linux-kernel - [PATCH v3 6/6] sched/rt: Fix pushing unfit tasks to a better CPU

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Message-Id: <20200302132721.8353-7-qais.yousef@arm.com>
Date:   Mon,  2 Mar 2020 13:27:21 +0000
From:   Qais Yousef <qais.yousef@....com>
To:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Pavan Kondeti <pkondeti@...eaurora.org>
Cc:     Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel@...r.kernel.org, Qais Yousef <qais.yousef@....com>
Subject: [PATCH v3 6/6] sched/rt: Fix pushing unfit tasks to a better CPU

If a task was running on unfit CPU we could ignore migrating if the
priority level of the new fitting CPU is the *same* as the unfit one.

Add an extra check to select_task_rq_rt() to allow the push in case:

	* p->prio == new_cpu.highest_priority
	* task_fits(p, new_cpu)

Fixes: 804d402fb6f6 ("sched/rt: Make RT capacity-aware")
Signed-off-by: Qais Yousef <qais.yousef@....com>
---
 kernel/sched/rt.c | 41 ++++++++++++++++++++++++++++-------------
 1 file changed, 28 insertions(+), 13 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index ce230bec6847..8aaa442e4867 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1474,20 +1474,35 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
 	if (test || !rt_task_fits_capacity(p, cpu)) {
 		int target = find_lowest_rq(p);
 
-		/*
-		 * Bail out if we were forcing a migration to find a better
-		 * fitting CPU but our search failed.
-		 */
-		if (!test && target != -1 && !rt_task_fits_capacity(p, target))
-			goto out_unlock;
+		if (target != -1) {
+			bool fit_target = rt_task_fits_capacity(p, target);
 
-		/*
-		 * Don't bother moving it if the destination CPU is
-		 * not running a lower priority task.
-		 */
-		if (target != -1 &&
-		    p->prio < cpu_rq(target)->rt.highest_prio.curr)
-			cpu = target;
+			/*
+			 * Bail out if we were forcing a migration to find a
+			 * better fitting CPU but our search failed.
+			 */
+			if (!test && !fit_target)
+				goto out_unlock;
+
+			/*
+			 * Don't bother moving it if the destination CPU is
+			 * not running a lower priority task.
+			 */
+			if (p->prio < cpu_rq(target)->rt.highest_prio.curr) {
+
+				cpu = target;
+
+			} else if (p->prio == cpu_rq(target)->rt.highest_prio.curr) {
+
+				/*
+				 * If the priority is the same and the new CPU
+				 * is a better fit, then move, otherwise don't
+				 * bother here either.
+				 */
+				if (fit_target)
+					cpu = target;
+			}
+		}
 	}
 
 out_unlock:
-- 
2.17.1