Merge pull request #3753 from poettering/tasks-max-scale

Add support for relative TasksMax= specifications, and bump default for services

17 files changed, 239 insertions, 60 deletions

diff --git a/man/logind.conf.xml b/man/logind.conf.xml
index fe92277a1f..adba5a4131 100644
--- a/man/logind.conf.xml
+++ b/man/logind.conf.xml
@@ -315,12 +315,11 @@
       <varlistentry>
         <term><varname>UserTasksMax=</varname></term>
 
-        <listitem><para>Sets the maximum number of OS tasks each user
-        may run concurrently. This controls the
-        <varname>TasksMax=</varname> setting of the per-user slice
-        unit, see
+        <listitem><para>Sets the maximum number of OS tasks each user may run concurrently. This controls the
+        <varname>TasksMax=</varname> setting of the per-user slice unit, see
         <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>
-        for details. Defaults to 12288 (12K).</para></listitem>
+        for details. Defaults to 33%, which equals 10813 with the kernel's defaults on the host, but might be smaller
+        in OS containers.</para></listitem>
       </varlistentry>
 
       <varlistentry>
diff --git a/man/systemd-system.conf.xml b/man/systemd-system.conf.xml
index 8833e73c72..1bb40fd234 100644
--- a/man/systemd-system.conf.xml
+++ b/man/systemd-system.conf.xml
@@ -325,12 +325,11 @@
       <varlistentry>
         <term><varname>DefaultTasksMax=</varname></term>
 
-        <listitem><para>Configure the default value for the per-unit
-        <varname>TasksMax=</varname> setting. See
+        <listitem><para>Configure the default value for the per-unit <varname>TasksMax=</varname> setting. See
         <citerefentry><refentrytitle>systemd.resource-control</refentrytitle><manvolnum>5</manvolnum></citerefentry>
-        for details. This setting applies to all unit types that
-        support resource control settings, with the exception of slice
-        units. Defaults to 512.</para></listitem>
+        for details. This setting applies to all unit types that support resource control settings, with the exception
+        of slice units. Defaults to 15%, which equals 4915 with the kernel's defaults on the host, but might be smaller
+        in OS containers.</para></listitem>
       </varlistentry>
 
       <varlistentry>
diff --git a/man/systemd.resource-control.xml b/man/systemd.resource-control.xml
index 7263c0b329..bf44a68345 100644
--- a/man/systemd.resource-control.xml
+++ b/man/systemd.resource-control.xml
@@ -327,15 +327,12 @@
         <term><varname>TasksMax=<replaceable>N</replaceable></varname></term>
 
         <listitem>
-          <para>Specify the maximum number of tasks that may be
-          created in the unit. This ensures that the number of tasks
-          accounted for the unit (see above) stays below a specific
-          limit. If assigned the special value
-          <literal>infinity</literal>, no tasks limit is applied. This
-          controls the <literal>pids.max</literal> control group
-          attribute. For details about this control group attribute,
-          see <ulink
-          url="https://www.kernel.org/doc/Documentation/cgroup-v1/pids.txt">pids.txt</ulink>.</para>
+          <para>Specify the maximum number of tasks that may be created in the unit. This ensures that the number of
+          tasks accounted for the unit (see above) stays below a specific limit. This either takes an absolute number
+          of tasks or a percentage value that is taken relative to the configured maximum number of tasks on the
+          system.  If assigned the special value <literal>infinity</literal>, no tasks limit is applied. This controls
+          the <literal>pids.max</literal> control group attribute. For details about this control group attribute, see
+          <ulink url="https://www.kernel.org/doc/Documentation/cgroup-v1/pids.txt">pids.txt</ulink>.</para>
 
           <para>Implies <literal>TasksAccounting=true</literal>. The
           system default for this setting may be controlled with
diff --git a/src/basic/util.c b/src/basic/util.c
index 09d16697b7..c4b6fc2925 100644
--- a/src/basic/util.c
+++ b/src/basic/util.c
@@ -832,6 +832,61 @@ uint64_t physical_memory_scale(uint64_t v, uint64_t max) {
         return r;
 }
 
+uint64_t system_tasks_max(void) {
+
+#if SIZEOF_PID_T == 4
+#define TASKS_MAX ((uint64_t) (INT32_MAX-1))
+#elif SIZEOF_PID_T == 2
+#define TASKS_MAX ((uint64_t) (INT16_MAX-1))
+#else
+#error "Unknown pid_t size"
+#endif
+
+        _cleanup_free_ char *value = NULL, *root = NULL;
+        uint64_t a = TASKS_MAX, b = TASKS_MAX;
+
+        /* Determine the maximum number of tasks that may run on this system. We check three sources to determine this
+         * limit:
+         *
+         * a) the maximum value for the pid_t type
+         * b) the cgroups pids_max attribute for the system
+         * c) the kernel's configure maximum PID value
+         *
+         * And then pick the smallest of the three */
+
+        if (read_one_line_file("/proc/sys/kernel/pid_max", &value) >= 0)
+                (void) safe_atou64(value, &a);
+
+        if (cg_get_root_path(&root) >= 0) {
+                value = mfree(value);
+
+                if (cg_get_attribute("pids", root, "pids.max", &value) >= 0)
+                        (void) safe_atou64(value, &b);
+        }
+
+        return MIN3(TASKS_MAX,
+                    a <= 0 ? TASKS_MAX : a,
+                    b <= 0 ? TASKS_MAX : b);
+}
+
+uint64_t system_tasks_max_scale(uint64_t v, uint64_t max) {
+        uint64_t t, m;
+
+        assert(max > 0);
+
+        /* Multiply the system's task value by the fraction v/max. Hence, if max==100 this calculates percentages
+         * relative to the system's maximum number of tasks. Returns UINT64_MAX on overflow. */
+
+        t = system_tasks_max();
+        assert(t > 0);
+
+        m = t * v;
+        if (m / t != v) /* overflow? */
+                return UINT64_MAX;
+
+        return m / max;
+}
+
 int update_reboot_parameter_and_warn(const char *param) {
         int r;
 
diff --git a/src/basic/util.h b/src/basic/util.h
index db105197e8..8500c3077c 100644
--- a/src/basic/util.h
+++ b/src/basic/util.h
@@ -186,6 +186,9 @@ int namespace_enter(int pidns_fd, int mntns_fd, int netns_fd, int userns_fd, int
 uint64_t physical_memory(void);
 uint64_t physical_memory_scale(uint64_t v, uint64_t max);
 
+uint64_t system_tasks_max(void);
+uint64_t system_tasks_max_scale(uint64_t v, uint64_t max);
+
 int update_reboot_parameter_and_warn(const char *param);
 
 int version(void);
diff --git a/src/core/dbus-cgroup.c b/src/core/dbus-cgroup.c
index 6167ce92cd..85b0c86a2f 100644
--- a/src/core/dbus-cgroup.c
+++ b/src/core/dbus-cgroup.c
@@ -856,7 +856,7 @@ int bus_cgroup_set_property(
 
                 return 1;
 
-        } else if (STR_IN_SET(name, "MemoryLowByPhysicalMemory", "MemoryHighByPhysicalMemory", "MemoryMaxByPhysicalMemory")) {
+        } else if (STR_IN_SET(name, "MemoryLowScale", "MemoryHighScale", "MemoryMaxScale")) {
                 uint32_t raw;
                 uint64_t v;
 
@@ -872,7 +872,7 @@ int bus_cgroup_set_property(
                         const char *e;
 
                         /* Chop off suffix */
-                        assert_se(e = endswith(name, "ByPhysicalMemory"));
+                        assert_se(e = endswith(name, "Scale"));
                         name = strndupa(name, e - name);
 
                         if (streq(name, "MemoryLow"))
@@ -883,7 +883,8 @@ int bus_cgroup_set_property(
                                 c->memory_max = v;
 
                         unit_invalidate_cgroup(u, CGROUP_MASK_MEMORY);
-                        unit_write_drop_in_private_format(u, mode, name, "%s=%" PRIu32 "%%", name, (uint32_t) (DIV_ROUND_UP((uint64_t) raw * 100, (uint64_t) UINT32_MAX)));
+                        unit_write_drop_in_private_format(u, mode, name, "%s=%" PRIu32 "%%", name,
+                                                          (uint32_t) (DIV_ROUND_UP((uint64_t) raw * 100U, (uint64_t) UINT32_MAX)));
                 }
 
                 return 1;
@@ -909,7 +910,7 @@ int bus_cgroup_set_property(
 
                 return 1;
 
-        } else if (streq(name, "MemoryLimitByPhysicalMemory")) {
+        } else if (streq(name, "MemoryLimitScale")) {
                 uint64_t limit;
                 uint32_t raw;
 
@@ -924,7 +925,8 @@ int bus_cgroup_set_property(
                 if (mode != UNIT_CHECK) {
                         c->memory_limit = limit;
                         unit_invalidate_cgroup(u, CGROUP_MASK_MEMORY);
-                        unit_write_drop_in_private_format(u, mode, "MemoryLimit", "MemoryLimit=%" PRIu32 "%%", (uint32_t) (DIV_ROUND_UP((uint64_t) raw * 100, (uint64_t) UINT32_MAX)));
+                        unit_write_drop_in_private_format(u, mode, "MemoryLimit", "MemoryLimit=%" PRIu32 "%%",
+                                                          (uint32_t) (DIV_ROUND_UP((uint64_t) raw * 100U, (uint64_t) UINT32_MAX)));
                 }
 
                 return 1;
@@ -1060,6 +1062,8 @@ int bus_cgroup_set_property(
                 r = sd_bus_message_read(message, "t", &limit);
                 if (r < 0)
                         return r;
+                if (limit <= 0)
+                        return sd_bus_error_set_errnof(error, EINVAL, "%s= is too small", name);
 
                 if (mode != UNIT_CHECK) {
                         c->tasks_max = limit;
@@ -1072,6 +1076,26 @@ int bus_cgroup_set_property(
                 }
 
                 return 1;
+        } else if (streq(name, "TasksMaxScale")) {
+                uint64_t limit;
+                uint32_t raw;
+
+                r = sd_bus_message_read(message, "u", &raw);
+                if (r < 0)
+                        return r;
+
+                limit = system_tasks_max_scale(raw, UINT32_MAX);
+                if (limit <= 0 || limit >= UINT64_MAX)
+                        return sd_bus_error_set_errnof(error, EINVAL, "%s= is out of range", name);
+
+                if (mode != UNIT_CHECK) {
+                        c->tasks_max = limit;
+                        unit_invalidate_cgroup(u, CGROUP_MASK_PIDS);
+                        unit_write_drop_in_private_format(u, mode, name, "TasksMax=%" PRIu32 "%%",
+                                                          (uint32_t) (DIV_ROUND_UP((uint64_t) raw * 100U, (uint64_t) UINT32_MAX)));
+                }
+
+                return 1;
         }
 
         if (u->transient && u->load_state == UNIT_STUB) {
diff --git a/src/core/load-fragment.c b/src/core/load-fragment.c
index 782e420e4c..ae306de4ae 100644
--- a/src/core/load-fragment.c
+++ b/src/core/load-fragment.c
@@ -2823,8 +2823,8 @@ int config_parse_memory_limit(
                 } else
                         bytes = physical_memory_scale(r, 100U);
 
-                if (bytes < 1) {
-                        log_syntax(unit, LOG_ERR, filename, line, 0, "Memory limit '%s' too small. Ignoring.", rvalue);
+                if (bytes <= 0 || bytes >= UINT64_MAX) {
+                        log_syntax(unit, LOG_ERR, filename, line, 0, "Memory limit '%s' out of range. Ignoring.", rvalue);
                         return 0;
                 }
         }
@@ -2861,9 +2861,18 @@ int config_parse_tasks_max(
                 return 0;
         }
 
-        r = safe_atou64(rvalue, &u);
-        if (r < 0 || u < 1) {
-                log_syntax(unit, LOG_ERR, filename, line, r, "Maximum tasks value '%s' invalid. Ignoring.", rvalue);
+        r = parse_percent(rvalue);
+        if (r < 0) {
+                r = safe_atou64(rvalue, &u);
+                if (r < 0) {
+                        log_syntax(unit, LOG_ERR, filename, line, r, "Maximum tasks value '%s' invalid. Ignoring.", rvalue);
+                        return 0;
+                }
+        } else
+                u = system_tasks_max_scale(r, 100U);
+
+        if (u <= 0 || u >= UINT64_MAX) {
+                log_syntax(unit, LOG_ERR, filename, line, 0, "Maximum tasks value '%s' out of range. Ignoring.", rvalue);
                 return 0;
         }
 
diff --git a/src/core/main.c b/src/core/main.c
index fc04fb8051..c59228ad53 100644
--- a/src/core/main.c
+++ b/src/core/main.c
@@ -127,7 +127,7 @@ static bool arg_default_io_accounting = false;
 static bool arg_default_blockio_accounting = false;
 static bool arg_default_memory_accounting = false;
 static bool arg_default_tasks_accounting = true;
-static uint64_t arg_default_tasks_max = UINT64_C(512);
+static uint64_t arg_default_tasks_max = UINT64_MAX;
 static sd_id128_t arg_machine_id = {};
 
 noreturn static void freeze_or_reboot(void) {
@@ -1298,6 +1298,11 @@ static int fixup_environment(void) {
         _cleanup_free_ char *term = NULL;
         int r;
 
+        /* We expect the environment to be set correctly
+         * if run inside a container. */
+        if (detect_container() > 0)
+                return 0;
+
         /* When started as PID1, the kernel uses /dev/console
          * for our stdios and uses TERM=linux whatever the
          * backend device used by the console. We try to make
@@ -1314,7 +1319,7 @@ static int fixup_environment(void) {
         if (r == 0) {
                 term = strdup(default_term_for_tty("/dev/console") + 5);
                 if (!term)
-                        return -errno;
+                        return -ENOMEM;
         }
 
         if (setenv("TERM", term, 1) < 0)
@@ -1508,13 +1513,10 @@ int main(int argc, char *argv[]) {
         }
 
         if (arg_system) {
-                /* We expect the environment to be set correctly
-                 * if run inside a container. */
-                if (detect_container() <= 0)
-                        if (fixup_environment() < 0) {
-                                error_message = "Failed to fix up PID1 environment";
-                                goto finish;
-                        }
+                if (fixup_environment() < 0) {
+                        error_message = "Failed to fix up PID1 environment";
+                        goto finish;
+                }
 
                 /* Try to figure out if we can use colors with the console. No
                  * need to do that for user instances since they never log
@@ -1556,6 +1558,8 @@ int main(int argc, char *argv[]) {
         (void) reset_all_signal_handlers();
         (void) ignore_signals(SIGNALS_IGNORE, -1);
 
+        arg_default_tasks_max = system_tasks_max_scale(15U, 100U); /* 15% the system PIDs equals 4915 by default. */
+
         if (parse_config_file() < 0) {
                 error_message = "Failed to parse config file";
                 goto finish;
diff --git a/src/core/manager.c b/src/core/manager.c
index a0181e2138..4d84a0b37e 100644
--- a/src/core/manager.c
+++ b/src/core/manager.c
@@ -569,7 +569,7 @@ int manager_new(UnitFileScope scope, bool test_run, Manager **_m) {
         m->exit_code = _MANAGER_EXIT_CODE_INVALID;
         m->default_timer_accuracy_usec = USEC_PER_MINUTE;
         m->default_tasks_accounting = true;
-        m->default_tasks_max = UINT64_C(512);
+        m->default_tasks_max = UINT64_MAX;
 
 #ifdef ENABLE_EFI
         if (MANAGER_IS_SYSTEM(m) && detect_container() <= 0)
diff --git a/src/core/system.conf b/src/core/system.conf
index db8b7acd78..c6bb050aac 100644
--- a/src/core/system.conf
+++ b/src/core/system.conf
@@ -42,7 +42,7 @@
 #DefaultBlockIOAccounting=no
 #DefaultMemoryAccounting=no
 #DefaultTasksAccounting=yes
-#DefaultTasksMax=512
+#DefaultTasksMax=15%
 #DefaultLimitCPU=
 #DefaultLimitFSIZE=
 #DefaultLimitDATA=
diff --git a/src/login/logind-gperf.gperf b/src/login/logind-gperf.gperf
index 6bd08adc05..0b6a5f3cf4 100644
--- a/src/login/logind-gperf.gperf
+++ b/src/login/logind-gperf.gperf
@@ -36,4 +36,4 @@ Login.RuntimeDirectorySize,        config_parse_tmpfs_size,    0, offsetof(Manag
 Login.RemoveIPC,                   config_parse_bool,          0, offsetof(Manager, remove_ipc)
 Login.InhibitorsMax,               config_parse_uint64,        0, offsetof(Manager, inhibitors_max)
 Login.SessionsMax,                 config_parse_uint64,        0, offsetof(Manager, sessions_max)
-Login.UserTasksMax,                config_parse_uint64,        0, offsetof(Manager, user_tasks_max)
+Login.UserTasksMax,                config_parse_user_tasks_max,0, offsetof(Manager, user_tasks_max)
diff --git a/src/login/logind-user.c b/src/login/logind-user.c
index de44d369cf..f7d83909a0 100644
--- a/src/login/logind-user.c
+++ b/src/login/logind-user.c
@@ -870,3 +870,48 @@ int config_parse_tmpfs_size(
 
         return 0;
 }
+
+int config_parse_user_tasks_max(
+                const char* unit,
+                const char *filename,
+                unsigned line,
+                const char *section,
+                unsigned section_line,
+                const char *lvalue,
+                int ltype,
+                const char *rvalue,
+                void *data,
+                void *userdata) {
+
+        uint64_t *m = data;
+        uint64_t k;
+        int r;
+
+        assert(filename);
+        assert(lvalue);
+        assert(rvalue);
+        assert(data);
+
+        /* First, try to parse as percentage */
+        r = parse_percent(rvalue);
+        if (r > 0 && r < 100)
+                k = system_tasks_max_scale(r, 100U);
+        else {
+
+                /* If the passed argument was not a percentage, or out of range, parse as byte size */
+
+                r = safe_atou64(rvalue, &k);
+                if (r < 0) {
+                        log_syntax(unit, LOG_ERR, filename, line, r, "Failed to parse tasks maximum, ignoring: %s", rvalue);
+                        return 0;
+                }
+        }
+
+        if (k <= 0 || k >= UINT64_MAX) {
+                log_syntax(unit, LOG_ERR, filename, line, 0, "Tasks maximum out of range, ignoring: %s", rvalue);
+                return 0;
+        }
+
+        *m = k;
+        return 0;
+}
diff --git a/src/login/logind.c b/src/login/logind.c
index d01dd110ea..5ce36d28c7 100644
--- a/src/login/logind.c
+++ b/src/login/logind.c
@@ -62,7 +62,7 @@ static void manager_reset_config(Manager *m) {
         m->idle_action = HANDLE_IGNORE;
 
         m->runtime_dir_size = physical_memory_scale(10U, 100U); /* 10% */
-        m->user_tasks_max = 12288;
+        m->user_tasks_max = system_tasks_max_scale(33U, 100U); /* 33% */
         m->sessions_max = 8192;
         m->inhibitors_max = 8192;
 
diff --git a/src/login/logind.conf.in b/src/login/logind.conf.in
index 32c0844cb6..6f720b7708 100644
--- a/src/login/logind.conf.in
+++ b/src/login/logind.conf.in
@@ -34,4 +34,4 @@
 #RemoveIPC=yes
 #InhibitorsMax=8192
 #SessionsMax=8192
-#UserTasksMax=12288
+#UserTasksMax=33%
diff --git a/src/login/logind.h b/src/login/logind.h
index 90431eb4b0..086fa1eeb5 100644
--- a/src/login/logind.h
+++ b/src/login/logind.h
@@ -187,6 +187,7 @@ const struct ConfigPerfItem* logind_gperf_lookup(const char *key, unsigned lengt
 int manager_set_lid_switch_ignore(Manager *m, usec_t until);
 
 int config_parse_tmpfs_size(const char *unit, const char *filename, unsigned line, const char *section, unsigned section_line, const char *lvalue, int ltype, const char *rvalue, void *data, void *userdata);
+int config_parse_user_tasks_max(const char *unit, const char *filename, unsigned line, const char *section, unsigned section_line, const char *lvalue, int ltype, const char *rvalue, void *data, void *userdata);
 
 int manager_get_session_from_creds(Manager *m, sd_bus_message *message, const char *name, sd_bus_error *error, Session **ret);
 int manager_get_user_from_creds(Manager *m, sd_bus_message *message, uid_t uid, sd_bus_error *error, User **ret);
diff --git a/src/shared/bus-unit-util.c b/src/shared/bus-unit-util.c
index 94ffa8af87..ea020b517b 100644
--- a/src/shared/bus-unit-util.c
+++ b/src/shared/bus-unit-util.c
@@ -132,10 +132,10 @@ int bus_append_unit_property_assignment(sd_bus_message *m, const char *assignmen
                                 char *n;
 
                                 /* When this is a percentage we'll convert this into a relative value in the range
-                                 * 0…UINT32_MAX and pass it in the MemoryLowByPhysicalMemory property (and related
+                                 * 0…UINT32_MAX and pass it in the MemoryLowScale property (and related
                                  * ones). This way the physical memory size can be determined server-side */
 
-                                n = strjoina(field, "ByPhysicalMemory");
+                                n = strjoina(field, "Scale");
                                 r = sd_bus_message_append(m, "sv", n, "u", (uint32_t) (((uint64_t) UINT32_MAX * r) / 100U));
                                 goto finish;
 
@@ -148,6 +148,26 @@ int bus_append_unit_property_assignment(sd_bus_message *m, const char *assignmen
 
                 r = sd_bus_message_append(m, "sv", field, "t", bytes);
                 goto finish;
+        } else if (streq(field, "TasksMax")) {
+                uint64_t t;
+
+                if (isempty(eq) || streq(eq, "infinity"))
+                        t = (uint64_t) -1;
+                else {
+                        r = parse_percent(eq);
+                        if (r >= 0) {
+                                r = sd_bus_message_append(m, "sv", "TasksMaxScale", "u", (uint32_t) (((uint64_t) UINT32_MAX * r) / 100U));
+                                goto finish;
+                        } else {
+                                r = safe_atou64(eq, &t);
+                                if (r < 0)
+                                        return log_error_errno(r, "Failed to parse maximum tasks specification %s", assignment);
+                        }
+
+                }
+
+                r = sd_bus_message_append(m, "sv", "TasksMax", "t", t);
+                goto finish;
         }
 
         r = sd_bus_message_append_basic(m, SD_BUS_TYPE_STRING, field);
@@ -191,21 +211,6 @@ int bus_append_unit_property_assignment(sd_bus_message *m, const char *assignmen
 
                 r = sd_bus_message_append(m, "v", "b", r);
 
-        } else if (streq(field, "TasksMax")) {
-                uint64_t n;
-
-                if (isempty(eq) || streq(eq, "infinity"))
-                        n = (uint64_t) -1;
-                else {
-                        r = safe_atou64(eq, &n);
-                        if (r < 0) {
-                                log_error("Failed to parse maximum tasks specification %s", assignment);
-                                return -EINVAL;
-                        }
-                }
-
-                r = sd_bus_message_append(m, "v", "t", n);
-
         } else if (STR_IN_SET(field, "CPUShares", "StartupCPUShares")) {
                 uint64_t u;
 
diff --git a/src/test/test-util.c b/src/test/test-util.c
index e177612a9f..1b5cba86c1 100644
--- a/src/test/test-util.c
+++ b/src/test/test-util.c
@@ -308,7 +308,43 @@ static void test_physical_memory_scale(void) {
 
         /* overflow */
         assert_se(physical_memory_scale(UINT64_MAX/4, UINT64_MAX) == UINT64_MAX);
+}
+
+static void test_system_tasks_max(void) {
+        uint64_t t;
+
+        t = system_tasks_max();
+        assert_se(t > 0);
+        assert_se(t < UINT64_MAX);
+
+        log_info("Max tasks: %" PRIu64, t);
+}
+
+static void test_system_tasks_max_scale(void) {
+        uint64_t t;
+
+        t = system_tasks_max();
+
+        assert_se(system_tasks_max_scale(0, 100) == 0);
+        assert_se(system_tasks_max_scale(100, 100) == t);
+
+        assert_se(system_tasks_max_scale(0, 1) == 0);
+        assert_se(system_tasks_max_scale(1, 1) == t);
+        assert_se(system_tasks_max_scale(2, 1) == 2*t);
+
+        assert_se(system_tasks_max_scale(0, 2) == 0);
+        assert_se(system_tasks_max_scale(1, 2) == t/2);
+        assert_se(system_tasks_max_scale(2, 2) == t);
+        assert_se(system_tasks_max_scale(3, 2) == (3*t)/2);
+        assert_se(system_tasks_max_scale(4, 2) == t*2);
+
+        assert_se(system_tasks_max_scale(0, UINT32_MAX) == 0);
+        assert_se(system_tasks_max_scale((UINT32_MAX-1)/2, UINT32_MAX-1) == t/2);
+        assert_se(system_tasks_max_scale(UINT32_MAX, UINT32_MAX) == t);
+
+        /* overflow */
 
+        assert_se(system_tasks_max_scale(UINT64_MAX/4, UINT64_MAX) == UINT64_MAX);
 }
 
 int main(int argc, char *argv[]) {
@@ -327,6 +363,8 @@ int main(int argc, char *argv[]) {
         test_raw_clone();
         test_physical_memory();
         test_physical_memory_scale();
+        test_system_tasks_max();
+        test_system_tasks_max_scale();
 
         return 0;
 }