On Wed, Jan 29, 2020 at 11:32:56AM -0000 tip-bot2 for Konstantin Khlebnikov wrote:
> 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@xxxxxxxxxxxxxx>
> AuthorDate: Sat, 25 Jan 2020 17:50:38 +03:00
> Committer: Ingo Molnar <mingo@xxxxxxxxxx>
> 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@xxxxxxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> 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);
>
Heh, I misread it the first time and didn't see the !ret condition. But I think
it should be just "ret = ".
But thanks for getting this fixed.
Cheers,
Phil
> - 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;
>
--
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