On Thu, 3 Nov 2022 at 04:01, Song Zhang <zhangsong34@xxxxxxxxxx> wrote: > > Thanks for your reply! > > On 2022/11/3 2:01, Vincent Guittot wrote: > > On Wed, 2 Nov 2022 at 04:54, Song Zhang <zhangsong34@xxxxxxxxxx> wrote: > >> > > > > This really looks like a v3 of > > https://lore.kernel.org/all/20220810015636.3865248-1-zhangsong34@xxxxxxxxxx/ > > > > Please keep versioning. > > > >> Add a new sysctl interface: > >> /proc/sys/kernel/sched_prio_load_balance_enabled > > > > We don't want to add more sysctl knobs for the scheduler, we even > > removed some. Knob usually means that you want to fix your use case > > but the solution doesn't make sense for all cases. > > > > OK, I will remove this knobs later. > > >> > >> 0: default behavior > >> 1: enable priority load balance for CFS > >> > >> For co-location with idle and non-idle tasks, when CFS do load balance, > >> it is reasonable to prefer migrating non-idle tasks and migrating idle > >> tasks lastly. This will reduce the interference by SCHED_IDLE tasks > >> as much as possible. > > > > I don't agree that it's always the best choice to migrate a non-idle task 1st. > > > > CPU0 has 1 non idle task and CPU1 has 1 non idle task and hundreds of > > idle task and there is an imbalance between the 2 CPUS: migrating the > > non idle task from CPU1 to CPU0 is not the best choice > > > > If the non idle task on CPU1 is running or cache hot, it cannot be > migrated and idle tasks can also be migrated from CPU1 to CPU0. So I > think it does not matter. What I mean is that migrating non idle tasks first is not a universal win and not always what we want. > > >> > >> Testcase: > >> - Spawn large number of idle(SCHED_IDLE) tasks occupy CPUs > > > > What do you mean by a large number ? > > > >> - Let non-idle tasks compete with idle tasks for CPU time. > >> > >> Using schbench to test non-idle tasks latency: > >> $ ./schbench -m 1 -t 10 -r 30 -R 200 > > > > How many CPUs do you have ? > > > > OK, some details may not be mentioned. > My virtual machine has 8 CPUs running with a schbench process and 5000 > idle tasks. The idle task is a while dead loop process below: How can you care about latency when you start 10 workers on 8 vCPUs with 5000 non idle threads ? > > $ cat idle_process.c > int main() > { > int i = 0; > while(1) { > usleep(500); > for(i = 0; i < 1000000; i++); > } > } > > You can compile and spawn 5000 idle(SCHED_IDLE) tasks occupying 8 CPUs > and execute schbench command to test it. > > >> > >> Test result: > >> 1.Default behavior > >> Latency percentiles (usec) runtime 30 (s) (4562 total samples) > >> 50.0th: 62528 (2281 samples) > >> 75.0th: 623616 (1141 samples) > >> 90.0th: 764928 (687 samples) > >> 95.0th: 824320 (225 samples) > >> *99.0th: 920576 (183 samples) > >> 99.5th: 953344 (23 samples) > >> 99.9th: 1008640 (18 samples) > >> min=9, max=1074466 > >> > >> 2.Enable priority load balance > >> Latency percentiles (usec) runtime 30 (s) (4391 total samples) > >> 50.0th: 22624 (2204 samples) > >> 75.0th: 48832 (1092 samples) > >> 90.0th: 85376 (657 samples) > >> 95.0th: 113280 (220 samples) > >> *99.0th: 182528 (175 samples) > >> 99.5th: 206592 (22 samples) > >> 99.9th: 290304 (17 samples) > >> min=6, max=351815 > >> > >> From percentile details, we see the benefit of priority load balance > >> that 95% of non-idle tasks latencies stays no more than 113ms, while > > > > But even 113ms seems quite a large number if there is anything else > > but 10 schbench workers and a bunch of idle threads that are running. > > > >> non-idle tasks latencies has got almost 50% over 600ms if priority > >> load balance not enabled. > > > > Als have you considered enabling sched_feature LB_MIN ? > > > > I have tried to echo LB_MIN > /sys/kernel/debug/sched/features, but this > feature seems make no sense. > > >> > >> Signed-off-by: Song Zhang <zhangsong34@xxxxxxxxxx> > >> --- > >> include/linux/sched/sysctl.h | 4 +++ > >> init/Kconfig | 10 ++++++ > >> kernel/sched/core.c | 3 ++ > >> kernel/sched/fair.c | 61 +++++++++++++++++++++++++++++++++++- > >> kernel/sched/sched.h | 3 ++ > >> kernel/sysctl.c | 11 +++++++ > >> 6 files changed, 91 insertions(+), 1 deletion(-) > >> > >> diff --git a/include/linux/sched/sysctl.h b/include/linux/sched/sysctl.h > >> index 303ee7dd0c7e..9b3673269ecc 100644 > >> --- a/include/linux/sched/sysctl.h > >> +++ b/include/linux/sched/sysctl.h > >> @@ -32,6 +32,10 @@ extern unsigned int sysctl_numa_balancing_promote_rate_limit; > >> #define sysctl_numa_balancing_mode 0 > >> #endif > >> > >> +#ifdef CONFIG_SCHED_PRIO_LB > >> +extern unsigned int sysctl_sched_prio_load_balance_enabled; > >> +#endif > >> + > >> int sysctl_numa_balancing(struct ctl_table *table, int write, void *buffer, > >> size_t *lenp, loff_t *ppos); > >> > >> diff --git a/init/Kconfig b/init/Kconfig > >> index 694f7c160c9c..b0dfe6701218 100644 > >> --- a/init/Kconfig > >> +++ b/init/Kconfig > >> @@ -1026,6 +1026,16 @@ config CFS_BANDWIDTH > >> restriction. > >> See Documentation/scheduler/sched-bwc.rst for more information. > >> > >> +config SCHED_PRIO_LB > >> + bool "Priority load balance for CFS" > >> + depends on SMP > >> + default n > >> + help > >> + This feature enable CFS priority load balance to reduce > >> + non-idle tasks latency interferenced by SCHED_IDLE tasks. > >> + It prefer migrating non-idle tasks firstly and > >> + migrating SCHED_IDLE tasks lastly. > >> + > >> config RT_GROUP_SCHED > >> bool "Group scheduling for SCHED_RR/FIFO" > >> depends on CGROUP_SCHED > >> diff --git a/kernel/sched/core.c b/kernel/sched/core.c > >> index 5800b0623ff3..9be35431fdd5 100644 > >> --- a/kernel/sched/core.c > >> +++ b/kernel/sched/core.c > >> @@ -9731,6 +9731,9 @@ void __init sched_init(void) > >> rq->max_idle_balance_cost = sysctl_sched_migration_cost; > >> > >> INIT_LIST_HEAD(&rq->cfs_tasks); > >> +#ifdef CONFIG_SCHED_PRIO_LB > >> + INIT_LIST_HEAD(&rq->cfs_idle_tasks); > >> +#endif > >> > >> rq_attach_root(rq, &def_root_domain); > >> #ifdef CONFIG_NO_HZ_COMMON > >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > >> index e4a0b8bd941c..bdeb04324f0c 100644 > >> --- a/kernel/sched/fair.c > >> +++ b/kernel/sched/fair.c > >> @@ -139,6 +139,10 @@ static int __init setup_sched_thermal_decay_shift(char *str) > >> } > >> __setup("sched_thermal_decay_shift=", setup_sched_thermal_decay_shift); > >> > >> +#ifdef CONFIG_SCHED_PRIO_LB > >> +unsigned int sysctl_sched_prio_load_balance_enabled; > >> +#endif > >> + > >> #ifdef CONFIG_SMP > >> /* > >> * For asym packing, by default the lower numbered CPU has higher priority. > >> @@ -3199,6 +3203,21 @@ static inline void update_scan_period(struct task_struct *p, int new_cpu) > >> > >> #endif /* CONFIG_NUMA_BALANCING */ > >> > >> +#ifdef CONFIG_SCHED_PRIO_LB > >> +static void > >> +adjust_rq_cfs_tasks( > >> + void (*list_op)(struct list_head *, struct list_head *), > >> + struct rq *rq, > >> + struct sched_entity *se) > >> +{ > >> + if (sysctl_sched_prio_load_balance_enabled && > >> + task_has_idle_policy(task_of(se))) > >> + (*list_op)(&se->group_node, &rq->cfs_idle_tasks); > >> + else > >> + (*list_op)(&se->group_node, &rq->cfs_tasks); > >> +} > >> +#endif > >> + > >> static void > >> account_entity_enqueue(struct cfs_rq *cfs_rq, struct sched_entity *se) > >> { > >> @@ -3208,7 +3227,11 @@ account_entity_enqueue(struct cfs_rq *cfs_rq, struct sched_entity *se) > >> struct rq *rq = rq_of(cfs_rq); > >> > >> account_numa_enqueue(rq, task_of(se)); > >> +#ifdef CONFIG_SCHED_PRIO_LB > >> + adjust_rq_cfs_tasks(list_add, rq, se); > >> +#else > >> list_add(&se->group_node, &rq->cfs_tasks); > >> +#endif > >> } > >> #endif > >> cfs_rq->nr_running++; > >> @@ -7631,7 +7654,11 @@ done: __maybe_unused; > >> * the list, so our cfs_tasks list becomes MRU > >> * one. > >> */ > >> +#ifdef CONFIG_SCHED_PRIO_LB > >> + adjust_rq_cfs_tasks(list_move, rq, &p->se); > >> +#else > >> list_move(&p->se.group_node, &rq->cfs_tasks); > >> +#endif > >> #endif > >> > >> if (hrtick_enabled_fair(rq)) > >> @@ -8156,11 +8183,18 @@ static void detach_task(struct task_struct *p, struct lb_env *env) > >> static struct task_struct *detach_one_task(struct lb_env *env) > >> { > >> struct task_struct *p; > >> + struct list_head *tasks = &env->src_rq->cfs_tasks; > >> +#ifdef CONFIG_SCHED_PRIO_LB > >> + bool has_detach_idle_tasks = false; > >> +#endif > >> > >> lockdep_assert_rq_held(env->src_rq); > >> > >> +#ifdef CONFIG_SCHED_PRIO_LB > >> +again: > >> +#endif > >> list_for_each_entry_reverse(p, > >> - &env->src_rq->cfs_tasks, se.group_node) { > >> + tasks, se.group_node) { > >> if (!can_migrate_task(p, env)) > >> continue; > >> > >> @@ -8175,6 +8209,13 @@ static struct task_struct *detach_one_task(struct lb_env *env) > >> schedstat_inc(env->sd->lb_gained[env->idle]); > >> return p; > >> } > >> +#ifdef CONFIG_SCHED_PRIO_LB > >> + if (sysctl_sched_prio_load_balance_enabled && !has_detach_idle_tasks) { > >> + has_detach_idle_tasks = true; > >> + tasks = &env->src_rq->cfs_idle_tasks; > >> + goto again; > >> + } > >> +#endif > >> return NULL; > >> } > >> > >> @@ -8190,6 +8231,9 @@ static int detach_tasks(struct lb_env *env) > >> unsigned long util, load; > >> struct task_struct *p; > >> int detached = 0; > >> +#ifdef CONFIG_SCHED_PRIO_LB > >> + bool has_detach_idle_tasks = false; > >> +#endif > >> > >> lockdep_assert_rq_held(env->src_rq); > >> > >> @@ -8205,6 +8249,9 @@ static int detach_tasks(struct lb_env *env) > >> if (env->imbalance <= 0) > >> return 0; > >> > >> +#ifdef CONFIG_SCHED_PRIO_LB > >> +again: > >> +#endif > >> while (!list_empty(tasks)) { > >> /* > >> * We don't want to steal all, otherwise we may be treated likewise, > >> @@ -8310,6 +8357,14 @@ static int detach_tasks(struct lb_env *env) > >> list_move(&p->se.group_node, tasks); > >> } > >> > >> +#ifdef CONFIG_SCHED_PRIO_LB > >> + if (sysctl_sched_prio_load_balance_enabled && > >> + !has_detach_idle_tasks && env->imbalance > 0) { > >> + has_detach_idle_tasks = true; > >> + tasks = &env->src_rq->cfs_idle_tasks; > >> + goto again; > >> + } > >> +#endif > >> /* > >> * Right now, this is one of only two places we collect this stat > >> * so we can safely collect detach_one_task() stats here rather > >> @@ -11814,7 +11869,11 @@ static void set_next_task_fair(struct rq *rq, struct task_struct *p, bool first) > >> * Move the next running task to the front of the list, so our > >> * cfs_tasks list becomes MRU one. > >> */ > >> +#ifdef CONFIG_SCHED_PRIO_LB > >> + adjust_rq_cfs_tasks(list_move, rq, se); > >> +#else > >> list_move(&se->group_node, &rq->cfs_tasks); > >> +#endif > >> } > >> #endif > >> > >> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > >> index 1644242ecd11..1b831c05ba30 100644 > >> --- a/kernel/sched/sched.h > >> +++ b/kernel/sched/sched.h > >> @@ -1053,6 +1053,9 @@ struct rq { > >> int online; > >> > >> struct list_head cfs_tasks; > >> +#ifdef CONFIG_SCHED_PRIO_LB > >> + struct list_head cfs_idle_tasks; > >> +#endif > >> > >> struct sched_avg avg_rt; > >> struct sched_avg avg_dl; > >> diff --git a/kernel/sysctl.c b/kernel/sysctl.c > >> index 188c305aeb8b..5fc0f9ffb675 100644 > >> --- a/kernel/sysctl.c > >> +++ b/kernel/sysctl.c > >> @@ -2090,6 +2090,17 @@ static struct ctl_table kern_table[] = { > >> .extra1 = SYSCTL_ONE, > >> .extra2 = SYSCTL_INT_MAX, > >> }, > >> +#endif > >> +#ifdef CONFIG_SCHED_PRIO_LB > >> + { > >> + .procname = "sched_prio_load_balance_enabled", > >> + .data = &sysctl_sched_prio_load_balance_enabled, > >> + .maxlen = sizeof(unsigned int), > >> + .mode = 0644, > >> + .proc_handler = proc_dointvec_minmax, > >> + .extra1 = SYSCTL_ZERO, > >> + .extra2 = SYSCTL_ONE, > >> + }, > >> #endif > >> { } > >> }; > >> -- > >> 2.27.0 > >> > > .
