linux-kernel - [PATCH] sched/deadline: Add reporting of runtime left & abs deadline to sched

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Message-ID: <20250912053937.31636-1-tommaso.cucinotta@santannapisa.it>
Date: Fri, 12 Sep 2025 07:38:28 +0200
From: Tommaso Cucinotta <tommaso.cucinotta@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	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>,
	Tommaso Cucinotta <tommaso.cucinotta@...il.com>
Subject: [PATCH] sched/deadline: Add reporting of runtime left & abs deadline to sched_getattr() for DEADLINE tasks

I'm resending this patch proposal after having addressed a few Juri's
comments, and a rebase on top of the post-august-break tip sched/core.

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@...tannapisa.it>