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Date: Thu, 3 Nov 2022 11:01:40 +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
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.
>>
>> 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:
$ 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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