parisc: Add native high-resolution sched_clock() implementation

Add a native implementation for the sched_clock() function which utilizes the
processor-internal cycle counter (Control Register 16) as high-resolution time
source.

With this patch we now get much more fine-grained resolutions in various
in-kernel time measurements (e.g. when viewing the function tracing logs), and
probably a more accurate scheduling on SMP systems.

There are a few specific implementation details in this patch:

1. On a 32bit kernel we emulate the higher 32bits of the required 64-bit
resolution of sched_clock() by increasing a per-cpu counter at every
wrap-around of the 32bit cycle counter.

2. In a SMP system, the cycle counters of the various CPUs are not syncronized
(similiar to the TSC in a x86_64 system). To cope with this we define
HAVE_UNSTABLE_SCHED_CLOCK and let the upper layers do the adjustment work.

3. Since we need HAVE_UNSTABLE_SCHED_CLOCK, we need to provide a cmpxchg64()
function even on a 32-bit kernel.

4. A 64-bit SMP kernel which is started on a UP system will mark the
sched_clock() implementation as "stable", which means that we don't expect any
jumps in the returned counter. This is true because we then run only on one
CPU.

Signed-off-by: Helge Deller <deller@gmx.de>
This commit is contained in:
Helge Deller 2016-04-20 21:34:15 +02:00
parent 64e2a42bca
commit 54b6680090
4 changed files with 70 additions and 9 deletions

View file

@ -33,6 +33,7 @@ config PARISC
select HAVE_ARCH_AUDITSYSCALL
select HAVE_ARCH_SECCOMP_FILTER
select HAVE_ARCH_TRACEHOOK
select HAVE_UNSTABLE_SCHED_CLOCK if (SMP || !64BIT)
select ARCH_NO_COHERENT_DMA_MMAP
select CPU_NO_EFFICIENT_FFS

View file

@ -52,8 +52,7 @@ extern void __cmpxchg_called_with_bad_pointer(void);
/* __cmpxchg_u32/u64 defined in arch/parisc/lib/bitops.c */
extern unsigned long __cmpxchg_u32(volatile unsigned int *m, unsigned int old,
unsigned int new_);
extern unsigned long __cmpxchg_u64(volatile unsigned long *ptr,
unsigned long old, unsigned long new_);
extern u64 __cmpxchg_u64(volatile u64 *ptr, u64 old, u64 new_);
/* don't worry...optimizer will get rid of most of this */
static inline unsigned long
@ -61,7 +60,7 @@ __cmpxchg(volatile void *ptr, unsigned long old, unsigned long new_, int size)
{
switch (size) {
#ifdef CONFIG_64BIT
case 8: return __cmpxchg_u64((unsigned long *)ptr, old, new_);
case 8: return __cmpxchg_u64((u64 *)ptr, old, new_);
#endif
case 4: return __cmpxchg_u32((unsigned int *)ptr,
(unsigned int)old, (unsigned int)new_);
@ -86,7 +85,7 @@ static inline unsigned long __cmpxchg_local(volatile void *ptr,
{
switch (size) {
#ifdef CONFIG_64BIT
case 8: return __cmpxchg_u64((unsigned long *)ptr, old, new_);
case 8: return __cmpxchg_u64((u64 *)ptr, old, new_);
#endif
case 4: return __cmpxchg_u32(ptr, old, new_);
default:
@ -111,4 +110,6 @@ static inline unsigned long __cmpxchg_local(volatile void *ptr,
#define cmpxchg64_local(ptr, o, n) __cmpxchg64_local_generic((ptr), (o), (n))
#endif
#define cmpxchg64(ptr, o, n) __cmpxchg_u64(ptr, o, n)
#endif /* _ASM_PARISC_CMPXCHG_H_ */

View file

@ -38,6 +38,18 @@
static unsigned long clocktick __read_mostly; /* timer cycles per tick */
#ifndef CONFIG_64BIT
/*
* The processor-internal cycle counter (Control Register 16) is used as time
* source for the sched_clock() function. This register is 64bit wide on a
* 64-bit kernel and 32bit on a 32-bit kernel. Since sched_clock() always
* requires a 64bit counter we emulate on the 32-bit kernel the higher 32bits
* with a per-cpu variable which we increase every time the counter
* wraps-around (which happens every ~4 secounds).
*/
static DEFINE_PER_CPU(unsigned long, cr16_high_32_bits);
#endif
/*
* We keep time on PA-RISC Linux by using the Interval Timer which is
* a pair of registers; one is read-only and one is write-only; both
@ -108,6 +120,12 @@ irqreturn_t __irq_entry timer_interrupt(int irq, void *dev_id)
*/
mtctl(next_tick, 16);
#if !defined(CONFIG_64BIT)
/* check for overflow on a 32bit kernel (every ~4 seconds). */
if (unlikely(next_tick < now))
this_cpu_inc(cr16_high_32_bits);
#endif
/* Skip one clocktick on purpose if we missed next_tick.
* The new CR16 must be "later" than current CR16 otherwise
* itimer would not fire until CR16 wrapped - e.g 4 seconds
@ -219,6 +237,12 @@ void __init start_cpu_itimer(void)
unsigned int cpu = smp_processor_id();
unsigned long next_tick = mfctl(16) + clocktick;
#if defined(CONFIG_HAVE_UNSTABLE_SCHED_CLOCK) && defined(CONFIG_64BIT)
/* With multiple 64bit CPUs online, the cr16's are not syncronized. */
if (cpu != 0)
clear_sched_clock_stable();
#endif
mtctl(next_tick, 16); /* kick off Interval Timer (CR16) */
per_cpu(cpu_data, cpu).it_value = next_tick;
@ -246,15 +270,52 @@ void read_persistent_clock(struct timespec *ts)
}
}
/*
* sched_clock() framework
*/
static u32 cyc2ns_mul __read_mostly;
static u32 cyc2ns_shift __read_mostly;
u64 sched_clock(void)
{
u64 now;
/* Get current cycle counter (Control Register 16). */
#ifdef CONFIG_64BIT
now = mfctl(16);
#else
now = mfctl(16) + (((u64) this_cpu_read(cr16_high_32_bits)) << 32);
#endif
/* return the value in ns (cycles_2_ns) */
return mul_u64_u32_shr(now, cyc2ns_mul, cyc2ns_shift);
}
/*
* timer interrupt and sched_clock() initialization
*/
void __init time_init(void)
{
unsigned long current_cr16_khz;
current_cr16_khz = PAGE0->mem_10msec/10; /* kHz */
clocktick = (100 * PAGE0->mem_10msec) / HZ;
/* calculate mult/shift values for cr16 */
clocks_calc_mult_shift(&cyc2ns_mul, &cyc2ns_shift, current_cr16_khz,
NSEC_PER_MSEC, 0);
#if defined(CONFIG_HAVE_UNSTABLE_SCHED_CLOCK) && defined(CONFIG_64BIT)
/* At bootup only one 64bit CPU is online and cr16 is "stable" */
set_sched_clock_stable();
#endif
start_cpu_itimer(); /* get CPU 0 started */
/* register at clocksource framework */
current_cr16_khz = PAGE0->mem_10msec/10; /* kHz */
clocksource_register_khz(&clocksource_cr16, current_cr16_khz);
}

View file

@ -55,11 +55,10 @@ unsigned long __xchg8(char x, char *ptr)
}
#ifdef CONFIG_64BIT
unsigned long __cmpxchg_u64(volatile unsigned long *ptr, unsigned long old, unsigned long new)
u64 __cmpxchg_u64(volatile u64 *ptr, u64 old, u64 new)
{
unsigned long flags;
unsigned long prev;
u64 prev;
_atomic_spin_lock_irqsave(ptr, flags);
if ((prev = *ptr) == old)
@ -67,7 +66,6 @@ unsigned long __cmpxchg_u64(volatile unsigned long *ptr, unsigned long old, unsi
_atomic_spin_unlock_irqrestore(ptr, flags);
return prev;
}
#endif
unsigned long __cmpxchg_u32(volatile unsigned int *ptr, unsigned int old, unsigned int new)
{