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Date: Fri, 4 Nov 2022 10:05:50 +0800 From: Song Zhang <zhangsong34@...wei.com> To: Vincent Guittot <vincent.guittot@...aro.org> CC: <mingo@...hat.com>, <peterz@...radead.org>, <juri.lelli@...hat.com>, <mcgrof@...nel.org>, <keescook@...omium.org>, <yzaikin@...gle.com>, <dietmar.eggemann@....com>, <rostedt@...dmis.org>, <bsegall@...gle.com>, <mgorman@...e.de>, <bristot@...hat.com>, <vschneid@...hat.com>, <linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org> Subject: Re: [PATCH] sched/fair: Introduce priority load balance for CFS On 2022/11/3 17:22, Vincent Guittot wrote: > On Thu, 3 Nov 2022 at 10:20, Song Zhang <zhangsong34@...wei.com> wrote: >> >> >> >> On 2022/11/3 16:33, Vincent Guittot wrote: >>> On Thu, 3 Nov 2022 at 04:01, Song Zhang <zhangsong34@...wei.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@...wei.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. > > yes spawn 5000 idle tasks but my point remains the same > But I really don't understand what you are most focused on, and what else should I do. >> >>>> >>>> $ 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@...wei.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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