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authorKonstantin Khlebnikov <khlebnikov@yandex-team.ru>2020-01-25 17:50:38 +0300
committerIngo Molnar <mingo@kernel.org>2020-01-28 23:37:09 +0300
commitb4fb015eeff7f3e5518a7dbe8061169a3e2f2bc7 (patch)
treecae717cb16d5852d37b3ab04e71c0a45cb11e96b /kernel/sched/rt.c
parentbec2860a2bd6cd38ea34434d04f4033eb32f0f31 (diff)
downloadlinux-b4fb015eeff7f3e5518a7dbe8061169a3e2f2bc7.tar.xz
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
Diffstat (limited to 'kernel/sched/rt.c')
-rw-r--r--kernel/sched/rt.c24
1 files changed, 11 insertions, 13 deletions
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 4043abe45459..55a4a5042292 100644
--- 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;