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From: Qais Yousef <qais.yousef@arm.com>
To: Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Pavan Kondeti <pkondeti@codeaurora.org>
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 v2 2/6] sched/rt: Re-instate old behavior in select_task_rq_rt
Date: Sun, 23 Feb 2020 18:39:57 +0000	[thread overview]
Message-ID: <20200223184001.14248-3-qais.yousef@arm.com> (raw)
In-Reply-To: <20200223184001.14248-1-qais.yousef@arm.com>

When RT Capacity Aware support was added, the logic in select_task_rq_rt
was modified to force a search for a fitting CPU if the task currently
doesn't run on one.

But if the search failed, and the search was only triggered to fulfill
the fitness request; we could end up selecting a new CPU unnecessarily.

Fix this and re-instate the original behavior by ensuring we bail out
in that case.

This behavior change only affected asymmetric systems that are using
util_clamp to implement capacity aware. None asymmetric systems weren't
affected.

Reported-by: Pavan Kondeti <pkondeti@codeaurora.org>
Fixes: 804d402fb6f6 ("sched/rt: Make RT capacity-aware")
LINK: https://lore.kernel.org/lkml/20200218041620.GD28029@codeaurora.org/
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
---
 kernel/sched/rt.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 4043abe45459..2c3fae637cef 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1474,6 +1474,13 @@ 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 && !rt_task_fits_capacity(p, target))
+			goto out_unlock;
+
 		/*
 		 * Don't bother moving it if the destination CPU is
 		 * not running a lower priority task.
@@ -1482,6 +1489,8 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
 		    p->prio < cpu_rq(target)->rt.highest_prio.curr)
 			cpu = target;
 	}
+
+out_unlock:
 	rcu_read_unlock();
 
 out:
-- 
2.17.1


  parent reply	other threads:[~2020-02-23 18:40 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-23 18:39 [PATCH v2 0/6] RT Capacity Awareness Fixes & Improvements Qais Yousef
2020-02-23 18:39 ` [PATCH v2 1/6] sched/rt: cpupri_find: implement fallback mechanism for !fit case Qais Yousef
2020-02-23 18:39 ` Qais Yousef [this message]
2020-02-25 15:21   ` [PATCH v2 2/6] sched/rt: Re-instate old behavior in select_task_rq_rt Dietmar Eggemann
2020-02-26 11:34     ` Qais Yousef
2020-02-23 18:39 ` [PATCH v2 3/6] sched/rt: Optimize cpupri_find on non-heterogenous systems Qais Yousef
2020-02-23 18:39 ` [PATCH v2 4/6] sched/rt: allow pulling unfitting task Qais Yousef
2020-02-23 18:40 ` [PATCH v2 5/6] sched/rt: Better manage pushing unfit tasks on wakeup Qais Yousef
2020-02-24  6:10   ` Pavan Kondeti
2020-02-24 12:11     ` Qais Yousef
2020-02-24 16:04       ` Pavan Kondeti
2020-02-24 17:41         ` Qais Yousef
2020-02-25  3:55           ` Pavan Kondeti
2020-02-26 16:02             ` Qais Yousef
2020-02-27  3:36               ` Pavan Kondeti
2020-02-27 10:29                 ` Qais Yousef
2020-02-23 18:40 ` [PATCH v2 6/6] sched/rt: Remove unnecessary assignment in inc/dec_rt_migration Qais Yousef
2020-02-23 23:16   ` Dietmar Eggemann
2020-02-24 12:31     ` Qais Yousef
2020-02-24 13:03       ` Dietmar Eggemann
2020-02-24 13:47         ` Qais Yousef

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