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timer: add new feature FixedRandomDelay=

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FixedRandomDelay=yes will use
`siphash24(sd_id128_get_machine() || MANAGER_IS_SYSTEM(m) || getuid() || u->id)`,
where || is concatenation, instead of a random number to choose a value between
0 and RandomizedDelaySec= as the timer delay.
This essentially sets up a fixed, but seemingly random, offset for each timer
iteration rather than having a random offset recalculated each time it fires.

Closes #10355

Co-author: Anita Zhang <the.anitazha@gmail.com>
This commit is contained in:
Kristijan Gjoshev
2020-11-05 10:59:33 -08:00
committed by Anita Zhang
parent 5cecbae158
commit acf24a1a84
10 changed files with 62 additions and 2 deletions
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src/core/timer.c
+33 -1
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@@ -169,6 +169,36 @@ static int timer_setup_persistent(Timer *t) {
return 0;
}
static uint64_t timer_get_fixed_delay_hash(Timer *t) {
static const uint8_t hash_key[] = {
0x51, 0x0a, 0xdb, 0x76, 0x29, 0x51, 0x42, 0xc2,
0x80, 0x35, 0xea, 0xe6, 0x8e, 0x3a, 0x37, 0xbd
};
struct siphash state;
sd_id128_t machine_id;
uid_t uid;
int r;
assert(t);
uid = getuid();
r = sd_id128_get_machine(&machine_id);
if (r < 0) {
log_unit_debug_errno(UNIT(t), r,
"Failed to get machine ID for the fixed delay calculation, proceeding with 0: %m");
machine_id = SD_ID128_NULL;
}
siphash24_init(&state, hash_key);
siphash24_compress(&machine_id, sizeof(sd_id128_t), &state);
siphash24_compress_boolean(MANAGER_IS_SYSTEM(UNIT(t)->manager), &state);
siphash24_compress(&uid, sizeof(uid_t), &state);
siphash24_compress_string(UNIT(t)->id, &state);
return siphash24_finalize(&state);
}
static int timer_load(Unit *u) {
Timer *t = TIMER(u);
int r;
@@ -215,6 +245,7 @@ static void timer_dump(Unit *u, FILE *f, const char *prefix) {
"%sWakeSystem: %s\n"
"%sAccuracy: %s\n"
"%sRemainAfterElapse: %s\n"
"%sFixedRandomDelay: %s\n"
"%sOnClockChange: %s\n"
"%sOnTimeZoneChange: %s\n",
prefix, timer_state_to_string(t->state),
@@ -224,6 +255,7 @@ static void timer_dump(Unit *u, FILE *f, const char *prefix) {
prefix, yes_no(t->wake_system),
prefix, format_timespan(buf, sizeof(buf), t->accuracy_usec, 1),
prefix, yes_no(t->remain_after_elapse),
prefix, yes_no(t->fixed_random_delay),
prefix, yes_no(t->on_clock_change),
prefix, yes_no(t->on_timezone_change));
@@ -332,7 +364,7 @@ static void add_random(Timer *t, usec_t *v) {
if (*v == USEC_INFINITY)
return;
add = random_u64() % t->random_usec;
add = (t->fixed_random_delay ? timer_get_fixed_delay_hash(t) : random_u64()) % t->random_usec;
if (*v + add < *v) /* overflow */
*v = (usec_t) -2; /* Highest possible value, that is not USEC_INFINITY */