[PATCH v2 28/35] sched: support preempt=full under PREEMPT_AUTO

[Ankur Arora]

The default preemption policy for preempt-full under PREEMPT_AUTO is
to minimize latency, and thus to always schedule eagerly. This is
identical to CONFIG_PREEMPT, and so should result in similar
performance.

Comparing scheduling/IPC workload:

 # perf stat -a -e cs --repeat 10 -- perf bench sched messaging -g 20 -t -l 5000

 PREEMPT_AUTO, preempt=full

         3,080,508            context-switches      ( +-  0.64% )
	   3.65171 +- 0.00654 seconds time elapsed  ( +-  0.18% )

 PREEMPT_DYNAMIC, preempt=full

	 3,087,527            context-switches      ( +-  0.33% )
	   3.60163 +- 0.00633 seconds time elapsed  ( +-  0.18% )

Looking at the breakup between voluntary and involuntary
context-switches, we see almost identical behaviour as well.

 PREEMPT_AUTO, preempt=full

           2087910.00 +- 34720.95 voluntary context-switches   ( +- 1.660% )
            784437.60 +- 19827.79 involuntary context-switches ( +- 2.520% )

 PREEMPT_DYNAMIC, preempt=full

           2102879.60 +- 22767.11 voluntary context-switches   ( +- 1.080% )
            801189.90 +- 21324.18 involuntary context-switches ( +- 2.660% )

Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Ziljstra <peterz@xxxxxxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Originally-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/
Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
---
 kernel/sched/core.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c3ba33c77053..c25cccc09b65 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1035,9 +1035,10 @@ void wake_up_q(struct wake_q_head *head)
  * For preemption models other than PREEMPT_AUTO: always schedule
  * eagerly.
  *
- * For PREEMPT_AUTO: schedule idle threads eagerly, allow everything else
- * to finish its time quanta, and mark for rescheduling at the next exit
- * to user.
+ * For PREEMPT_AUTO: schedule idle threads eagerly, and under full
+ * preemption, all tasks eagerly. Otherwise, allow everything else
+ * to finish its time quanta, and mark for rescheduling at the next
+ * exit to user.
  */
 static resched_t resched_opt_translate(struct task_struct *curr,
 				       enum resched_opt opt)
@@ -1048,6 +1049,9 @@ static resched_t resched_opt_translate(struct task_struct *curr,
 	if (opt == RESCHED_FORCE)
 		return RESCHED_NOW;
 
+	if (preempt_model_preemptible())
+		return RESCHED_NOW;
+
 	if (is_idle_task(curr))
 		return RESCHED_NOW;
 
@@ -8997,7 +9001,9 @@ static void __sched_dynamic_update(int mode)
 			pr_warn("%s: preempt=full is not recommended with CONFIG_PREEMPT_RCU=n",
 				PREEMPT_MODE);
 
-		preempt_dynamic_mode = preempt_dynamic_undefined;
+		if (mode != preempt_dynamic_mode)
+			pr_info("%s: full\n", PREEMPT_MODE);
+		preempt_dynamic_mode = mode;
 		break;
 	}
 }
-- 
2.31.1