kernel: optimize ms-to-ticks for certain tick frequencies
Some tick frequencies lend themselves to optimized conversions from ms
to ticks and vice-versa.
- 1000Hz which does not need any conversion
- 500Hz, 250Hz, 125Hz where the division/multiplication are a straight
shift since they are power-of-two factors of 1000.
In addition, some more generally used values are made to use optimized
conversion equations rather than the generic one that uses 64-bit math,
and often results in calling compiler intrinsics.
These values are: 100Hz, 50Hz, 25Hz, 20Hz, 10Hz, 1Hz (the last one used
in some testing).
Avoiding the 64-bit math intrisics has the additional benefit, in
addition to increased performance, of using a significant lower amount
of stack space: 52 bytes on ARM Cortex-M and 80 bytes on x86.
Change-Id: I080eb338a2637d6b1c6838c119af1a9fa37fe869
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>
diff --git a/include/kernel.h b/include/kernel.h
index 9f67834..35b4510 100644
--- a/include/kernel.h
+++ b/include/kernel.h
@@ -32,6 +32,7 @@
#include <misc/__assert.h>
#include <misc/dlist.h>
#include <misc/slist.h>
+#include <misc/util.h>
#ifdef __cplusplus
extern "C" {
@@ -634,14 +635,48 @@
* @cond INTERNAL_HIDDEN
*/
+/* kernel clocks */
+
+#if (sys_clock_ticks_per_sec == 1000) || \
+ (sys_clock_ticks_per_sec == 500) || \
+ (sys_clock_ticks_per_sec == 250) || \
+ (sys_clock_ticks_per_sec == 125) || \
+ (sys_clock_ticks_per_sec == 100) || \
+ (sys_clock_ticks_per_sec == 50) || \
+ (sys_clock_ticks_per_sec == 25) || \
+ (sys_clock_ticks_per_sec == 20) || \
+ (sys_clock_ticks_per_sec == 10) || \
+ (sys_clock_ticks_per_sec == 1)
+
+ #define _ms_per_tick (MSEC_PER_SEC / sys_clock_ticks_per_sec)
+#else
+ /* yields horrible 64-bit math on many architectures: try to avoid */
+ #define _NON_OPTIMIZED_TICKS_PER_SEC
+#endif
+
+#ifdef _NON_OPTIMIZED_TICKS_PER_SEC
+extern int32_t _ms_to_ticks(int32_t ms);
+#else
+static ALWAYS_INLINE int32_t _ms_to_ticks(int32_t ms)
+{
+ return (int32_t)ceiling_fraction((uint32_t)ms, _ms_per_tick);
+}
+#endif
+
/* added tick needed to account for tick in progress */
#define _TICK_ALIGN 1
-static int64_t __ticks_to_ms(int64_t ticks)
+static inline int64_t __ticks_to_ms(int64_t ticks)
{
-#if CONFIG_SYS_CLOCK_EXISTS
+#ifdef CONFIG_SYS_CLOCK_EXISTS
+
+#ifdef _NON_OPTIMIZED_TICKS_PER_SEC
return (MSEC_PER_SEC * (uint64_t)ticks) / sys_clock_ticks_per_sec;
#else
+ return (uint64_t)ticks * _ms_per_tick;
+#endif
+
+#else
__ASSERT(ticks == 0, "");
return 0;
#endif
diff --git a/include/legacy.h b/include/legacy.h
index f3d525e..b4f4624 100644
--- a/include/legacy.h
+++ b/include/legacy.h
@@ -2892,10 +2892,6 @@
*/
#define nano_task_stack_pop nano_stack_pop
-/* kernel clocks */
-
-extern int32_t _ms_to_ticks(int32_t ms);
-
/**
* @brief Return the current system tick count.
*
diff --git a/kernel/Kconfig b/kernel/Kconfig
index 406420d..3e175ea 100644
--- a/kernel/Kconfig
+++ b/kernel/Kconfig
@@ -399,6 +399,30 @@
help
This option specifies the frequency of the system clock in Hz.
+ Depending on the choice made, an amount of possibly expensive math must
+ occur when converting ticks to milliseconds and vice-versa. Some values
+ are optimized, and yield significantly less math.
+
+ The optimal values from a computational point-of-view are 1000, 500,
+ 250 and 125, since in these cases there is either no computation
+ required, or it is all done via bit-shifting. These also give a
+ granularity from 1ms to 8ms.
+
+ Other good values are 100, 50, 25, 20 and 10. In this case, some math
+ is required but is minimized. These are also values that necessitate a
+ reduced number of clock interrupts per second, at the cost of
+ granularity (10ms to 100ms).
+
+ All other values require some extensive 64-bit math, and in some
+ configurations even require calls to compiler built-in functions, and
+ can require a non-trivial extra amount of stack space (e.g. around 80
+ bytes on x86).
+
+ Using the legacy API also incurs an extra penalty, since when asking
+ for a timeout, a translation is made from ticks to milliseconds to call
+ the native kernel APIs, and then another translation is made back to
+ ticks, since the kernel is tick-based.
+
config SYS_CLOCK_HW_CYCLES_PER_SEC
int "System clock's h/w timer frequency"
help
diff --git a/kernel/include/ksched.h b/kernel/include/ksched.h
index 008ae85..7cfc08d 100644
--- a/kernel/include/ksched.h
+++ b/kernel/include/ksched.h
@@ -31,7 +31,9 @@
extern void _pend_current_thread(_wait_q_t *wait_q, int32_t timeout);
extern void _move_thread_to_end_of_prio_q(struct k_thread *thread);
extern int __must_switch_threads(void);
+#ifdef _NON_OPTIMIZED_TICKS_PER_SEC
extern int32_t _ms_to_ticks(int32_t ms);
+#endif
extern void idle(void *, void *, void *);
/* find which one is the next thread to run */
diff --git a/kernel/sched.c b/kernel/sched.c
index 2755d81..c6674c0 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -19,6 +19,7 @@
#include <atomic.h>
#include <ksched.h>
#include <wait_q.h>
+#include <misc/util.h>
/* the only struct _kernel instance */
struct _kernel _kernel = {0};
@@ -191,15 +192,14 @@
/* convert milliseconds to ticks */
-#define ceiling(numerator, divider) \
- (((numerator) + ((divider) - 1)) / (divider))
-
+#ifdef _NON_OPTIMIZED_TICKS_PER_SEC
int32_t _ms_to_ticks(int32_t ms)
{
int64_t ms_ticks_per_sec = (int64_t)ms * sys_clock_ticks_per_sec;
- return (int32_t)ceiling(ms_ticks_per_sec, MSEC_PER_SEC);
+ return (int32_t)ceiling_fraction(ms_ticks_per_sec, MSEC_PER_SEC);
}
+#endif
/* pend the specified thread: it must *not* be in the ready queue */
/* must be called with interrupts locked */
diff --git a/kernel/sys_clock.c b/kernel/sys_clock.c
index c8f28d7..26cea33 100644
--- a/kernel/sys_clock.c
+++ b/kernel/sys_clock.c
@@ -24,6 +24,12 @@
#include <drivers/system_timer.h>
#ifdef CONFIG_SYS_CLOCK_EXISTS
+#ifdef _NON_OPTIMIZED_TICKS_PER_SEC
+#warning "non-optimized system clock frequency chosen: performance may suffer"
+#endif
+#endif
+
+#ifdef CONFIG_SYS_CLOCK_EXISTS
int sys_clock_us_per_tick = 1000000 / sys_clock_ticks_per_sec;
int sys_clock_hw_cycles_per_tick =
CONFIG_SYS_CLOCK_HW_CYCLES_PER_SEC / sys_clock_ticks_per_sec;
