Commit 27601055 authored by Cassio Neri's avatar Cassio Neri Committed by Thomas Gleixner

time: Improve performance of time64_to_tm()

The current implementation of time64_to_tm() contains unnecessary loops,
branches and look-up tables. The new one uses an arithmetic-based algorithm
appeared in [1] and is approximately 3x faster (YMMV).

The drawback is that the new code isn't intuitive and contains many 'magic
numbers' (not unusual for this type of algorithm). However, [1] justifies
all those numbers and, given this function's history, the code is unlikely
to need much maintenance, if any at all.

Add a KUnit test for it which checks every day in a 160,000 years interval
centered at 1970-01-01 against the expected result.

[1] Neri, Schneider, "Euclidean Affine Functions and Applications to
Calendar Algorithms". https://arxiv.org/abs/2102.06959Signed-off-by: default avatarCassio Neri <cassio.neri@gmail.com>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210622213616.313046-1-cassio.neri@gmail.com
parent 4e82d2e2
......@@ -64,6 +64,15 @@ config LEGACY_TIMER_TICK
lack support for the generic clockevent framework.
New platforms should use generic clockevents instead.
config TIME_KUNIT_TEST
tristate "KUnit test for kernel/time functions" if !KUNIT_ALL_TESTS
depends on KUNIT
default KUNIT_ALL_TESTS
help
Enable this option to test RTC library functions.
If unsure, say N.
if GENERIC_CLOCKEVENTS
menu "Timers subsystem"
......
......@@ -22,3 +22,4 @@ obj-$(CONFIG_DEBUG_FS) += timekeeping_debug.o
obj-$(CONFIG_TEST_UDELAY) += test_udelay.o
obj-$(CONFIG_TIME_NS) += namespace.o
obj-$(CONFIG_TEST_CLOCKSOURCE_WATCHDOG) += clocksource-wdtest.o
obj-$(CONFIG_TIME_KUNIT_TEST) += time_test.o
// SPDX-License-Identifier: LGPL-2.1+
#include <kunit/test.h>
#include <linux/time.h>
/*
* Traditional implementation of leap year evaluation.
*/
static bool is_leap(long year)
{
return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
}
/*
* Gets the last day of a month.
*/
static int last_day_of_month(long year, int month)
{
if (month == 2)
return 28 + is_leap(year);
if (month == 4 || month == 6 || month == 9 || month == 11)
return 30;
return 31;
}
/*
* Advances a date by one day.
*/
static void advance_date(long *year, int *month, int *mday, int *yday)
{
if (*mday != last_day_of_month(*year, *month)) {
++*mday;
++*yday;
return;
}
*mday = 1;
if (*month != 12) {
++*month;
++*yday;
return;
}
*month = 1;
*yday = 0;
++*year;
}
/*
* Checks every day in a 160000 years interval centered at 1970-01-01
* against the expected result.
*/
static void time64_to_tm_test_date_range(struct kunit *test)
{
/*
* 80000 years = (80000 / 400) * 400 years
* = (80000 / 400) * 146097 days
* = (80000 / 400) * 146097 * 86400 seconds
*/
time64_t total_secs = ((time64_t) 80000) / 400 * 146097 * 86400;
long year = 1970 - 80000;
int month = 1;
int mdday = 1;
int yday = 0;
struct tm result;
time64_t secs;
s64 days;
for (secs = -total_secs; secs <= total_secs; secs += 86400) {
time64_to_tm(secs, 0, &result);
days = div_s64(secs, 86400);
#define FAIL_MSG "%05ld/%02d/%02d (%2d) : %ld", \
year, month, mdday, yday, days
KUNIT_ASSERT_EQ_MSG(test, year - 1900, result.tm_year, FAIL_MSG);
KUNIT_ASSERT_EQ_MSG(test, month - 1, result.tm_mon, FAIL_MSG);
KUNIT_ASSERT_EQ_MSG(test, mdday, result.tm_mday, FAIL_MSG);
KUNIT_ASSERT_EQ_MSG(test, yday, result.tm_yday, FAIL_MSG);
advance_date(&year, &month, &mdday, &yday);
}
}
static struct kunit_case time_test_cases[] = {
KUNIT_CASE(time64_to_tm_test_date_range),
{}
};
static struct kunit_suite time_test_suite = {
.name = "time_test_cases",
.test_cases = time_test_cases,
};
kunit_test_suite(time_test_suite);
......@@ -22,47 +22,16 @@
/*
* Converts the calendar time to broken-down time representation
* Based on code from glibc-2.6
*
* 2009-7-14:
* Moved from glibc-2.6 to kernel by Zhaolei<zhaolei@cn.fujitsu.com>
* 2021-06-02:
* Reimplemented by Cassio Neri <cassio.neri@gmail.com>
*/
#include <linux/time.h>
#include <linux/module.h>
/*
* Nonzero if YEAR is a leap year (every 4 years,
* except every 100th isn't, and every 400th is).
*/
static int __isleap(long year)
{
return (year) % 4 == 0 && ((year) % 100 != 0 || (year) % 400 == 0);
}
/* do a mathdiv for long type */
static long math_div(long a, long b)
{
return a / b - (a % b < 0);
}
/* How many leap years between y1 and y2, y1 must less or equal to y2 */
static long leaps_between(long y1, long y2)
{
long leaps1 = math_div(y1 - 1, 4) - math_div(y1 - 1, 100)
+ math_div(y1 - 1, 400);
long leaps2 = math_div(y2 - 1, 4) - math_div(y2 - 1, 100)
+ math_div(y2 - 1, 400);
return leaps2 - leaps1;
}
/* How many days come before each month (0-12). */
static const unsigned short __mon_yday[2][13] = {
/* Normal years. */
{0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365},
/* Leap years. */
{0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366}
};
#include <linux/kernel.h>
#define SECS_PER_HOUR (60 * 60)
#define SECS_PER_DAY (SECS_PER_HOUR * 24)
......@@ -77,9 +46,11 @@ static const unsigned short __mon_yday[2][13] = {
*/
void time64_to_tm(time64_t totalsecs, int offset, struct tm *result)
{
long days, rem, y;
u32 u32tmp, day_of_century, year_of_century, day_of_year, month, day;
u64 u64tmp, udays, century, year;
bool is_Jan_or_Feb, is_leap_year;
long days, rem;
int remainder;
const unsigned short *ip;
days = div_s64_rem(totalsecs, SECS_PER_DAY, &remainder);
rem = remainder;
......@@ -103,27 +74,68 @@ void time64_to_tm(time64_t totalsecs, int offset, struct tm *result)
if (result->tm_wday < 0)
result->tm_wday += 7;
y = 1970;
while (days < 0 || days >= (__isleap(y) ? 366 : 365)) {
/* Guess a corrected year, assuming 365 days per year. */
long yg = y + math_div(days, 365);
/* Adjust DAYS and Y to match the guessed year. */
days -= (yg - y) * 365 + leaps_between(y, yg);
y = yg;
}
result->tm_year = y - 1900;
result->tm_yday = days;
ip = __mon_yday[__isleap(y)];
for (y = 11; days < ip[y]; y--)
continue;
days -= ip[y];
result->tm_mon = y;
result->tm_mday = days + 1;
/*
* The following algorithm is, basically, Proposition 6.3 of Neri
* and Schneider [1]. In a few words: it works on the computational
* (fictitious) calendar where the year starts in March, month = 2
* (*), and finishes in February, month = 13. This calendar is
* mathematically convenient because the day of the year does not
* depend on whether the year is leap or not. For instance:
*
* March 1st 0-th day of the year;
* ...
* April 1st 31-st day of the year;
* ...
* January 1st 306-th day of the year; (Important!)
* ...
* February 28th 364-th day of the year;
* February 29th 365-th day of the year (if it exists).
*
* After having worked out the date in the computational calendar
* (using just arithmetics) it's easy to convert it to the
* corresponding date in the Gregorian calendar.
*
* [1] "Euclidean Affine Functions and Applications to Calendar
* Algorithms". https://arxiv.org/abs/2102.06959
*
* (*) The numbering of months follows tm more closely and thus,
* is slightly different from [1].
*/
udays = ((u64) days) + 2305843009213814918ULL;
u64tmp = 4 * udays + 3;
century = div64_u64_rem(u64tmp, 146097, &u64tmp);
day_of_century = (u32) (u64tmp / 4);
u32tmp = 4 * day_of_century + 3;
u64tmp = 2939745ULL * u32tmp;
year_of_century = upper_32_bits(u64tmp);
day_of_year = lower_32_bits(u64tmp) / 2939745 / 4;
year = 100 * century + year_of_century;
is_leap_year = year_of_century ? !(year_of_century % 4) : !(century % 4);
u32tmp = 2141 * day_of_year + 132377;
month = u32tmp >> 16;
day = ((u16) u32tmp) / 2141;
/*
* Recall that January 1st is the 306-th day of the year in the
* computational (not Gregorian) calendar.
*/
is_Jan_or_Feb = day_of_year >= 306;
/* Convert to the Gregorian calendar and adjust to Unix time. */
year = year + is_Jan_or_Feb - 6313183731940000ULL;
month = is_Jan_or_Feb ? month - 12 : month;
day = day + 1;
day_of_year += is_Jan_or_Feb ? -306 : 31 + 28 + is_leap_year;
/* Convert to tm's format. */
result->tm_year = (long) (year - 1900);
result->tm_mon = (int) month;
result->tm_mday = (int) day;
result->tm_yday = (int) day_of_year;
}
EXPORT_SYMBOL(time64_to_tm);
Markdown is supported
0%
or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment