linux-kernel - [GIT PULL] scheduler fix

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Message-ID: <20171027191629.t66wcealehgu5l7u@gmail.com>
Date:   Fri, 27 Oct 2017 21:16:29 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [GIT PULL] scheduler fix

Linus,

Please pull the latest sched-urgent-for-linus git tree from:

   git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git sched-urgent-for-linus

   # HEAD: 88796e7e5c457cae72833196cb98e6895dd107e2 sched/swait: Document it clearly that the swait facilities are special and shouldn't be used

Update the <linux/swait.h> documentation to discourage their use.

 Thanks,

	Ingo

------------------>
Davidlohr Bueso (1):
      sched/swait: Document it clearly that the swait facilities are special and shouldn't be used


 include/linux/swait.h | 27 ++++++++++++++++-----------
 1 file changed, 16 insertions(+), 11 deletions(-)

diff --git a/include/linux/swait.h b/include/linux/swait.h
index 73e97a08d3d0..cf30f5022472 100644
--- a/include/linux/swait.h
+++ b/include/linux/swait.h
@@ -9,13 +9,16 @@
 /*
  * Simple wait queues
  *
- * While these are very similar to the other/complex wait queues (wait.h) the
- * most important difference is that the simple waitqueue allows for
- * deterministic behaviour -- IOW it has strictly bounded IRQ and lock hold
- * times.
+ * While these are very similar to regular wait queues (wait.h) the most
+ * important difference is that the simple waitqueue allows for deterministic
+ * behaviour -- IOW it has strictly bounded IRQ and lock hold times.
  *
- * In order to make this so, we had to drop a fair number of features of the
- * other waitqueue code; notably:
+ * Mainly, this is accomplished by two things. Firstly not allowing swake_up_all
+ * from IRQ disabled, and dropping the lock upon every wakeup, giving a higher
+ * priority task a chance to run.
+ *
+ * Secondly, we had to drop a fair number of features of the other waitqueue
+ * code; notably:
  *
  *  - mixing INTERRUPTIBLE and UNINTERRUPTIBLE sleeps on the same waitqueue;
  *    all wakeups are TASK_NORMAL in order to avoid O(n) lookups for the right
@@ -24,12 +27,14 @@
  *  - the exclusive mode; because this requires preserving the list order
  *    and this is hard.
  *
- *  - custom wake functions; because you cannot give any guarantees about
- *    random code.
- *
- * As a side effect of this; the data structures are slimmer.
+ *  - custom wake callback functions; because you cannot give any guarantees
+ *    about random code. This also allows swait to be used in RT, such that
+ *    raw spinlock can be used for the swait queue head.
  *
- * One would recommend using this wait queue where possible.
+ * As a side effect of these; the data structures are slimmer albeit more ad-hoc.
+ * For all the above, note that simple wait queues should _only_ be used under
+ * very specific realtime constraints -- it is best to stick with the regular
+ * wait queues in most cases.
  */
 
 struct task_struct;