Rework the interface between priority propagation (lending) and the

schedulers a bit to ensure more correct handling of priorities and fewer
priority inversions:
- Add two functions to the sched(9) API to handle priority lending:
  sched_lend_prio() and sched_unlend_prio().  The turnstile code uses these
  functions to ask the scheduler to lend a thread a set priority and to
  tell the scheduler when it thinks it is ok for a thread to stop borrowing
  priority.  The unlend case is slightly complex in that the turnstile code
  tells the scheduler what the minimum priority of the thread needs to be
  to satisfy the requirements of any other threads blocked on locks owned
  by the thread in question.  The scheduler then decides where the thread
  can go back to normal mode (if it's normal priority is high enough to
  satisfy the pending lock requests) or it it should continue to use the
  priority specified to the sched_unlend_prio() call.  This involves adding
  a new per-thread flag TDF_BORROWING that replaces the ULE-only kse flag
  for priority elevation.
- Schedulers now refuse to lower the priority of a thread that is currently
  borrowing another therad's priority.
- If a scheduler changes the priority of a thread that is currently sitting
  on a turnstile, it will call a new function turnstile_adjust() to inform
  the turnstile code of the change.  This function resorts the thread on
  the priority list of the turnstile if needed, and if the thread ends up
  at the head of the list (due to having the highest priority) and its
  priority was raised, then it will propagate that new priority to the
  owner of the lock it is blocked on.

Some additional fixes specific to the 4BSD scheduler include:
- Common code for updating the priority of a thread when the user priority
  of its associated kse group has been consolidated in a new static
  function resetpriority_thread().  One change to this function is that
  it will now only adjust the priority of a thread if it already has a
  time sharing priority, thus preserving any boosts from a tsleep() until
  the thread returns to userland.  Also, resetpriority() no longer calls
  maybe_resched() on each thread in the group. Instead, the code calling
  resetpriority() is responsible for calling resetpriority_thread() on
  any threads that need to be updated.
- schedcpu() now uses resetpriority_thread() instead of just calling
  sched_prio() directly after it updates a kse group's user priority.
- sched_clock() now uses resetpriority_thread() rather than writing
  directly to td_priority.
- sched_nice() now updates all the priorities of the threads after the
  group priority has been adjusted.

Discussed with:	bde
Reviewed by:	ups, jeffr
Tested on:	4bsd, ule
Tested on:	i386, alpha, sparc64

1 files changed, 1 insertions, 0 deletions

diff --git a/sys/sys/proc.h b/sys/sys/proc.h
index 542f62fd5023..6278ae8f50e8 100644
--- a/sys/sys/proc.h
+++ b/sys/sys/proc.h
@@ -327,6 +327,7 @@ struct thread {
  * Flags kept in td_flags:
  * To change these you MUST have the scheduler lock.
  */
+#define	TDF_BORROWING	0x00000001 /* Thread is borrowing pri from another. */
 #define	TDF_UNUSED0	0x00000001 /* --available -- */
 #define	TDF_INPANIC	0x00000002 /* Caused a panic, let it drive crashdump. */
 #define	TDF_SINTR	0x00000008 /* Sleep is interruptible. */