Commit 2e7af192 authored by Tommaso Cucinotta's avatar Tommaso Cucinotta Committed by Peter Zijlstra
Browse files

sched/deadline: Add reporting of runtime left & abs deadline to sched_getattr() for DEADLINE tasks 



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).

See also the notes from discussion held at OSPM 2025 on the topic
"Making user space aware of current deadline-scheduler parameters".

Signed-off-by: default avatarTommaso Cucinotta <tommaso.cucinotta@santannapisa.it>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: default avatarMatteo Martelli <matteo.martelli@codethink.co.uk>
Link: https://patch.msgid.link/20250912053937.31636-2-tommaso.cucinotta@santannapisa.it
parent fd54d81c

include/uapi/linux/sched.h

+3 −0
Original line number Diff line number Diff line
@@ -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 */

kernel/sched/deadline.c

+16 −3
Original line number Diff line number Diff line
@@ -3617,13 +3617,26 @@ 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;

	if (flags & SCHED_GETATTR_FLAG_DL_DYNAMIC) {

		guard(raw_spinlock_irq)(&rq->__lock);

		update_rq_clock(rq);

		if (task_current(rq, p))

			update_curr_dl(rq);



		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;

kernel/sched/sched.h

+1 −1
Original line number Diff line number Diff line
@@ -356,7 +356,7 @@ 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);

kernel/sched/syscalls.c

+11 −5
Original line number Diff line number Diff line
@@ -881,10 +881,10 @@ 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 {
@@ -950,7 +950,7 @@ 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);

}
@@ -1035,7 +1035,7 @@ 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) {
@@ -1043,6 +1043,12 @@ 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)

				return -EINVAL;

		}



		retval = security_task_getscheduler(p);

		if (retval)

			return retval;
@@ -1050,7 +1056,7 @@ SYSCALL_DEFINE4(sched_getattr, pid_t, pid, struct sched_attr __user *, uattr, 
		kattr.sched_policy = p->policy;

		if (p->sched_reset_on_fork)

			kattr.sched_flags |= SCHED_FLAG_RESET_ON_FORK;

		get_params(p, &kattr);

		get_params(p, &kattr, flags);

		kattr.sched_flags &= SCHED_FLAG_ALL;



#ifdef CONFIG_UCLAMP_TASK