linux-kernel - [PATCH] sched: fix RLIMIT_RTTIME when PI-boosting to RT

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Message-Id: <1424305436-6716-1-git-send-email-brian@peloton-tech.com>
Date:	Wed, 18 Feb 2015 16:23:56 -0800
From:	brian@...oton-tech.com
To:	mingo@...hat.com, peterz@...radead.org
Cc:	austin@...oton-tech.com, linux-kernel@...r.kernel.org,
	brian@...oton-tech.com
Subject: [PATCH] sched: fix RLIMIT_RTTIME when PI-boosting to RT

From: Brian Silverman <brian@...oton-tech.com>

When non-realtime tasks get priority-inheritance boosted to a realtime
scheduling class, RLIMIT_RTTIME starts to apply to them. However, the
counter used for checking this (the same one used for SCHED_RR
timeslices) was not getting reset. This meant that tasks running with a
non-realtime scheduling class which are repeatedly boosted to a realtime
one, but never block while they are running realtime, eventually hit the
timeout without ever running for a time over the limit. This patch
resets the realtime timeslice counter when un-PI-boosting from an RT to
a non-RT scheduling class.

I have some test code with two threads and a shared PTHREAD_PRIO_INHERIT
mutex which induces priority boosting and spins while boosted that gets
killed by a SIGXCPU on non-fixed kernels but doesn't with this patch
applied. It happens much faster with a CONFIG_PREEMPT_RT kernel, and
does happen eventually with PREEMPT_VOLUNTARY kernels.

Signed-off-by: Brian Silverman <brian@...oton-tech.com>
---
I am not subscribed to the list so please CC me on any responses.

 kernel/sched/core.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 87b9814..16ad0ed 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3192,6 +3192,8 @@ void rt_mutex_setprio(struct task_struct *p, int prio)
 	} else {
 		if (dl_prio(oldprio))
 			p->dl.dl_boosted = 0;
+		if (rt_prio(oldprio))
+			p->rt.timeout = 0;
 		p->sched_class = &fair_sched_class;
 	}

-- 
1.7.10.4

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