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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.

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: 804d402fb6 ("sched/rt: Make RT capacity-aware")
Link: https://lkml.kernel.org/r/20200302132721.8353-3-qais.yousef@arm.com
This commit is contained in:
Qais Yousef 2020-03-02 13:27:17 +00:00 committed by Ingo Molnar
parent d9cb236b94
commit b28bc1e002
1 changed files with 9 additions and 0 deletions
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9
kernel/sched/rt.c
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@ -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 && 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.
@ -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: