git: 40efe74352cf - main - 4bsd: Simplistic time-sharing for interrupt threads.

The branch main has been updated by jhb:

URL: https://cgit.FreeBSD.org/src/commit/?id=40efe74352cfdddc26ddb6d1512efeffc44bacc3

commit 40efe74352cfdddc26ddb6d1512efeffc44bacc3
Author:     John Baldwin <jhb@FreeBSD.org>
AuthorDate: 2022-07-14 20:14:17 +0000
Commit:     John Baldwin <jhb@FreeBSD.org>
CommitDate: 2022-07-14 20:14:17 +0000

    4bsd: Simplistic time-sharing for interrupt threads.
    
    If an interrupt thread runs for a full quantum without yielding the
    CPU, demote its priority and schedule a preemption to give other
    ithreads a turn.
    
    Reviewed by:    kib
    Sponsored by:   Netflix
    Differential Revision:  https://reviews.freebsd.org/D35645
---
 sys/kern/sched_4bsd.c | 28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/sys/kern/sched_4bsd.c b/sys/kern/sched_4bsd.c
index 779e1a7b4ef5..b85c32d54605 100644
--- a/sys/kern/sched_4bsd.c
+++ b/sys/kern/sched_4bsd.c
@@ -703,6 +703,10 @@ sched_rr_interval(void)
 	return (imax(1, (sched_slice * hz + realstathz / 2) / realstathz));
 }
 
+SCHED_STAT_DEFINE(ithread_demotions, "Interrupt thread priority demotions");
+SCHED_STAT_DEFINE(ithread_preemptions,
+    "Interrupt thread preemptions due to time-sharing");
+
 /*
  * We adjust the priority of the current process.  The priority of a
  * process gets worse as it accumulates CPU time.  The cpu usage
@@ -739,7 +743,20 @@ sched_clock_tick(struct thread *td)
 	 */
 	if (!TD_IS_IDLETHREAD(td) && --ts->ts_slice <= 0) {
 		ts->ts_slice = sched_slice;
-		td->td_flags |= TDF_NEEDRESCHED | TDF_SLICEEND;
+
+		/*
+		 * If an ithread uses a full quantum, demote its
+		 * priority and preempt it.
+		 */
+		if (PRI_BASE(td->td_pri_class) == PRI_ITHD) {
+			SCHED_STAT_INC(ithread_preemptions);
+			td->td_owepreempt = 1;
+			if (td->td_base_pri + RQ_PPQ < PRI_MAX_ITHD) {
+				SCHED_STAT_INC(ithread_demotions);
+				sched_prio(td, td->td_base_pri + RQ_PPQ);
+			}
+		} else
+			td->td_flags |= TDF_NEEDRESCHED | TDF_SLICEEND;
 	}
 
 	stat = DPCPU_PTR(idlestat);
@@ -1134,6 +1151,15 @@ sched_wakeup(struct thread *td, int srqflags)
 	td->td_slptick = 0;
 	ts->ts_slptime = 0;
 	ts->ts_slice = sched_slice;
+
+	/*
+	 * When resuming an idle ithread, restore its base ithread
+	 * priority.
+	 */
+	if (PRI_BASE(td->td_pri_class) == PRI_ITHD &&
+	    td->td_base_pri != td->td_base_ithread_pri)
+		sched_prio(td, td->td_base_ithread_pri);
+
 	sched_add(td, srqflags);
 }