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sched/deadline: document new sched_getattr() feature for retrieving current parameters for DEADLINE tasks

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Document in Documentation/sched/sched-deadline.rst the new capability of
sched_getattr() to retrieve, for DEADLINE tasks, the runtime left and absolute
deadline (setting the flags syscall parameter to 1), in addition to the static
parameters (obtained with flags=0).

Signed-off-by: Tommaso Cucinotta <tommaso.cucinotta@santannapisa.it>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20260304102843.1373905-2-tommaso.cucinotta@santannapisa.it>
This commit is contained in:
Tommaso Cucinotta 2026-03-04 11:28:16 +01:00 • committed by Jonathan Corbet
commit b8e7e03370
1 changed files with 15 additions and 4 deletions
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19
Documentation/scheduler/sched-deadline.rst
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@ -628,10 +628,21 @@ Deadline Task Scheduling
* the new scheduling related syscalls that manipulate it, i.e.,
sched_setattr() and sched_getattr() are implemented.
For debugging purposes, the leftover runtime and absolute deadline of a
SCHED_DEADLINE task can be retrieved through /proc/<pid>/sched (entries
dl.runtime and dl.deadline, both values in ns). A programmatic way to
retrieve these values from production code is under discussion.
The leftover runtime and absolute deadline of a SCHED_DEADLINE task can be
read using the sched_getattr() syscall, setting the last syscall parameter
flags to the SCHED_GETATTR_FLAG_DL_DYNAMIC=1 value. This updates the
runtime left, converts the absolute deadline in CLOCK_MONOTONIC reference,
then returns these parameters to user-space. The absolute deadline is
returned as the number of nanoseconds since the CLOCK_MONOTONIC time
reference (boot instant), as a u64 in the sched_deadline field of sched_attr,
which can represent nearly 585 years since boot time (calling sched_getattr()
with flags=0 causes retrieval of the static parameters instead).
For debugging purposes, these parameters can also be retrieved through
/proc/<pid>/sched (entries dl.runtime and dl.deadline, both values in ns),
but: this is highly inefficient; the returned runtime left is not updated as
done by sched_getattr(); the deadline is provided in kernel rq_clock time
reference, that is not directly usable from user-space.
4.3 Default behavior