Commit 27d84b63 authored Mar 02, 2020 by Qais Yousef

FROMGIT: sched/rt: Re-instate old behavior in select_task_rq_rt()

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.

Bug: 120440300
LINK: https://lore.kernel.org/lkml/20200218041620.GD28029@codeaurora.org/


Reported-by: Pavan Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Fixes: 804d402fb6f6 ("sched/rt: Make RT capacity-aware")
Link: https://lkml.kernel.org/r/20200302132721.8353-3-qais.yousef@arm.com
(cherry picked from commit b28bc1e002c23ff8a4999c4a2fb1d4d412bc6f5e
 https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git

 sched/core)
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Change-Id: I670ab7f95a3bd8b4790e1cafe89308ead524367e

parent ca79ac3c

kernel/sched/rt.c

+9 −0

Original line number	Diff line number	Diff line
		@@ -1478,6 +1478,13 @@ 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 && target != -1 && !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.
		@@ -1486,6 +1493,8 @@ 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: