sched/rt: Optimize checking group RT scheduler constraints

Group RT scheduler contains protection against setting zero runtime for
cgroup with RT tasks. Right now function tg_set_rt_bandwidth() iterates
over all CPU cgroups and calls tg_has_rt_tasks() for any cgroup which
runtime is zero (not only for changed one). Default RT runtime is zero,
thus tg_has_rt_tasks() will is called for almost at CPU cgroups.

This protection already is slightly racy: runtime limit could be changed
between cpu_cgroup_can_attach() and cpu_cgroup_attach() because changing
cgroup attribute does not lock cgroup_mutex while attach does not lock
rt_constraints_mutex. Changing task scheduler class also races with
changing rt runtime: check in __sched_setscheduler() isn't protected.

Function tg_has_rt_tasks() iterates over all threads in the system.
This gives NR_CGROUPS * NR_TASKS operations under single tasklist_lock
locked for read tg_set_rt_bandwidth(). Any concurrent attempt of locking
tasklist_lock for write (for example fork) will stuck with disabled irqs.

This patch makes two optimizations:
1) Remove locking tasklist_lock and iterate only tasks in cgroup
2) Call tg_has_rt_tasks() iff rt runtime changes from non-zero to zero

All changed code is under CONFIG_RT_GROUP_SCHED.

Testcase:

 # mkdir /sys/fs/cgroup/cpu/test{1..10000}
 # echo 0 | tee /sys/fs/cgroup/cpu/test*/cpu.rt_runtime_us

At the same time without patch fork time will be >100ms:

 # perf trace -e clone --duration 100 stress-ng --fork 1

Also remote ping will show timings >100ms caused by irq latency.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/157996383820.4651.11292439232549211693.stgit@buzz

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 4043abe..55a4a50 100644 (file)
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -2449,10 +2449,11 @@ const struct sched_class rt_sched_class = {
  */
 static DEFINE_MUTEX(rt_constraints_mutex);
 
-/* Must be called with tasklist_lock held */
 static inline int tg_has_rt_tasks(struct task_group *tg)
 {
-       struct task_struct *g, *p;
+       struct task_struct *task;
+       struct css_task_iter it;
+       int ret = 0;
 
        /*
         * Autogroups do not have RT tasks; see autogroup_create().
@@ -2460,12 +2461,12 @@ static inline int tg_has_rt_tasks(struct task_group *tg)
        if (task_group_is_autogroup(tg))
                return 0;
 
-       for_each_process_thread(g, p) {
-               if (rt_task(p) && task_group(p) == tg)
-                       return 1;
-       }
+       css_task_iter_start(&tg->css, 0, &it);
+       while (!ret && (task = css_task_iter_next(&it)))
+               ret |= rt_task(task);
+       css_task_iter_end(&it);
 
-       return 0;
+       return ret;
 }
 
 struct rt_schedulable_data {
@@ -2496,9 +2497,10 @@ static int tg_rt_schedulable(struct task_group *tg, void *data)
                return -EINVAL;
 
        /*
-        * Ensure we don't starve existing RT tasks.
+        * Ensure we don't starve existing RT tasks if runtime turns zero.
         */
-       if (rt_bandwidth_enabled() && !runtime && tg_has_rt_tasks(tg))
+       if (rt_bandwidth_enabled() && !runtime &&
+           tg->rt_bandwidth.rt_runtime && tg_has_rt_tasks(tg))
                return -EBUSY;
 
        total = to_ratio(period, runtime);
@@ -2564,7 +2566,6 @@ static int tg_set_rt_bandwidth(struct task_group *tg,
                return -EINVAL;
 
        mutex_lock(&rt_constraints_mutex);
-       read_lock(&tasklist_lock);
        err = __rt_schedulable(tg, rt_period, rt_runtime);
        if (err)
                goto unlock;
@@ -2582,7 +2583,6 @@ static int tg_set_rt_bandwidth(struct task_group *tg,
        }
        raw_spin_unlock_irq(&tg->rt_bandwidth.rt_runtime_lock);
 unlock:
-       read_unlock(&tasklist_lock);
        mutex_unlock(&rt_constraints_mutex);
 
        return err;
@@ -2641,9 +2641,7 @@ static int sched_rt_global_constraints(void)
        int ret = 0;
 
        mutex_lock(&rt_constraints_mutex);
-       read_lock(&tasklist_lock);
        ret = __rt_schedulable(NULL, 0, 0);
-       read_unlock(&tasklist_lock);
        mutex_unlock(&rt_constraints_mutex);
 
        return ret;