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sched_clock: fix cpu_clock()
This patch fixes 3 issues:
a) it removes the dependency on jiffies, because jiffies are incremented
by a single CPU, and the tick is not synchronized between CPUs. Therefore
relying on it to calculate a window to clip whacky TSC values doesn't work
as it can drift around.
So instead use [GTOD, GTOD+TICK_NSEC) as the window.
b) __update_sched_clock() did (roughly speaking):
delta = sched_clock() - scd->tick_raw;
clock += delta;
Which gives exponential growth, instead of linear.
c) allows the sched_clock_cpu() value to warp the u64 without breaking.
the results are more reliable sched_clock() deltas:
before after sched_clock
cpu_clock: 15750 51312 51488
cpu_clock: 59719 51052 50947
cpu_clock: 15879 51249 51061
cpu_clock: 1 50933 51198
cpu_clock: 1 50931 51039
cpu_clock: 1 51093 50981
cpu_clock: 1 51043 51040
cpu_clock: 1 50959 50938
cpu_clock: 1 50981 51011
cpu_clock: 1 51364 51212
cpu_clock: 1 51219 51273
cpu_clock: 1 51389 51048
cpu_clock: 1 51285 51611
cpu_clock: 1 50964 51137
cpu_clock: 1 50973 50968
cpu_clock: 1 50967 50972
cpu_clock: 1 58910 58485
cpu_clock: 1 51082 51025
cpu_clock: 1 50957 50958
cpu_clock: 1 50958 50957
cpu_clock: 1006128 51128 50971
cpu_clock: 1 51107 51155
cpu_clock: 1 51371 51081
cpu_clock: 1 51104 51365
cpu_clock: 1 51363 51309
cpu_clock: 1 51107 51160
cpu_clock: 1 51139 51100
cpu_clock: 1 51216 51136
cpu_clock: 1 51207 51215
cpu_clock: 1 51087 51263
cpu_clock: 1 51249 51177
cpu_clock: 1 51519 51412
cpu_clock: 1 51416 51255
cpu_clock: 1 51591 51594
cpu_clock: 1 50966 51374
cpu_clock: 1 50966 50966
cpu_clock: 1 51291 50948
cpu_clock: 1 50973 50867
cpu_clock: 1 50970 50970
cpu_clock: 998306 50970 50971
cpu_clock: 1 50971 50970
cpu_clock: 1 50970 50970
cpu_clock: 1 50971 50971
cpu_clock: 1 50970 50970
cpu_clock: 1 51351 50970
cpu_clock: 1 50970 51352
cpu_clock: 1 50971 50970
cpu_clock: 1 50970 50970
cpu_clock: 1 51321 50971
cpu_clock: 1 50974 51324
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Peter Zijlstra
Ingo Molnar
parent
efc2dead2c
commit
354879bb97
1 changed files with 34 additions and 50 deletions
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@ -12,19 +12,17 @@
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*
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* Create a semi stable clock from a mixture of other events, including:
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* - gtod
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* - jiffies
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* - sched_clock()
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* - explicit idle events
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*
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* We use gtod as base and the unstable clock deltas. The deltas are filtered,
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* making it monotonic and keeping it within an expected window. This window
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* is set up using jiffies.
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* making it monotonic and keeping it within an expected window.
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*
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* Furthermore, explicit sleep and wakeup hooks allow us to account for time
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* that is otherwise invisible (TSC gets stopped).
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*
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* The clock: sched_clock_cpu() is monotonic per cpu, and should be somewhat
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* consistent between cpus (never more than 1 jiffies difference).
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* consistent between cpus (never more than 2 jiffies difference).
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*/
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#include <linux/sched.h>
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#include <linux/percpu.h>
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@ -54,7 +52,6 @@ struct sched_clock_data {
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*/
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raw_spinlock_t lock;
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unsigned long tick_jiffies;
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u64 tick_raw;
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u64 tick_gtod;
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u64 clock;
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@ -75,14 +72,12 @@ static inline struct sched_clock_data *cpu_sdc(int cpu)
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void sched_clock_init(void)
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{
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u64 ktime_now = ktime_to_ns(ktime_get());
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unsigned long now_jiffies = jiffies;
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int cpu;
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for_each_possible_cpu(cpu) {
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struct sched_clock_data *scd = cpu_sdc(cpu);
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scd->lock = (raw_spinlock_t)__RAW_SPIN_LOCK_UNLOCKED;
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scd->tick_jiffies = now_jiffies;
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scd->tick_raw = 0;
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scd->tick_gtod = ktime_now;
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scd->clock = ktime_now;
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@ -91,47 +86,52 @@ void sched_clock_init(void)
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sched_clock_running = 1;
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}
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/*
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* min,max except they take wrapping into account
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*/
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static inline u64 wrap_min(u64 x, u64 y)
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{
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return (s64)(x - y) < 0 ? x : y;
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}
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static inline u64 wrap_max(u64 x, u64 y)
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{
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return (s64)(x - y) > 0 ? x : y;
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}
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/*
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* update the percpu scd from the raw @now value
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*
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* - filter out backward motion
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* - use jiffies to generate a min,max window to clip the raw values
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* - use the GTOD tick value to create a window to filter crazy TSC values
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*/
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static u64 __update_sched_clock(struct sched_clock_data *scd, u64 now)
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{
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unsigned long now_jiffies = jiffies;
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long delta_jiffies = now_jiffies - scd->tick_jiffies;
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u64 clock = scd->clock;
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u64 min_clock, max_clock;
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s64 delta = now - scd->tick_raw;
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u64 clock, min_clock, max_clock;
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WARN_ON_ONCE(!irqs_disabled());
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min_clock = scd->tick_gtod + delta_jiffies * TICK_NSEC;
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if (unlikely(delta < 0)) {
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clock++;
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goto out;
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}
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if (unlikely(delta < 0))
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delta = 0;
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max_clock = min_clock + TICK_NSEC;
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/*
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* scd->clock = clamp(scd->tick_gtod + delta,
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* max(scd->tick_gtod, scd->clock),
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* scd->tick_gtod + TICK_NSEC);
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*/
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if (unlikely(clock + delta > max_clock)) {
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if (clock < max_clock)
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clock = max_clock;
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else
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clock++;
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} else {
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clock += delta;
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}
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clock = scd->tick_gtod + delta;
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min_clock = wrap_max(scd->tick_gtod, scd->clock);
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max_clock = scd->tick_gtod + TICK_NSEC;
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out:
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if (unlikely(clock < min_clock))
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clock = min_clock;
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clock = wrap_max(clock, min_clock);
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clock = wrap_min(clock, max_clock);
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scd->tick_jiffies = now_jiffies;
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scd->clock = clock;
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return clock;
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return scd->clock;
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}
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static void lock_double_clock(struct sched_clock_data *data1,
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@ -171,7 +171,7 @@ u64 sched_clock_cpu(int cpu)
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* larger time as the latest time for both
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* runqueues. (this creates monotonic movement)
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*/
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if (likely(remote_clock < this_clock)) {
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if (likely((s64)(remote_clock - this_clock) < 0)) {
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clock = this_clock;
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scd->clock = clock;
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} else {
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@ -207,14 +207,9 @@ void sched_clock_tick(void)
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now = sched_clock();
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__raw_spin_lock(&scd->lock);
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__update_sched_clock(scd, now);
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/*
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* update tick_gtod after __update_sched_clock() because that will
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* already observe 1 new jiffy; adding a new tick_gtod to that would
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* increase the clock 2 jiffies.
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*/
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scd->tick_raw = now;
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scd->tick_gtod = now_gtod;
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__update_sched_clock(scd, now);
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__raw_spin_unlock(&scd->lock);
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}
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@ -232,18 +227,7 @@ EXPORT_SYMBOL_GPL(sched_clock_idle_sleep_event);
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*/
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void sched_clock_idle_wakeup_event(u64 delta_ns)
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{
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struct sched_clock_data *scd = this_scd();
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/*
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* Override the previous timestamp and ignore all
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* sched_clock() deltas that occured while we idled,
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* and use the PM-provided delta_ns to advance the
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* rq clock:
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*/
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__raw_spin_lock(&scd->lock);
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scd->clock += delta_ns;
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__raw_spin_unlock(&scd->lock);
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sched_clock_tick();
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touch_softlockup_watchdog();
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}
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EXPORT_SYMBOL_GPL(sched_clock_idle_wakeup_event);
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