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Message-ID: <aIy4vmuBh9QemI63@jlelli-thinkpadt14gen4.remote.csb>
Date: Fri, 1 Aug 2025 14:53:18 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Tommaso Cucinotta <tommaso.cucinotta@...il.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Valentin Schneider <vschneid@...hat.com>,
linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>
Subject: Re: [PATCH] sched/deadline: sched_getattr(...flags=1) returns the
runtime left and abs deadline for DEADLINE tasks
Hi Tommaso,
On 15/07/25 18:39, Tommaso Cucinotta wrote:
> The SCHED_DEADLINE scheduler allows reading the statically configured
> run-time, deadline, and period parameters through the sched_getattr()
> system call. However, there is no immediate way to access, from user space,
> the current parameters used within the scheduler: the instantaneous runtime
> left in the current cycle, as well as the current absolute deadline.
>
> The `flags' sched_getattr() parameter, so far mandated to contain zero,
> now supports the SCHED_GETATTR_FLAG_DL_DYNAMIC=1 flag, to request
> retrieval of the leftover runtime and absolute deadline, converted to a
> CLOCK_MONOTONIC reference, instead of the statically configured parameters.
>
> This feature is useful for adaptive SCHED_DEADLINE tasks that need to
> modify their behavior depending on whether or not there is enough runtime
> left in the current period, and/or what is the current absolute deadline.
>
> Notes:
> - before returning the instantaneous parameters, the runtime is updated;
> - the abs deadline is returned shifted from rq_clock() to ktime_get_ns(),
> in CLOCK_MONOTONIC reference; this causes multiple invocations from the
> same period to return values that may differ for a few ns (showing some
> small drift), albeit the deadline doesn't move, in rq_clock() reference;
> - the abs deadline value returned to user-space, as unsigned 64-bit value,
> can represent nearly 585 years since boot time;
> - setting flags=0 provides the old behavior (retrieve static parameters).
As we discussed this offline before your submission you know that I was
already on board with the idea, so I would really like to hear what
Peter and others think about this.
Still a few comments below.
$SUBJECT can maybe simply be "sched/deadline: Add reporting of remaining
time/abs deadline to sched_getattr".
> See also the notes from discussion held at OSPM 2025 on the topic
> "Making user space aware of current deadline-scheduler parameters":
> https://lwn.net/Articles/1022054/
I would probably remove the link from the changelog as it might
disappear/change in the future.
> Signed-off-by: Tommaso Cucinotta <tommaso.cucinotta@...tannapisa.it>
> ---
> include/uapi/linux/sched.h | 3 +++
> kernel/sched/deadline.c | 18 +++++++++++++++---
> kernel/sched/sched.h | 2 +-
> kernel/sched/syscalls.c | 16 +++++++++++-----
> 4 files changed, 30 insertions(+), 9 deletions(-)
>
> diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
> index 359a14cc..52b69ce8 100644
> --- a/include/uapi/linux/sched.h
> +++ b/include/uapi/linux/sched.h
> @@ -146,4 +146,7 @@ struct clone_args {
> SCHED_FLAG_KEEP_ALL | \
> SCHED_FLAG_UTIL_CLAMP)
>
> +/* Only for sched_getattr() own flag param, if task is SCHED_DEADLINE */
> +#define SCHED_GETATTR_FLAG_DL_DYNAMIC 0x01
> +
> #endif /* _UAPI_LINUX_SCHED_H */
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 9c7d9528..1b5cd7fa 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -3290,13 +3290,25 @@ void __setparam_dl(struct task_struct *p, const struct sched_attr *attr)
> dl_se->dl_density = to_ratio(dl_se->dl_deadline, dl_se->dl_runtime);
> }
>
> -void __getparam_dl(struct task_struct *p, struct sched_attr *attr)
> +void __getparam_dl(struct task_struct *p, struct sched_attr *attr, unsigned int flags)
> {
> struct sched_dl_entity *dl_se = &p->dl;
> + struct rq *rq = task_rq(p);
> + u64 adj_deadline;
>
> attr->sched_priority = p->rt_priority;
> - attr->sched_runtime = dl_se->dl_runtime;
> - attr->sched_deadline = dl_se->dl_deadline;
> + if (flags & SCHED_GETATTR_FLAG_DL_DYNAMIC) {
> + guard(raw_spinlock_irq)(&rq->__lock);
> + update_rq_clock(rq);
> + update_curr_dl(rq);
I p is not current maybe we don't need to call update_curr_dl() as p
will still have sensible dynamic parameters updated last time it
blocked?
Also, even though this is superuser stuff and all, mildly fear this
could be used as a DOS attack vector? Do we want to be super defensive
and add some kind of rate limiting?
> +
> + attr->sched_runtime = dl_se->runtime;
> + adj_deadline = dl_se->deadline - rq_clock(rq) + ktime_get_ns();
> + attr->sched_deadline = adj_deadline;
> + } else {
> + attr->sched_runtime = dl_se->dl_runtime;
> + attr->sched_deadline = dl_se->dl_deadline;
> + }
> attr->sched_period = dl_se->dl_period;
> attr->sched_flags &= ~SCHED_DL_FLAGS;
> attr->sched_flags |= dl_se->flags;
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 3058fb62..f69bf019 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -353,7 +353,7 @@ extern int sched_dl_global_validate(void);
> extern void sched_dl_do_global(void);
> extern int sched_dl_overflow(struct task_struct *p, int policy, const struct sched_attr *attr);
> extern void __setparam_dl(struct task_struct *p, const struct sched_attr *attr);
> -extern void __getparam_dl(struct task_struct *p, struct sched_attr *attr);
> +extern void __getparam_dl(struct task_struct *p, struct sched_attr *attr, unsigned int flags);
> extern bool __checkparam_dl(const struct sched_attr *attr);
> extern bool dl_param_changed(struct task_struct *p, const struct sched_attr *attr);
> extern int dl_cpuset_cpumask_can_shrink(const struct cpumask *cur, const struct cpumask *trial);
> diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
> index 77ae87f3..c80b3568 100644
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
> @@ -928,10 +928,10 @@ static int sched_copy_attr(struct sched_attr __user *uattr, struct sched_attr *a
> return -E2BIG;
> }
>
> -static void get_params(struct task_struct *p, struct sched_attr *attr)
> +static void get_params(struct task_struct *p, struct sched_attr *attr, unsigned int flags)
> {
> if (task_has_dl_policy(p)) {
> - __getparam_dl(p, attr);
> + __getparam_dl(p, attr, flags);
> } else if (task_has_rt_policy(p)) {
> attr->sched_priority = p->rt_priority;
> } else {
> @@ -997,7 +997,7 @@ SYSCALL_DEFINE3(sched_setattr, pid_t, pid, struct sched_attr __user *, uattr,
> return -ESRCH;
>
> if (attr.sched_flags & SCHED_FLAG_KEEP_PARAMS)
> - get_params(p, &attr);
> + get_params(p, &attr, 0);
>
> return sched_setattr(p, &attr);
> }
> @@ -1082,7 +1082,7 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
> int retval;
>
> if (unlikely(!uattr || pid < 0 || usize > PAGE_SIZE ||
> - usize < SCHED_ATTR_SIZE_VER0 || flags))
> + usize < SCHED_ATTR_SIZE_VER0))
> return -EINVAL;
>
> scoped_guard (rcu) {
> @@ -1090,6 +1090,12 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr,
> if (!p)
> return -ESRCH;
>
> + if (flags) {
> + if (!task_has_dl_policy(p)
> + || flags != SCHED_GETATTR_FLAG_DL_DYNAMIC)
Nit pick. Formatting usually has line break after '||'.
> + return -EINVAL;
> + }
> +
Thanks!
Juri
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