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Message-ID: <20240917214337.GA10143@ranerica-svr.sc.intel.com>
Date: Tue, 17 Sep 2024 14:43:37 -0700
From: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To: Qais Yousef <qyousef@...alina.io>
Cc: Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
Vincent Guittot <vincent.guittot@...aro.org>,
"Rafael J. Wysocki" <rafael@...nel.org>,
Viresh Kumar <viresh.kumar@...aro.org>,
Juri Lelli <juri.lelli@...hat.com>,
Steven Rostedt <rostedt@...dmis.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
John Stultz <jstultz@...gle.com>, linux-pm@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 11/16] sched/qos: Add rampup multiplier QoS
On Tue, Aug 20, 2024 at 05:35:07PM +0100, Qais Yousef wrote:
> Bursty tasks are hard to predict. To use resources efficiently, the
> system would like to be exact as much as possible. But this poses
> a challenge for these bursty tasks that need to get access to more
> resources quickly.
>
> The new SCHED_QOS_RAMPUP_MULTIPLIER allows userspace to do that. As the
> name implies, it only helps them to transition to a higher performance
> state when they get _busier_. That is perfectly periodic tasks by
> definition are not going through a transition and will run at a constant
> performance level. It is the tasks that need to transition from one
> periodic state to another periodic state that is at a higher level that
> this rampup_multiplier will help with. It also slows down the ewma decay
> of util_est which should help those bursty tasks to keep their faster
> rampup.
>
> This should work complimentary with uclamp. uclamp tells the system
> about min and max perf requirements which can be applied immediately.
>
> rampup_multiplier is about reactiveness of the task to change.
> Specifically to a change for a higher performance level. The task might
> necessary need to have a min perf requirements, but it can have sudden
> burst of changes that require higher perf level and it needs the system
> to provide this faster.
>
> TODO: update the sched_qos docs
>
> Signed-off-by: Qais Yousef <qyousef@...alina.io>
> ---
> include/linux/sched.h | 7 ++++
> include/uapi/linux/sched.h | 2 ++
> kernel/sched/core.c | 66 ++++++++++++++++++++++++++++++++++++++
> kernel/sched/fair.c | 6 ++--
> kernel/sched/syscalls.c | 38 ++++++++++++++++++++--
> 5 files changed, 115 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 2e8c5a9ffa76..a30ee43a25fb 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -404,6 +404,11 @@ struct sched_info {
> #endif /* CONFIG_SCHED_INFO */
> };
>
> +struct sched_qos {
> + DECLARE_BITMAP(user_defined, SCHED_QOS_MAX);
> + unsigned int rampup_multiplier;
> +};
> +
> /*
> * Integer metrics need fixed point arithmetic, e.g., sched/fair
> * has a few: load, load_avg, util_avg, freq, and capacity.
> @@ -882,6 +887,8 @@ struct task_struct {
>
> struct sched_info sched_info;
>
> + struct sched_qos sched_qos;
> +
> struct list_head tasks;
> #ifdef CONFIG_SMP
> struct plist_node pushable_tasks;
> diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> index 67ef99f64ddc..0baba91ba5b8 100644
> --- a/include/uapi/linux/sched.h
> +++ b/include/uapi/linux/sched.h
> @@ -104,6 +104,8 @@ struct clone_args {
> };
>
> enum sched_qos_type {
> + SCHED_QOS_RAMPUP_MULTIPLIER,
> + SCHED_QOS_MAX,
> };
> #endif
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c91e6a62c7ab..54faa845cb29 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -152,6 +152,8 @@ __read_mostly int sysctl_resched_latency_warn_once = 1;
> */
> const_debug unsigned int sysctl_sched_nr_migrate = SCHED_NR_MIGRATE_BREAK;
>
> +unsigned int sysctl_sched_qos_default_rampup_multiplier = 1;
> +
> __read_mostly int scheduler_running;
>
> #ifdef CONFIG_SCHED_CORE
> @@ -4488,6 +4490,47 @@ static int sysctl_schedstats(struct ctl_table *table, int write, void *buffer,
> #endif /* CONFIG_SCHEDSTATS */
>
> #ifdef CONFIG_SYSCTL
> +static void sched_qos_sync_sysctl(void)
> +{
> + struct task_struct *g, *p;
> +
> + guard(rcu)();
> + for_each_process_thread(g, p) {
> + struct rq_flags rf;
> + struct rq *rq;
> +
> + rq = task_rq_lock(p, &rf);
> + if (!test_bit(SCHED_QOS_RAMPUP_MULTIPLIER, p->sched_qos.user_defined))
> + p->sched_qos.rampup_multiplier = sysctl_sched_qos_default_rampup_multiplier;
> + task_rq_unlock(rq, p, &rf);
> + }
> +}
> +
> +static int sysctl_sched_qos_handler(struct ctl_table *table, int write,
> + void *buffer, size_t *lenp, loff_t *ppos)
> +{
> + unsigned int old_rampup_mult;
> + int result;
> +
> + old_rampup_mult = sysctl_sched_qos_default_rampup_multiplier;
> +
> + result = proc_dointvec(table, write, buffer, lenp, ppos);
> + if (result)
> + goto undo;
> + if (!write)
> + return 0;
> +
> + if (old_rampup_mult != sysctl_sched_qos_default_rampup_multiplier) {
> + sched_qos_sync_sysctl();
> + }
> +
> + return 0;
> +
> +undo:
> + sysctl_sched_qos_default_rampup_multiplier = old_rampup_mult;
> + return result;
> +}
> +
> static struct ctl_table sched_core_sysctls[] = {
> #ifdef CONFIG_SCHEDSTATS
> {
> @@ -4534,6 +4577,13 @@ static struct ctl_table sched_core_sysctls[] = {
> .extra2 = SYSCTL_FOUR,
> },
> #endif /* CONFIG_NUMA_BALANCING */
> + {
> + .procname = "sched_qos_default_rampup_multiplier",
> + .data = &sysctl_sched_qos_default_rampup_multiplier,
> + .maxlen = sizeof(unsigned int),
IIUC, user space needs to select a value between 0 and (2^32 - 1). Does
this mean that it will need fine-tuning for each product and application?
Could there be some translation to a fewer number of QoS levels that are
qualitatively?
Also, I think about Intel processors. They work with hardware-controlled
performance scaling. The proposed interface would help us to communicate
per-task multipliers to hardware, but they would be used as hints to
hardware and not acted upon by the kernel to scale frequency.
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