On Friday 07 December 2007 12:19, Stefano Brivio wrote:
> This patch fixes a regression introduced by:
>
> commit bb29ab26863c022743143f27956cc0ca362f258c
> Author: Ingo Molnar <[email protected]>
> Date: Mon Jul 9 18:51:59 2007 +0200
>
> This caused the jiffies counter to leap back and forth on cpufreq changes
> on my x86 box. I'd say that we can't always assume that TSC does "small
> errors" only, when marked unstable. On cpufreq changes these errors can be
> huge.
>
> The original bug report can be found here:
> http://bugzilla.kernel.org/show_bug.cgi?id=9475
>
>
> Signed-off-by: Stefano Brivio <[email protected]>
While your fix should probably go into 2.6.24...
This particular issue has aggravated me enough times. Let's
fix the damn thing properly already... I think what would work best
is a relatively simple change to the API along these lines:
Index: linux-2.6/arch/x86/kernel/tsc_32.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/tsc_32.c
+++ linux-2.6/arch/x86/kernel/tsc_32.c
@@ -92,27 +92,35 @@ static inline void set_cyc2ns_scale(unsi
/*
* Scheduler clock - returns current time in nanosec units.
*/
-unsigned long long native_sched_clock(void)
+u64 native_sched_clock(u64 *sample)
{
- unsigned long long this_offset;
+ u64 now, delta;
/*
- * Fall back to jiffies if there's no TSC available:
+ * Fall back to the default sched_clock() implementation (keep in
+ * synch with kernel/sched.c) if there's no TSC available:
* ( But note that we still use it if the TSC is marked
* unstable. We do this because unlike Time Of Day,
* the scheduler clock tolerates small errors and it's
* very important for it to be as fast as the platform
* can achive it. )
*/
- if (unlikely(!tsc_enabled && !tsc_unstable))
- /* No locking but a rare wrong value is not a big deal: */
- return (jiffies_64 - INITIAL_JIFFIES) * (1000000000 / HZ);
+ if (unlikely(!tsc_enabled && !tsc_unstable)) {
+ now = (u64)jiffies;
+ delta = now - *sample;
+ *sample = now;
+
+ return delta * (NSEC_PER_SEC / HZ);
+
+ } else {
+ /* read the Time Stamp Counter: */
+ rdtscll(now);
+ delta = now - *sample;
+ *sample = now;
- /* read the Time Stamp Counter: */
- rdtscll(this_offset);
-
- /* return the value in ns */
- return cycles_2_ns(this_offset);
+ /* return the delta value in ns */
+ return cycles_2_ns(delta);
+ }
}
/* We need to define a real function for sched_clock, to override the
Index: linux-2.6/kernel/sched.c
===================================================================
--- linux-2.6.orig/kernel/sched.c
+++ linux-2.6/kernel/sched.c
@@ -72,9 +72,13 @@
* This is default implementation.
* Architectures and sub-architectures can override this.
*/
-unsigned long long __attribute__((weak)) sched_clock(void)
+u64 __attribute__((weak)) sched_clock(u64 *sample)
{
- return (unsigned long long)jiffies * (NSEC_PER_SEC / HZ);
+ u64 now = (u64)jiffies;
+ u64 delta = now - *sample;
+ *sample = now;
+
+ return delta * (NSEC_PER_SEC / HZ);
}
/*
@@ -314,7 +318,7 @@ struct rq {
unsigned long next_balance;
struct mm_struct *prev_mm;
- u64 clock, prev_clock_raw;
+ u64 clock, prev_clock_sample;
s64 clock_max_delta;
unsigned int clock_warps, clock_overflows;
@@ -385,9 +389,7 @@ static inline int cpu_of(struct rq *rq)
*/
static void __update_rq_clock(struct rq *rq)
{
- u64 prev_raw = rq->prev_clock_raw;
- u64 now = sched_clock();
- s64 delta = now - prev_raw;
+ u64 delta = sched_clock(&rq->prev_clock_sample);
u64 clock = rq->clock;
#ifdef CONFIG_SCHED_DEBUG
@@ -416,7 +418,6 @@ static void __update_rq_clock(struct rq
}
}
- rq->prev_clock_raw = now;
rq->clock = clock;
}
@@ -656,7 +657,6 @@ EXPORT_SYMBOL_GPL(sched_clock_idle_sleep
void sched_clock_idle_wakeup_event(u64 delta_ns)
{
struct rq *rq = cpu_rq(smp_processor_id());
- u64 now = sched_clock();
rq->idle_clock += delta_ns;
/*
@@ -666,7 +666,7 @@ void sched_clock_idle_wakeup_event(u64 d
* rq clock:
*/
spin_lock(&rq->lock);
- rq->prev_clock_raw = now;
+ (void)sched_clock(&rq->prev_clock_sample);
rq->clock += delta_ns;
spin_unlock(&rq->lock);
}
@@ -4967,7 +4967,7 @@ void __cpuinit init_idle(struct task_str
unsigned long flags;
__sched_fork(idle);
- idle->se.exec_start = sched_clock();
+ idle->se.exec_start = 0;
idle->prio = idle->normal_prio = MAX_PRIO;
idle->cpus_allowed = cpumask_of_cpu(cpu);
Index: linux-2.6/arch/x86/kernel/tsc_64.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/tsc_64.c
+++ linux-2.6/arch/x86/kernel/tsc_64.c
@@ -30,18 +30,21 @@ static unsigned long long cycles_2_ns(un
return (cyc * cyc2ns_scale) >> NS_SCALE;
}
-unsigned long long sched_clock(void)
+u64 sched_clock(u64 *sample)
{
- unsigned long a = 0;
+ u64 now, delta;
/* Could do CPU core sync here. Opteron can execute rdtsc speculatively,
* which means it is not completely exact and may not be monotonous
* between CPUs. But the errors should be too small to matter for
* scheduling purposes.
*/
+ rdtscll(now);
+ delta = now - *sample;
+ *sample = now;
- rdtscll(a);
- return cycles_2_ns(a);
+ /* return the delta value in ns */
+ return cycles_2_ns(delta);
}
static int tsc_unstable;
Index: linux-2.6/include/linux/sched.h
===================================================================
--- linux-2.6.orig/include/linux/sched.h
+++ linux-2.6/include/linux/sched.h
@@ -1433,7 +1433,7 @@ static inline int set_cpus_allowed(struc
}
#endif
-extern unsigned long long sched_clock(void);
+extern u64 sched_clock(u64 *sample);
/*
* For kernel-internal use: high-speed (but slightly incorrect) per-cpu
Index: linux-2.6/include/asm-x86/timer.h
===================================================================
--- linux-2.6.orig/include/asm-x86/timer.h
+++ linux-2.6/include/asm-x86/timer.h
@@ -5,7 +5,7 @@
#define TICK_SIZE (tick_nsec / 1000)
-unsigned long long native_sched_clock(void);
+unsigned u64 native_sched_clock(u64 *sample);
unsigned long native_calculate_cpu_khz(void);
extern int timer_ack;
Index: linux-2.6/kernel/lockdep.c
===================================================================
--- linux-2.6.orig/kernel/lockdep.c
+++ linux-2.6/kernel/lockdep.c
@@ -216,7 +216,7 @@ static void lock_release_holdtime(struct
if (!lock_stat)
return;
- holdtime = sched_clock() - hlock->holdtime_stamp;
+ holdtime = sched_clock(&hlock->holdtime_stamp);
stats = get_lock_stats(hlock->class);
if (hlock->read)
@@ -2413,7 +2413,7 @@ static int __lock_acquire(struct lockdep
hlock->hardirqs_off = hardirqs_off;
#ifdef CONFIG_LOCK_STAT
hlock->waittime_stamp = 0;
- hlock->holdtime_stamp = sched_clock();
+ (void)sched_clock(&hlock->holdtime_stamp);
#endif
if (check == 2 && !mark_irqflags(curr, hlock))
@@ -2780,7 +2780,7 @@ __lock_contended(struct lockdep_map *loc
return;
found_it:
- hlock->waittime_stamp = sched_clock();
+ (void)sched_clock(&hlock->waittime_stamp);
point = lock_contention_point(hlock->class, ip);
@@ -2825,9 +2825,8 @@ __lock_acquired(struct lockdep_map *lock
found_it:
cpu = smp_processor_id();
if (hlock->waittime_stamp) {
- now = sched_clock();
- waittime = now - hlock->waittime_stamp;
- hlock->holdtime_stamp = now;
+ waittime = sched_clock(&hlock->waittime_stamp);
+ hlock->holdtime_stamp = hlock->waittime_stamp;
}
stats = get_lock_stats(hlock->class);
Index: linux-2.6/kernel/printk.c
===================================================================
--- linux-2.6.orig/kernel/printk.c
+++ linux-2.6/kernel/printk.c
@@ -575,7 +575,7 @@ __setup("time", printk_time_setup);
__attribute__((weak)) unsigned long long printk_clock(void)
{
- return sched_clock();
+ return (unsigned long long)get_cycles();
}
/* Check if we have any console registered that can be called early in boot. */
Index: linux-2.6/kernel/sched_debug.c
===================================================================
--- linux-2.6.orig/kernel/sched_debug.c
+++ linux-2.6/kernel/sched_debug.c
@@ -176,7 +176,7 @@ static void print_cpu(struct seq_file *m
P(curr->pid);
PN(clock);
PN(idle_clock);
- PN(prev_clock_raw);
+ PN(prev_clock_sample);
P(clock_warps);
P(clock_overflows);
P(clock_deep_idle_events);
@@ -353,12 +353,12 @@ void proc_sched_show_task(struct task_st
#undef __P
{
- u64 t0, t1;
+ u64 sample, delta;
- t0 = sched_clock();
- t1 = sched_clock();
+ (void)sched_clock(&sample);
+ delta = sched_clock(&sample);
SEQ_printf(m, "%-35s:%21Ld\n",
- "clock-delta", (long long)(t1-t0));
+ "clock-delta", (long long)delta);
}
}
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