Skip to content

Commit 2760105

Browse files
cassioneriKAGA-KOKO
cassioneri
authored and
KAGA-KOKO
committed
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
1 parent 4e82d2e commit 2760105

4 files changed

Lines changed: 178 additions & 58 deletions

File tree

‎kernel/time/Kconfig‎

Lines changed: 9 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -64,6 +64,15 @@ config LEGACY_TIMER_TICK
6464
lack support for the generic clockevent framework.
6565
New platforms should use generic clockevents instead.
6666

67+
config TIME_KUNIT_TEST
68+
tristate "KUnit test for kernel/time functions" if !KUNIT_ALL_TESTS
69+
depends on KUNIT
70+
default KUNIT_ALL_TESTS
71+
help
72+
Enable this option to test RTC library functions.
73+
74+
If unsure, say N.
75+
6776
if GENERIC_CLOCKEVENTS
6877
menu "Timers subsystem"
6978

‎kernel/time/Makefile‎

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -22,3 +22,4 @@ obj-$(CONFIG_DEBUG_FS) += timekeeping_debug.o
2222
obj-$(CONFIG_TEST_UDELAY) += test_udelay.o
2323
obj-$(CONFIG_TIME_NS) += namespace.o
2424
obj-$(CONFIG_TEST_CLOCKSOURCE_WATCHDOG) += clocksource-wdtest.o
25+
obj-$(CONFIG_TIME_KUNIT_TEST) += time_test.o

‎kernel/time/time_test.c‎

Lines changed: 98 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,98 @@
1+
// SPDX-License-Identifier: LGPL-2.1+
2+
3+
#include <kunit/test.h>
4+
#include <linux/time.h>
5+
6+
/*
7+
* Traditional implementation of leap year evaluation.
8+
*/
9+
static bool is_leap(long year)
10+
{
11+
return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
12+
}
13+
14+
/*
15+
* Gets the last day of a month.
16+
*/
17+
static int last_day_of_month(long year, int month)
18+
{
19+
if (month == 2)
20+
return 28 + is_leap(year);
21+
if (month == 4 || month == 6 || month == 9 || month == 11)
22+
return 30;
23+
return 31;
24+
}
25+
26+
/*
27+
* Advances a date by one day.
28+
*/
29+
static void advance_date(long *year, int *month, int *mday, int *yday)
30+
{
31+
if (*mday != last_day_of_month(*year, *month)) {
32+
++*mday;
33+
++*yday;
34+
return;
35+
}
36+
37+
*mday = 1;
38+
if (*month != 12) {
39+
++*month;
40+
++*yday;
41+
return;
42+
}
43+
44+
*month = 1;
45+
*yday = 0;
46+
++*year;
47+
}
48+
49+
/*
50+
* Checks every day in a 160000 years interval centered at 1970-01-01
51+
* against the expected result.
52+
*/
53+
static void time64_to_tm_test_date_range(struct kunit *test)
54+
{
55+
/*
56+
* 80000 years = (80000 / 400) * 400 years
57+
* = (80000 / 400) * 146097 days
58+
* = (80000 / 400) * 146097 * 86400 seconds
59+
*/
60+
time64_t total_secs = ((time64_t) 80000) / 400 * 146097 * 86400;
61+
long year = 1970 - 80000;
62+
int month = 1;
63+
int mdday = 1;
64+
int yday = 0;
65+
66+
struct tm result;
67+
time64_t secs;
68+
s64 days;
69+
70+
for (secs = -total_secs; secs <= total_secs; secs += 86400) {
71+
72+
time64_to_tm(secs, 0, &result);
73+
74+
days = div_s64(secs, 86400);
75+
76+
#define FAIL_MSG "%05ld/%02d/%02d (%2d) : %ld", \
77+
year, month, mdday, yday, days
78+
79+
KUNIT_ASSERT_EQ_MSG(test, year - 1900, result.tm_year, FAIL_MSG);
80+
KUNIT_ASSERT_EQ_MSG(test, month - 1, result.tm_mon, FAIL_MSG);
81+
KUNIT_ASSERT_EQ_MSG(test, mdday, result.tm_mday, FAIL_MSG);
82+
KUNIT_ASSERT_EQ_MSG(test, yday, result.tm_yday, FAIL_MSG);
83+
84+
advance_date(&year, &month, &mdday, &yday);
85+
}
86+
}
87+
88+
static struct kunit_case time_test_cases[] = {
89+
KUNIT_CASE(time64_to_tm_test_date_range),
90+
{}
91+
};
92+
93+
static struct kunit_suite time_test_suite = {
94+
.name = "time_test_cases",
95+
.test_cases = time_test_cases,
96+
};
97+
98+
kunit_test_suite(time_test_suite);

‎kernel/time/timeconv.c‎

Lines changed: 70 additions & 58 deletions
Original file line numberDiff line numberDiff line change
@@ -22,47 +22,16 @@
2222

2323
/*
2424
* Converts the calendar time to broken-down time representation
25-
* Based on code from glibc-2.6
2625
*
2726
* 2009-7-14:
2827
* Moved from glibc-2.6 to kernel by Zhaolei<zhaolei@cn.fujitsu.com>
28+
* 2021-06-02:
29+
* Reimplemented by Cassio Neri <cassio.neri@gmail.com>
2930
*/
3031

3132
#include <linux/time.h>
3233
#include <linux/module.h>
33-
34-
/*
35-
* Nonzero if YEAR is a leap year (every 4 years,
36-
* except every 100th isn't, and every 400th is).
37-
*/
38-
static int __isleap(long year)
39-
{
40-
return (year) % 4 == 0 && ((year) % 100 != 0 || (year) % 400 == 0);
41-
}
42-
43-
/* do a mathdiv for long type */
44-
static long math_div(long a, long b)
45-
{
46-
return a / b - (a % b < 0);
47-
}
48-
49-
/* How many leap years between y1 and y2, y1 must less or equal to y2 */
50-
static long leaps_between(long y1, long y2)
51-
{
52-
long leaps1 = math_div(y1 - 1, 4) - math_div(y1 - 1, 100)
53-
+ math_div(y1 - 1, 400);
54-
long leaps2 = math_div(y2 - 1, 4) - math_div(y2 - 1, 100)
55-
+ math_div(y2 - 1, 400);
56-
return leaps2 - leaps1;
57-
}
58-
59-
/* How many days come before each month (0-12). */
60-
static const unsigned short __mon_yday[2][13] = {
61-
/* Normal years. */
62-
{0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365},
63-
/* Leap years. */
64-
{0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366}
65-
};
34+
#include <linux/kernel.h>
6635

6736
#define SECS_PER_HOUR (60 * 60)
6837
#define SECS_PER_DAY (SECS_PER_HOUR * 24)
@@ -77,9 +46,11 @@ static const unsigned short __mon_yday[2][13] = {
7746
*/
7847
void time64_to_tm(time64_t totalsecs, int offset, struct tm *result)
7948
{
80-
long days, rem, y;
49+
u32 u32tmp, day_of_century, year_of_century, day_of_year, month, day;
50+
u64 u64tmp, udays, century, year;
51+
bool is_Jan_or_Feb, is_leap_year;
52+
long days, rem;
8153
int remainder;
82-
const unsigned short *ip;
8354

8455
days = div_s64_rem(totalsecs, SECS_PER_DAY, &remainder);
8556
rem = remainder;
@@ -103,27 +74,68 @@ void time64_to_tm(time64_t totalsecs, int offset, struct tm *result)
10374
if (result->tm_wday < 0)
10475
result->tm_wday += 7;
10576

106-
y = 1970;
107-
108-
while (days < 0 || days >= (__isleap(y) ? 366 : 365)) {
109-
/* Guess a corrected year, assuming 365 days per year. */
110-
long yg = y + math_div(days, 365);
111-
112-
/* Adjust DAYS and Y to match the guessed year. */
113-
days -= (yg - y) * 365 + leaps_between(y, yg);
114-
y = yg;
115-
}
116-
117-
result->tm_year = y - 1900;
118-
119-
result->tm_yday = days;
120-
121-
ip = __mon_yday[__isleap(y)];
122-
for (y = 11; days < ip[y]; y--)
123-
continue;
124-
days -= ip[y];
125-
126-
result->tm_mon = y;
127-
result->tm_mday = days + 1;
77+
/*
78+
* The following algorithm is, basically, Proposition 6.3 of Neri
79+
* and Schneider [1]. In a few words: it works on the computational
80+
* (fictitious) calendar where the year starts in March, month = 2
81+
* (*), and finishes in February, month = 13. This calendar is
82+
* mathematically convenient because the day of the year does not
83+
* depend on whether the year is leap or not. For instance:
84+
*
85+
* March 1st 0-th day of the year;
86+
* ...
87+
* April 1st 31-st day of the year;
88+
* ...
89+
* January 1st 306-th day of the year; (Important!)
90+
* ...
91+
* February 28th 364-th day of the year;
92+
* February 29th 365-th day of the year (if it exists).
93+
*
94+
* After having worked out the date in the computational calendar
95+
* (using just arithmetics) it's easy to convert it to the
96+
* corresponding date in the Gregorian calendar.
97+
*
98+
* [1] "Euclidean Affine Functions and Applications to Calendar
99+
* Algorithms". https://arxiv.org/abs/2102.06959
100+
*
101+
* (*) The numbering of months follows tm more closely and thus,
102+
* is slightly different from [1].
103+
*/
104+
105+
udays = ((u64) days) + 2305843009213814918ULL;
106+
107+
u64tmp = 4 * udays + 3;
108+
century = div64_u64_rem(u64tmp, 146097, &u64tmp);
109+
day_of_century = (u32) (u64tmp / 4);
110+
111+
u32tmp = 4 * day_of_century + 3;
112+
u64tmp = 2939745ULL * u32tmp;
113+
year_of_century = upper_32_bits(u64tmp);
114+
day_of_year = lower_32_bits(u64tmp) / 2939745 / 4;
115+
116+
year = 100 * century + year_of_century;
117+
is_leap_year = year_of_century ? !(year_of_century % 4) : !(century % 4);
118+
119+
u32tmp = 2141 * day_of_year + 132377;
120+
month = u32tmp >> 16;
121+
day = ((u16) u32tmp) / 2141;
122+
123+
/*
124+
* Recall that January 1st is the 306-th day of the year in the
125+
* computational (not Gregorian) calendar.
126+
*/
127+
is_Jan_or_Feb = day_of_year >= 306;
128+
129+
/* Convert to the Gregorian calendar and adjust to Unix time. */
130+
year = year + is_Jan_or_Feb - 6313183731940000ULL;
131+
month = is_Jan_or_Feb ? month - 12 : month;
132+
day = day + 1;
133+
day_of_year += is_Jan_or_Feb ? -306 : 31 + 28 + is_leap_year;
134+
135+
/* Convert to tm's format. */
136+
result->tm_year = (long) (year - 1900);
137+
result->tm_mon = (int) month;
138+
result->tm_mday = (int) day;
139+
result->tm_yday = (int) day_of_year;
128140
}
129141
EXPORT_SYMBOL(time64_to_tm);

0 commit comments

Comments
 (0)