Messages in this thread | | | Date | Thu, 3 Nov 2022 17:20:22 +0800 | Subject | Re: [PATCH] sched/fair: Introduce priority load balance for CFS | From | Song Zhang <> |
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On 2022/11/3 16:33, Vincent Guittot wrote: > On Thu, 3 Nov 2022 at 04:01, Song Zhang <zhangsong34@huawei.com> 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@huawei.com> wrote: >>>> >>> >>> This really looks like a v3 of >>> https://lore.kernel.org/all/20220810015636.3865248-1-zhangsong34@huawei.com/ >>> >>> 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. >
But migrating online tasks first is mostly a trade-off that non-idle(Latency Sensitive) tasks can obtain more CPU time and minimize the interference caused by IDLE tasks. I think this makes sense in most cases, or you can point out what else I need to think about it ?
Best regards.
>> >>>> >>>> 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 ? >
No no no... spawn 5000 idle(SCHED_IDLE) processes not 5000 non-idle threads, and with 10 non-idle schbench workers on 8 vCPUs.
>> >> $ 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@huawei.com> >>>> --- >>>> 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 >>>> >>> . > .
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