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sched/rt: Optimize checking group RT scheduler constraints

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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
This commit is contained in:
Konstantin Khlebnikov 2020-01-25 17:50:38 +03:00 committed by Ingo Molnar
parent bec2860a2b
commit b4fb015eef
1 changed files with 11 additions and 13 deletions
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24
kernel/sched/rt.c
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@ -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;