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/2102.06959 Signed-off-by: Cassio Neri <cassio.neri@gmail.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Link: https://lore.kernel.org/r/20210622213616.313046-1-cassio.neri@gmail.com
This commit is contained in:
Cassio Neri
Thomas Gleixner
parent
4e82d2e20f
commit
2760105516
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@ -64,6 +64,15 @@ config LEGACY_TIMER_TICK
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lack support for the generic clockevent framework.
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New platforms should use generic clockevents instead.
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config TIME_KUNIT_TEST
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tristate "KUnit test for kernel/time functions" if !KUNIT_ALL_TESTS
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depends on KUNIT
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default KUNIT_ALL_TESTS
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help
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Enable this option to test RTC library functions.
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If unsure, say N.
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if GENERIC_CLOCKEVENTS
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menu "Timers subsystem"
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@ -22,3 +22,4 @@ obj-$(CONFIG_DEBUG_FS) += timekeeping_debug.o
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obj-$(CONFIG_TEST_UDELAY) += test_udelay.o
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obj-$(CONFIG_TIME_NS) += namespace.o
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obj-$(CONFIG_TEST_CLOCKSOURCE_WATCHDOG) += clocksource-wdtest.o
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obj-$(CONFIG_TIME_KUNIT_TEST) += time_test.o
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@ -0,0 +1,98 @@
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// SPDX-License-Identifier: LGPL-2.1+
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#include <kunit/test.h>
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#include <linux/time.h>
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/*
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* Traditional implementation of leap year evaluation.
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*/
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static bool is_leap(long year)
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{
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return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
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}
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/*
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* Gets the last day of a month.
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*/
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static int last_day_of_month(long year, int month)
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{
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if (month == 2)
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return 28 + is_leap(year);
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if (month == 4 || month == 6 || month == 9 || month == 11)
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return 30;
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return 31;
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}
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/*
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* Advances a date by one day.
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*/
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static void advance_date(long *year, int *month, int *mday, int *yday)
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{
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if (*mday != last_day_of_month(*year, *month)) {
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++*mday;
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++*yday;
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return;
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}
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*mday = 1;
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if (*month != 12) {
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++*month;
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++*yday;
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return;
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}
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*month = 1;
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*yday = 0;
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++*year;
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}
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/*
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* Checks every day in a 160000 years interval centered at 1970-01-01
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* against the expected result.
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*/
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static void time64_to_tm_test_date_range(struct kunit *test)
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{
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/*
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* 80000 years = (80000 / 400) * 400 years
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* = (80000 / 400) * 146097 days
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* = (80000 / 400) * 146097 * 86400 seconds
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*/
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time64_t total_secs = ((time64_t) 80000) / 400 * 146097 * 86400;
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long year = 1970 - 80000;
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int month = 1;
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int mdday = 1;
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int yday = 0;
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struct tm result;
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time64_t secs;
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s64 days;
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for (secs = -total_secs; secs <= total_secs; secs += 86400) {
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time64_to_tm(secs, 0, &result);
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days = div_s64(secs, 86400);
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#define FAIL_MSG "%05ld/%02d/%02d (%2d) : %ld", \
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year, month, mdday, yday, days
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KUNIT_ASSERT_EQ_MSG(test, year - 1900, result.tm_year, FAIL_MSG);
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KUNIT_ASSERT_EQ_MSG(test, month - 1, result.tm_mon, FAIL_MSG);
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KUNIT_ASSERT_EQ_MSG(test, mdday, result.tm_mday, FAIL_MSG);
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KUNIT_ASSERT_EQ_MSG(test, yday, result.tm_yday, FAIL_MSG);
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advance_date(&year, &month, &mdday, &yday);
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}
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}
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static struct kunit_case time_test_cases[] = {
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KUNIT_CASE(time64_to_tm_test_date_range),
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{}
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};
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static struct kunit_suite time_test_suite = {
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.name = "time_test_cases",
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.test_cases = time_test_cases,
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};
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kunit_test_suite(time_test_suite);
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@ -22,47 +22,16 @@
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/*
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* Converts the calendar time to broken-down time representation
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* Based on code from glibc-2.6
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*
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* 2009-7-14:
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* Moved from glibc-2.6 to kernel by Zhaolei<zhaolei@cn.fujitsu.com>
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* 2021-06-02:
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* Reimplemented by Cassio Neri <cassio.neri@gmail.com>
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*/
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#include <linux/time.h>
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#include <linux/module.h>
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/*
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* Nonzero if YEAR is a leap year (every 4 years,
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* except every 100th isn't, and every 400th is).
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*/
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static int __isleap(long year)
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{
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return (year) % 4 == 0 && ((year) % 100 != 0 || (year) % 400 == 0);
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}
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/* do a mathdiv for long type */
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static long math_div(long a, long b)
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{
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return a / b - (a % b < 0);
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}
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/* How many leap years between y1 and y2, y1 must less or equal to y2 */
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static long leaps_between(long y1, long y2)
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{
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long leaps1 = math_div(y1 - 1, 4) - math_div(y1 - 1, 100)
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+ math_div(y1 - 1, 400);
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long leaps2 = math_div(y2 - 1, 4) - math_div(y2 - 1, 100)
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+ math_div(y2 - 1, 400);
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return leaps2 - leaps1;
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}
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/* How many days come before each month (0-12). */
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static const unsigned short __mon_yday[2][13] = {
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/* Normal years. */
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{0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365},
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/* Leap years. */
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{0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366}
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};
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#include <linux/kernel.h>
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#define SECS_PER_HOUR (60 * 60)
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#define SECS_PER_DAY (SECS_PER_HOUR * 24)
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@ -77,9 +46,11 @@ static const unsigned short __mon_yday[2][13] = {
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*/
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void time64_to_tm(time64_t totalsecs, int offset, struct tm *result)
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{
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long days, rem, y;
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u32 u32tmp, day_of_century, year_of_century, day_of_year, month, day;
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u64 u64tmp, udays, century, year;
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bool is_Jan_or_Feb, is_leap_year;
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long days, rem;
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int remainder;
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const unsigned short *ip;
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days = div_s64_rem(totalsecs, SECS_PER_DAY, &remainder);
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rem = remainder;
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@ -103,27 +74,68 @@ void time64_to_tm(time64_t totalsecs, int offset, struct tm *result)
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if (result->tm_wday < 0)
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result->tm_wday += 7;
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y = 1970;
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/*
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* The following algorithm is, basically, Proposition 6.3 of Neri
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* and Schneider [1]. In a few words: it works on the computational
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* (fictitious) calendar where the year starts in March, month = 2
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* (*), and finishes in February, month = 13. This calendar is
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* mathematically convenient because the day of the year does not
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* depend on whether the year is leap or not. For instance:
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*
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* March 1st 0-th day of the year;
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* ...
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* April 1st 31-st day of the year;
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* ...
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* January 1st 306-th day of the year; (Important!)
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* ...
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* February 28th 364-th day of the year;
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* February 29th 365-th day of the year (if it exists).
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*
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* After having worked out the date in the computational calendar
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* (using just arithmetics) it's easy to convert it to the
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* corresponding date in the Gregorian calendar.
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*
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* [1] "Euclidean Affine Functions and Applications to Calendar
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* Algorithms". https://arxiv.org/abs/2102.06959
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*
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* (*) The numbering of months follows tm more closely and thus,
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* is slightly different from [1].
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*/
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while (days < 0 || days >= (__isleap(y) ? 366 : 365)) {
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/* Guess a corrected year, assuming 365 days per year. */
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long yg = y + math_div(days, 365);
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udays = ((u64) days) + 2305843009213814918ULL;
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/* Adjust DAYS and Y to match the guessed year. */
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days -= (yg - y) * 365 + leaps_between(y, yg);
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y = yg;
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}
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u64tmp = 4 * udays + 3;
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century = div64_u64_rem(u64tmp, 146097, &u64tmp);
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day_of_century = (u32) (u64tmp / 4);
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result->tm_year = y - 1900;
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u32tmp = 4 * day_of_century + 3;
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u64tmp = 2939745ULL * u32tmp;
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year_of_century = upper_32_bits(u64tmp);
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day_of_year = lower_32_bits(u64tmp) / 2939745 / 4;
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result->tm_yday = days;
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year = 100 * century + year_of_century;
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is_leap_year = year_of_century ? !(year_of_century % 4) : !(century % 4);
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ip = __mon_yday[__isleap(y)];
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for (y = 11; days < ip[y]; y--)
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continue;
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days -= ip[y];
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u32tmp = 2141 * day_of_year + 132377;
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month = u32tmp >> 16;
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day = ((u16) u32tmp) / 2141;
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result->tm_mon = y;
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result->tm_mday = days + 1;
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/*
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* Recall that January 1st is the 306-th day of the year in the
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* computational (not Gregorian) calendar.
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*/
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is_Jan_or_Feb = day_of_year >= 306;
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/* Convert to the Gregorian calendar and adjust to Unix time. */
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year = year + is_Jan_or_Feb - 6313183731940000ULL;
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month = is_Jan_or_Feb ? month - 12 : month;
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day = day + 1;
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day_of_year += is_Jan_or_Feb ? -306 : 31 + 28 + is_leap_year;
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/* Convert to tm's format. */
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result->tm_year = (long) (year - 1900);
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result->tm_mon = (int) month;
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result->tm_mday = (int) day;
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result->tm_yday = (int) day_of_year;
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}
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EXPORT_SYMBOL(time64_to_tm);
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