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Date: Sat, 25 Jan 2020 17:50:38 +0300 From: Konstantin Khlebnikov <khlebnikov@...dex-team.ru> To: linux-kernel@...r.kernel.org Cc: Juri Lelli <juri.lelli@...hat.com>, Vincent Guittot <vincent.guittot@...aro.org>, Peter Zijlstra <peterz@...radead.org>, Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>, Ingo Molnar <mingo@...hat.com>, Mel Gorman <mgorman@...e.de>, Dietmar Eggemann <dietmar.eggemann@....com> Subject: [PATCH] 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@...dex-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); - 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;
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