core: add new RandomSec= setting for time units

This allows configuration of a random time on top of the elapse events,
in order to spread time events in a network evenly across a range.

man/systemd.timer.xml

@@ -190,13 +190,12 @@
         <varname>OnUnitInactiveSec=</varname> and ending the time
         configured with <varname>AccuracySec=</varname> later. Within
         this time window, the expiry time will be placed at a
-        host-specific, randomized but stable position that is
+        host-specific, randomized, but stable position that is
         synchronized between all local timer units. This is done in
-        order to distribute the wake-up time in networked
-        installations, as well as optimizing power consumption to
-        suppress unnecessary CPU wake-ups. To get best accuracy, set
-        this option to 1us. Note that the timer is still subject to
-        the timer slack configured via
+        order to optimize power consumption to suppress unnecessary
+        CPU wake-ups. To get best accuracy, set this option to
+        1us. Note that the timer is still subject to the timer slack
+        configured via
         <citerefentry><refentrytitle>systemd-system.conf</refentrytitle><manvolnum>5</manvolnum></citerefentry>'s
         <varname>TimerSlackNSec=</varname> setting. See
         <citerefentry><refentrytitle>prctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
@@ -204,6 +203,38 @@
         this value as high as possible and as low as
         necessary.</para></listitem>
       </varlistentry>
+
+      <varlistentry>
+        <term><varname>RandomSec=</varname></term>
+
+        <listitem><para>Delay the timer by a randomly selected, evenly
+        distributed amount of time between 0 and the specified time
+        value. Defaults to 0, indicating that no randomized delay
+        shall be applied. Each timer unit will determine this delay
+        randomly each time it is started, and the delay will simply be
+        added on top of the next determined elapsing time. This is
+        useful to stretch dispatching of similarly configured timer
+        events over a certain amount time, to avoid that they all fire
+        at the same time, possibly resulting in resource
+        congestion. Note the relation to
+        <varname>AccuracySec=</varname> above: the latter allows the
+        service manager to coalesce timer events within a specified
+        time range in order to minimize wakeups, the former does the
+        opposite: it stretches timer events over a time range, to make
+        it unlikely that they fire simultaneously. If
+        <varname>RandomSec=</varname> and
+        <varname>AccuracySec=</varname> are used in conjunction, first
+        the a randomized time is added, and the result is then
+        possibly shifted further to coalesce it with other timer
+        events possibly happening on the system. As mentioned above
+        <varname>AccuracySec=</varname> defaults to 1min and
+        <varname>RandomSec=</varname> to 0, thus encouraging
+        coalescing of timer events. In order to optimally stretch
+        timer events over a certain range of time, make sure to set
+        <varname>RandomSec=</varname> to a higher value, and
+        <varname>AccuracySec=1us</varname>.</para></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><varname>Unit=</varname></term>
 

src/core/dbus-timer.c

@@ -180,6 +180,7 @@ const sd_bus_vtable bus_timer_vtable[] = {
         BUS_PROPERTY_DUAL_TIMESTAMP("LastTriggerUSec", offsetof(Timer, last_trigger), SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE),
         SD_BUS_PROPERTY("Result", "s", property_get_result, offsetof(Timer, result), SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE),
         SD_BUS_PROPERTY("AccuracyUSec", "t", bus_property_get_usec, offsetof(Timer, accuracy_usec), SD_BUS_VTABLE_PROPERTY_CONST),
+        SD_BUS_PROPERTY("RandomUSec", "t", bus_property_get_usec, offsetof(Timer, random_usec), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("Persistent", "b", bus_property_get_bool, offsetof(Timer, persistent), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("WakeSystem", "b", bus_property_get_bool, offsetof(Timer, wake_system), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("RemainAfterElapse", "b", bus_property_get_bool, offsetof(Timer, remain_after_elapse), SD_BUS_VTABLE_PROPERTY_CONST),
@@ -283,6 +284,22 @@ static int bus_timer_set_transient_property(
 
                 return 1;
 
+        } else if (streq(name, "RandomUSec")) {
+                usec_t u = 0;
+
+                r = sd_bus_message_read(message, "t", &u);
+                if (r < 0)
+                        return r;
+
+                if (mode != UNIT_CHECK) {
+                        char time[FORMAT_TIMESPAN_MAX];
+
+                        t->random_usec = u;
+                        unit_write_drop_in_private_format(UNIT(t), mode, name, "RandomSec=%s\n", format_timespan(time, sizeof(time), u, USEC_PER_MSEC));
+                }
+
+                return 1;
+
         } else if (streq(name, "WakeSystem")) {
                 int b;
 

src/core/load-fragment-gperf.gperf.m4

@@ -347,6 +347,7 @@ Timer.Persistent,                config_parse_bool,                  0,
 Timer.WakeSystem,                config_parse_bool,                  0,                             offsetof(Timer, wake_system)
 Timer.RemainAfterElapse,         config_parse_bool,                  0,                             offsetof(Timer, remain_after_elapse)
 Timer.AccuracySec,               config_parse_sec,                   0,                             offsetof(Timer, accuracy_usec)
+Timer.RandomSec,                 config_parse_sec,                   0,                             offsetof(Timer, random_usec)
 Timer.Unit,                      config_parse_trigger_unit,          0,                             0
 m4_dnl
 Path.PathExists,                 config_parse_path_spec,             0,                             0

src/core/timer.c

@@ -27,6 +27,7 @@
 #include "dbus-timer.h"
 #include "fs-util.h"
 #include "parse-util.h"
+#include "random-util.h"
 #include "special.h"
 #include "string-table.h"
 #include "string-util.h"
@@ -330,6 +331,28 @@ static usec_t monotonic_to_boottime(usec_t t) {
                 return 0;
 }
 
+static void add_random(Timer *t, usec_t *v) {
+        char s[FORMAT_TIMESPAN_MAX];
+        usec_t add;
+
+        assert(t);
+        assert(*v);
+
+        if (t->random_usec == 0)
+                return;
+        if (*v == USEC_INFINITY)
+                return;
+
+        add = random_u64() % t->random_usec;
+
+        if (*v + add < *v) /* overflow */
+                *v = (usec_t) -2; /* Highest possible value, that is not USEC_INFINITY */
+        else
+                *v += add;
+
+        log_unit_info(UNIT(t), "Adding %s random time.", format_timespan(s, sizeof(s), add, 0));
+}
+
 static void timer_enter_waiting(Timer *t, bool initial) {
         bool found_monotonic = false, found_realtime = false;
         usec_t ts_realtime, ts_monotonic;
@@ -452,6 +475,8 @@ static void timer_enter_waiting(Timer *t, bool initial) {
                 char buf[FORMAT_TIMESPAN_MAX];
                 usec_t left;
 
+                add_random(t, &t->next_elapse_monotonic_or_boottime);
+
                 left = t->next_elapse_monotonic_or_boottime > ts_monotonic ? t->next_elapse_monotonic_or_boottime - ts_monotonic : 0;
                 log_unit_debug(UNIT(t), "Monotonic timer elapses in %s.", format_timespan(buf, sizeof(buf), left, 0));
 
@@ -486,6 +511,9 @@ static void timer_enter_waiting(Timer *t, bool initial) {
 
         if (found_realtime) {
                 char buf[FORMAT_TIMESTAMP_MAX];
+
+                add_random(t, &t->next_elapse_realtime);
+
                 log_unit_debug(UNIT(t), "Realtime timer elapses at %s.", format_timestamp(buf, sizeof(buf), t->next_elapse_realtime));
 
                 if (t->realtime_event_source) {

src/core/timer.h

@@ -58,6 +58,7 @@ struct Timer {
         Unit meta;
 
         usec_t accuracy_usec;
+        usec_t random_usec;
 
         LIST_HEAD(TimerValue, values);
         usec_t next_elapse_realtime;

src/shared/bus-util.c

@@ -1442,6 +1442,23 @@ int bus_append_unit_property_assignment(sd_bus_message *m, const char *assignmen
                         return bus_log_create_error(r);
 
                 return 0;
+
+        } else if (streq(field, "RandomSec")) {
+                usec_t t;
+
+                r = parse_sec(eq, &t);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to parse RandomSec= parameter: %s", eq);
+
+                r = sd_bus_message_append_basic(m, SD_BUS_TYPE_STRING, "RandomUSec");
+                if (r < 0)
+                        return bus_log_create_error(r);
+
+                r = sd_bus_message_append(m, "v", "t", t);
+                if (r < 0)
+                        return bus_log_create_error(r);
+
+                return 0;
         }
 
         r = sd_bus_message_append_basic(m, SD_BUS_TYPE_STRING, field);