linux-kernel - [PATCH] sched/deadline: document behavior of sched

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Date:   Fri,  9 Sep 2016 19:45:17 +0200
From:   Tommaso Cucinotta <tommaso.cucinotta@...up.it>
To:     Luca Abeni <luca.abeni@...tn.it>,
        Juri Lelli <juri.lelli@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     linux-kernel@...r.kernel.org, linux-dl@...is.sssup.it,
        Tommaso Cucinotta <tommaso.cucinotta@...up.it>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: [PATCH] sched/deadline: document behavior of sched_yield()

This is a documentation only patch, explaining the
behavior of sched_yield() when a SCHED_DEADLINE
task calls it (give up remaining runtime and be
throttled until next period begins).

Signed-off-by: Tommaso Cucinotta <tommaso.cucinotta@...up.it>
Reviewed-by: Juri Lelli <juri.lelli@....com>
Reviewed-by: Luca Abeni <luca.abeni@...tn.it>
Reviewed-by: Daniel Bristot de Oliveira <bristot@...hat.com>
---
 Documentation/scheduler/sched-deadline.txt | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/Documentation/scheduler/sched-deadline.txt b/Documentation/scheduler/sched-deadline.txt
index 53a2fe1..8e37b0b 100644
--- a/Documentation/scheduler/sched-deadline.txt
+++ b/Documentation/scheduler/sched-deadline.txt
@@ -16,6 +16,7 @@ CONTENTS
    4.1 System-wide settings
    4.2 Task interface
    4.3 Default behavior
+   4.4 Behavior of sched_yield()
  5. Tasks CPU affinity
    5.1 SCHED_DEADLINE and cpusets HOWTO
  6. Future plans
@@ -426,6 +427,23 @@ CONTENTS
  Finally, notice that in order not to jeopardize the admission control a
  -deadline task cannot fork.
 
+
+4.4 Behavior of sched_yield()
+-----------------------------
+
+ When a SCHED_DEADLINE task calls sched_yield(), it gives up its
+ remaining runtime and is immediately throttled, until the next
+ period, when its runtime will be replenished (a special flag
+ dl_yielded is set and used to handle correctly throttling and runtime
+ replenishment after a call to sched_yield()).
+
+ This behavior of sched_yield() allows the task to wake-up exactly at
+ the beginning of the next period. Also, this may be useful in the
+ future with bandwidth reclaiming mechanisms, where sched_yield() will
+ make the leftoever runtime available for reclamation by other
+ SCHED_DEADLINE tasks.
+
+
 5. Tasks CPU affinity
 =====================
 
-- 
2.7.4