[PATCH v10 12/16] sched/core: uclamp: Extend CPU's cgroup controller

Patrick Bellasi
2019-06-21T08:43:29Z

The cgroup CPU bandwidth controller allows to assign a specified
(maximum) bandwidth to the tasks of a group. However this bandwidth is
defined and enforced only on a temporal base, without considering the
actual frequency a CPU is running on. Thus, the amount of computation
completed by a task within an allocated bandwidth can be very different
depending on the actual frequency the CPU is running that task.
The amount of computation can be affected also by the specific CPU a
task is running on, especially when running on asymmetric capacity
systems like Arm's big.LITTLE.

With the availability of schedutil, the scheduler is now able
to drive frequency selections based on actual task utilization.
Moreover, the utilization clamping support provides a mechanism to
bias the frequency selection operated by schedutil depending on
constraints assigned to the tasks currently RUNNABLE on a CPU.

Giving the mechanisms described above, it is now possible to extend the
cpu controller to specify the minimum (or maximum) utilization which
should be considered for tasks RUNNABLE on a cpu.
This makes it possible to better defined the actual computational
power assigned to task groups, thus improving the cgroup CPU bandwidth
controller which is currently based just on time constraints.

Extend the CPU controller with a couple of new attributes uclamp.{min,max}
which allow to enforce utilization boosting and capping for all the
tasks in a group.

Specifically:

- uclamp.min: defines the minimum utilization which should be considered
i.e. the RUNNABLE tasks of this group will run at least at a
minimum frequency which corresponds to the uclamp.min
utilization

- uclamp.max: defines the maximum utilization which should be considered
i.e. the RUNNABLE tasks of this group will run up to a
maximum frequency which corresponds to the uclamp.max
utilization

These attributes:

a) are available only for non-root nodes, both on default and legacy
hierarchies, while system wide clamps are defined by a generic
interface which does not depends on cgroups. This system wide
interface enforces constraints on tasks in the root node.

b) enforce effective constraints at each level of the hierarchy which
are a restriction of the group requests considering its parent's
effective constraints. Root group effective constraints are defined
by the system wide interface.
This mechanism allows each (non-root) level of the hierarchy to:
- request whatever clamp values it would like to get
- effectively get only up to the maximum amount allowed by its parent

c) have higher priority than task-specific clamps, defined via
sched_setattr(), thus allowing to control and restrict task requests.

Add two new attributes to the cpu controller to collect "requested"
clamp values. Allow that at each non-root level of the hierarchy.
Validate local consistency by enforcing uclamp.min < uclamp.max.
Keep it simple by not caring now about "effective" values computation
and propagation along the hierarchy.

Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Tejun Heo <tj@kernel.org>

---
Changes in v10:
Message-ID: <https://lore.kernel.org/lkml/20190603122422.GA19426@darkstar/>
- rename cgroup attributes to be cpu.uclamp.{min,max}
Message-ID: <https://lore.kernel.org/lkml/20190605152754.GO374014@devbig004.ftw2.facebook.com/>
- use a percentage rational numbers for clamp attributes
Message-ID: <https://lore.kernel.org/lkml/20190605153955.GP374014@devbig004.ftw2.facebook.com/>
- update initialization of subgroups clamps to be none by default
---
Documentation/admin-guide/cgroup-v2.rst | 29 ++++
init/Kconfig | 22 +++
kernel/sched/core.c | 181 +++++++++++++++++++++++-
kernel/sched/sched.h | 6 +
4 files changed, 237 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index a5c845338d6d..4761d20c5cad 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -951,6 +951,12 @@ controller implements weight and absolute bandwidth limit models for
normal scheduling policy and absolute bandwidth allocation model for
realtime scheduling policy.

+Cycles distribution is based, by default, on a temporal base and it
+does not account for the frequency at which tasks are executed.
+The (optional) utilization clamping support allows to enforce a minimum
+bandwidth, which should always be provided by a CPU, and a maximum bandwidth,
+which should never be exceeded by a CPU.
+
WARNING: cgroup2 doesn't yet support control of realtime processes and
the cpu controller can only be enabled when all RT processes are in
the root cgroup. Be aware that system management software may already
@@ -1016,6 +1022,29 @@ All time durations are in microseconds.
Shows pressure stall information for CPU. See
Documentation/accounting/psi.txt for details.

+ cpu.uclamp.min
+ A read-write single value file which exists on non-root cgroups.
+ The default is "0", i.e. no utilization boosting.
+
+ The requested minimum utilization as a percentage rational number,
+ e.g. 12.34 for 12.34%.
+
+ This interface allows reading and setting minimum utilization clamp
+ values similar to the sched_setattr(2). This minimum utilization
+ value is used to clamp the task specific minimum utilization clamp.
+
+ cpu.uclamp.max
+ A read-write single value file which exists on non-root cgroups.
+ The default is "max". i.e. no utilization capping
+
+ The requested maximum utilization as a percentage rational number,
+ e.g. 98.76 for 98.76%.
+
+ This interface allows reading and setting maximum utilization clamp
+ values similar to the sched_setattr(2). This maximum utilization
+ value is used to clamp the task specific maximum utilization clamp.
+
+

Memory
------
diff --git a/init/Kconfig b/init/Kconfig
index bf96faf3fe43..68a21188786c 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -903,6 +903,28 @@ config RT_GROUP_SCHED

endif #CGROUP_SCHED

+config UCLAMP_TASK_GROUP
+ bool "Utilization clamping per group of tasks"
+ depends on CGROUP_SCHED
+ depends on UCLAMP_TASK
+ default n
+ help
+ This feature enables the scheduler to track the clamped utilization
+ of each CPU based on RUNNABLE tasks currently scheduled on that CPU.
+
+ When this option is enabled, the user can specify a min and max
+ CPU bandwidth which is allowed for each single task in a group.
+ The max bandwidth allows to clamp the maximum frequency a task
+ can use, while the min bandwidth allows to define a minimum
+ frequency a task will always use.
+
+ When task group based utilization clamping is enabled, an eventually
+ specified task-specific clamp value is constrained by the cgroup
+ specified clamp value. Both minimum and maximum task clamping cannot
+ be bigger than the corresponding clamping defined at task group level.
+
+ If in doubt, say N.
+
config CGROUP_PIDS
bool "PIDs controller"
help
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2226ddd1de04..0975f832066e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1138,8 +1138,12 @@ static void __init init_uclamp(void)

/* System defaults allow max clamp values for both indexes */
uclamp_se_set(&uc_max, uclamp_none(UCLAMP_MAX), false);
- for_each_clamp_id(clamp_id)
+ for_each_clamp_id(clamp_id) {
uclamp_default[clamp_id] = uc_max;
+#ifdef CONFIG_UCLAMP_TASK_GROUP
+ root_task_group.uclamp_req[clamp_id] = uc_max;
+#endif
+ }
}

#else /* CONFIG_UCLAMP_TASK */
@@ -6714,6 +6718,19 @@ void ia64_set_curr_task(int cpu, struct task_struct *p)
/* task_group_lock serializes the addition/removal of task groups */
static DEFINE_SPINLOCK(task_group_lock);

+static inline void alloc_uclamp_sched_group(struct task_group *tg,
+ struct task_group *parent)
+{
+#ifdef CONFIG_UCLAMP_TASK_GROUP
+ int clamp_id;
+
+ for_each_clamp_id(clamp_id) {
+ uclamp_se_set(&tg->uclamp_req[clamp_id],
+ uclamp_none(clamp_id), false);
+ }
+#endif
+}
+
static void sched_free_group(struct task_group *tg)
{
free_fair_sched_group(tg);
@@ -6737,6 +6754,8 @@ struct task_group *sched_create_group(struct task_group *parent)
if (!alloc_rt_sched_group(tg, parent))
goto err;

+ alloc_uclamp_sched_group(tg, parent);
+
return tg;

err:
@@ -6957,6 +6976,138 @@ static void cpu_cgroup_attach(struct cgroup_taskset *tset)
sched_move_task(task);
}

+#ifdef CONFIG_UCLAMP_TASK_GROUP
+static inline int uclamp_scale_from_percent(char *buf, u64 *value)
+{
+ *value = SCHED_CAPACITY_SCALE;
+
+ buf = strim(buf);
+ if (strncmp("max", buf, 4)) {
+ s64 percent;
+ int ret;
+
+ ret = cgroup_parse_float(buf, 2, &percent);
+ if (ret)
+ return ret;
+
+ percent <<= SCHED_CAPACITY_SHIFT;
+ *value = DIV_ROUND_CLOSEST_ULL(percent, 10000);
+ }
+
+ return 0;
+}
+
+static inline u64 uclamp_percent_from_scale(u64 value)
+{
+ return DIV_ROUND_CLOSEST_ULL(value * 10000, SCHED_CAPACITY_SCALE);
+}
+
+static ssize_t cpu_uclamp_min_write(struct kernfs_open_file *of,
+ char *buf, size_t nbytes,
+ loff_t off)
+{
+ struct task_group *tg;
+ u64 min_value;
+ int ret;
+
+ ret = uclamp_scale_from_percent(buf, &min_value);
+ if (ret)
+ return ret;
+ if (min_value > SCHED_CAPACITY_SCALE)
+ return -ERANGE;
+
+ rcu_read_lock();
+
+ tg = css_tg(of_css(of));
+ if (tg == &root_task_group) {
+ ret = -EINVAL;
+ goto out;
+ }
+ if (tg->uclamp_req[UCLAMP_MIN].value == min_value)
+ goto out;
+ if (tg->uclamp_req[UCLAMP_MAX].value < min_value) {
+ ret = -EINVAL;
+ goto out;
+ }
+
+ uclamp_se_set(&tg->uclamp_req[UCLAMP_MIN], min_value, false);
+
+out:
+ rcu_read_unlock();
+
+ return nbytes;
+}
+
+static ssize_t cpu_uclamp_max_write(struct kernfs_open_file *of,
+ char *buf, size_t nbytes,
+ loff_t off)
+{
+ struct task_group *tg;
+ u64 max_value;
+ int ret;
+
+ ret = uclamp_scale_from_percent(buf, &max_value);
+ if (ret)
+ return ret;
+ if (max_value > SCHED_CAPACITY_SCALE)
+ return -ERANGE;
+
+ rcu_read_lock();
+
+ tg = css_tg(of_css(of));
+ if (tg == &root_task_group) {
+ ret = -EINVAL;
+ goto out;
+ }
+ if (tg->uclamp_req[UCLAMP_MAX].value == max_value)
+ goto out;
+ if (tg->uclamp_req[UCLAMP_MIN].value > max_value) {
+ ret = -EINVAL;
+ goto out;
+ }
+
+ uclamp_se_set(&tg->uclamp_req[UCLAMP_MAX], max_value, false);
+
+out:
+ rcu_read_unlock();
+
+ return nbytes;
+}
+
+static inline void cpu_uclamp_print(struct seq_file *sf,
+ enum uclamp_id clamp_id)
+{
+ struct task_group *tg;
+ u64 util_clamp;
+ u64 percent;
+
+ rcu_read_lock();
+ tg = css_tg(seq_css(sf));
+ util_clamp = tg->uclamp_req[clamp_id].value;
+ rcu_read_unlock();
+
+ if (util_clamp == SCHED_CAPACITY_SCALE) {
+ seq_puts(sf, "max\n");
+ return;
+ }
+
+ percent = uclamp_percent_from_scale(util_clamp);
+ seq_printf(sf, "%llu.%llu\n", percent / 100, percent % 100);
+}
+
+static int cpu_uclamp_min_show(struct seq_file *sf, void *v)
+{
+ cpu_uclamp_print(sf, UCLAMP_MIN);
+ return 0;
+}
+
+static int cpu_uclamp_max_show(struct seq_file *sf, void *v)
+{
+ cpu_uclamp_print(sf, UCLAMP_MAX);
+ return 0;
+}
+#endif /* CONFIG_UCLAMP_TASK_GROUP */
+
#ifdef CONFIG_FAIR_GROUP_SCHED
static int cpu_shares_write_u64(struct cgroup_subsys_state *css,
struct cftype *cftype, u64 shareval)
@@ -7301,6 +7452,20 @@ static struct cftype cpu_legacy_files[] = {
.read_u64 = cpu_rt_period_read_uint,
.write_u64 = cpu_rt_period_write_uint,
},
+#endif
+#ifdef CONFIG_UCLAMP_TASK_GROUP
+ {
+ .name = "uclamp.min",
+ .flags = CFTYPE_NOT_ON_ROOT,
+ .seq_show = cpu_uclamp_min_show,
+ .write = cpu_uclamp_min_write,
+ },
+ {
+ .name = "uclamp.max",
+ .flags = CFTYPE_NOT_ON_ROOT,
+ .seq_show = cpu_uclamp_max_show,
+ .write = cpu_uclamp_max_write,
+ },
#endif
{ } /* Terminate */
};
@@ -7468,6 +7633,20 @@ static struct cftype cpu_files[] = {
.seq_show = cpu_max_show,
.write = cpu_max_write,
},
+#endif
+#ifdef CONFIG_UCLAMP_TASK_GROUP
+ {
+ .name = "uclamp.min",
+ .flags = CFTYPE_NOT_ON_ROOT,
+ .seq_show = cpu_uclamp_min_show,
+ .write = cpu_uclamp_min_write,
+ },
+ {
+ .name = "uclamp.max",
+ .flags = CFTYPE_NOT_ON_ROOT,
+ .seq_show = cpu_uclamp_max_show,
+ .write = cpu_uclamp_max_write,
+ },
#endif
{ } /* terminate */
};
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index f81e8930ff19..bdbefd50ff46 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -393,6 +393,12 @@ struct task_group {
#endif

struct cfs_bandwidth cfs_bandwidth;
+
+#ifdef CONFIG_UCLAMP_TASK_GROUP
+ /* Clamp values requested for a task group */
+ struct uclamp_se uclamp_req[UCLAMP_CNT];
+#endif
+
};

#ifdef CONFIG_FAIR_GROUP_SCHED
--
2.21.0