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From:   Qais Yousef <qais.yousef@arm.com>
To:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Pavan Kondeti <pkondeti@codeaurora.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc:     Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        linux-kernel@vger.kernel.org, Qais Yousef <qais.yousef@arm.com>
Subject: [PATCH 2/3] sched/rt: allow pulling unfitting task
Date:   Fri, 14 Feb 2020 16:39:48 +0000
Message-Id: <20200214163949.27850-3-qais.yousef@arm.com>
In-Reply-To: <20200214163949.27850-1-qais.yousef@arm.com>
References: <20200214163949.27850-1-qais.yousef@arm.com>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

When implemented RT Capacity Awareness; the logic was done such that if
a task was running on a fitting CPU, then it was sticky and we would try
our best to keep it there.

But as Steve suggested, to adhere to the strict priority rules of RT
class; allow pulling an RT task to unfitting CPU to ensure it gets a
chance to run ASAP. When doing so, mark the queue as overloaded to give
the system a chance to push the task to a better fitting CPU when a
chance arises.

Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
---
 kernel/sched/rt.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 4043abe45459..0c8bac134d3a 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1646,10 +1646,20 @@ static void put_prev_task_rt(struct rq *rq, struct task_struct *p)
 
 static int pick_rt_task(struct rq *rq, struct task_struct *p, int cpu)
 {
-	if (!task_running(rq, p) &&
-	    cpumask_test_cpu(cpu, p->cpus_ptr) &&
-	    rt_task_fits_capacity(p, cpu))
+	if (!task_running(rq, p) && cpumask_test_cpu(cpu, p->cpus_ptr)) {
+
+		/*
+		 * If the CPU doesn't fit the task, allow pulling but mark the
+		 * rq as overloaded so that we can push it again to a more
+		 * suitable CPU ASAP.
+		 */
+		if (!rt_task_fits_capacity(p, cpu)) {
+			rt_set_overload(rq);
+			rq->rt.overloaded = 1;
+		}
+
 		return 1;
+	}
 
 	return 0;
 }
-- 
2.17.1