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

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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: Tommaso Cucinotta <tommaso.cucinotta@santannapisa.it>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Matteo Martelli <matteo.martelli@codethink.co.uk>
Link: https://patch.msgid.link/20250912053937.31636-2-tommaso.cucinotta@santannapisa.it
This commit is contained in:
Tommaso Cucinotta 2025-09-12 07:38:29 +02:00 committed by Peter Zijlstra
parent fd54d81c2c
commit 2e7af19269
4 changed files with 31 additions and 9 deletions
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3
include/uapi/linux/sched.h
View File

@ -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 */

19
kernel/sched/deadline.c
View File

@ -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;
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);
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;

2
kernel/sched/sched.h
View File

@ -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);

16
kernel/sched/syscalls.c
View File

@ -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