Messages in this thread | | | Date | Mon, 27 Jan 2020 12:47:49 -0500 | From | Phil Auld <> | Subject | Re: [PATCH] sched/rt: optimize checking group rt scheduler constraints |
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Hi Konstantin,
On Sat, Jan 25, 2020 at 05:50:38PM +0300 Konstantin Khlebnikov wrote: > 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> > --- > 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 e591d40fd645..95d1d7be84ef 100644 > --- a/kernel/sched/rt.c > +++ b/kernel/sched/rt.c > @@ -2396,10 +2396,11 @@ 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(). > @@ -2407,12 +2408,12 @@ 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); >
Why not a break; in there alon with the change to "="?
> - return 0; > + return ret; > } > > struct rt_schedulable_data { > @@ -2443,9 +2444,10 @@ 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); > @@ -2511,7 +2513,6 @@ 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; > @@ -2529,7 +2530,6 @@ 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; > @@ -2588,9 +2588,7 @@ 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; >
Thanks, Phil
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