lkml.org 
[lkml]   [2020]   [Jan]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[tip: sched/core] sched/rt: Optimize checking group RT scheduler constraints
The following commit has been merged into the sched/core branch of tip:

Commit-ID: b4fb015eeff7f3e5518a7dbe8061169a3e2f2bc7
Gitweb: https://git.kernel.org/tip/b4fb015eeff7f3e5518a7dbe8061169a3e2f2bc7
Author: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
AuthorDate: Sat, 25 Jan 2020 17:50:38 +03:00
Committer: Ingo Molnar <mingo@kernel.org>
CommitterDate: Tue, 28 Jan 2020 21:37:09 +01:00

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
---
kernel/sched/rt.c | 24 +++++++++++-------------
1 file changed, 11 insertions(+), 13 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 4043abe..55a4a50 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;
\
 
 \ /
  Last update: 2020-01-29 12:34    [W:0.102 / U:0.220 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site