On Wed, 2008-01-16 at 14:36 -0800, john stultz wrote: > On Jan 16, 2008 6:56 AM, Mathieu Desnoyers <[EMAIL PROTECTED]> wrote: > > If you really want an seqlock free algorithm (I _do_ want this for > > tracing!) :) maybe going in the RCU direction could help (I refer to my > > RCU-based 32-to-64 bits lockless timestamp counter extension, which > > could be turned into the clocksource updater). > > Yea. After our earlier discussion and talking w/ Steven, I'm taking a > swing at this now. The lock-free method still doesn't apply to the > update_wall_time function, but does work fine for the monotonic cycle > uses. I'll send a patch for review as soon as I get things building.
So here's my first attempt at adding Mathieu's lock-free method to Steven's get_monotonic_cycles() interface. Completely un-tested, but it builds, so I figured I'd send it out for review. 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 <[EMAIL PROTECTED]> 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; + 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; 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)); } unsigned long notrace cycles_to_usecs(cycle_t cycles) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/