2008-01-17T02:36:18Z

* john stultz (johnstul@us.ibm.com) wrote:
> On Wed, 2008-01-16 at 18:39 -0500, Mathieu Desnoyers wrote:
> > I would disable preemption in clocksource_get_basecycles. We would not
> > want to be scheduled out while we hold a pointer to the old array
> > element.
> >
> > > + int num = cs->base_num;
> >
> > Since you deal with base_num in a shared manner (not per cpu), you will
> > need a smp_read_barrier_depend() here after the cs->base_num read.
> >
> > You should think about reading the cs->base_num first, and _after_ that
> > read the real clocksource. Here, the clocksource value is passed as
> > parameter. It means that the read clocksource may have been read in the
> > previous RCU window.
>
> Here's an updated version of the patch w/ the suggested memory barrier
> changes and favored (1-x) inversion change. ;) Let me know if you see
> any other holes, or have any other suggestions or ideas.
>
> Still un-tested (my test box will free up soon, I promise!), but builds.
>
> Signed-off-by: John Stultz <johstul@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 18:12:53.000000000 -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 +183,29 @@
> }
>
> /**
> - * 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 offset = (now - cs->cycle_last) & cs->mask;
> - offset += cs->cycle_accumulated;
> + int num;
> + cycle_t now, offset;
> +
> + preempt_disable();
> + num = cs->base_num;
> + smp_read_barrier_depends();
> + now = clocksource_read(cs);
> + offset = (now - cs->base[num].cycle_base_last);
> + offset &= cs->mask;
> + offset += cs->base[num].cycle_base;
> + preempt_enable();
> +
> return offset;
> }
>
> @@ -197,14 +215,26 @@
> * @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 = 1 - cs->base_num;

(nitpick)
right here, you could probably express 1-num with cs->base_num, since we
are the only ones supposed to touch it.

> + cycle_t offset = (now - cs->base[1-num].cycle_base_last);
> + offset &= cs->mask;

here too.

> + cs->base[num].cycle_base = cs->base[1-num].cycle_base + offset;
> + cs->base[num].cycle_base_last = now;
> + wmb();

As I just emailed : smp_smb() *should* be enough. I don't see which
architecture could reorder writes wrt local interrupts ? (please tell me
if I am grossly mistaken)

Mathieu

> + 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 17:51:50.000000000 -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;
> 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);
> }
>
> unsigned long notrace cycles_to_usecs(cycle_t cycles)
>
>

--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68