linux-kernel - Re: [PATCH 1/2] Fix stop_machine_run problem with naughty real time process

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Date:	Fri, 11 May 2007 18:18:13 +0900
From:	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Zwane Mwaikambo <zwane@....linux.org.uk>,
	Nathan Lynch <nathanl@...tin.ibm.com>,
	Joel Schopp <jschopp@...tin.ibm.com>,
	Ashok Raj <ashok.raj@...el.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>
Subject: Re: [PATCH 1/2] Fix stop_machine_run problem with naughty real time process

At Fri, 11 May 2007 17:49:20 +0900,
Satoru Takeuchi wrote:
> 
> Hi,
> 
> I wrote patches which fixes the problem regarding stop_machine_run() and
> cpu hotplug.

Sorry, there were extra tabs. Fixed.

Thanks,

Satoru

---
Fix stop_machine_run() problem with naughty real time process

stop_machine_run() does its work on "kstopmachine" thread having max priority.
However that thread get such priority after woken up. Therefore, in the
following case ...

  - "kstopmachine" try to run on CPU1
  - There is a real time process which doesn't relinquish CPU time voluntary on CPU1

... "kstopmachine" can't start to run and the CPU on which stop_machine_run() is runing
hangs up. To fix this problem, call sched_setscheduler() before waking up that thread.

Signed-off-by: Satoru Takeuchi <takeuchi_satoru@...fujitsu.com>

Index: linux-2.6.21/kernel/stop_machine.c
===================================================================
--- linux-2.6.21.orig/kernel/stop_machine.c	2007-05-11 13:45:34.000000000 +0900
+++ linux-2.6.21/kernel/stop_machine.c	2007-05-11 14:49:17.000000000 +0900
@@ -89,10 +89,6 @@ static void stopmachine_set_state(enum s
 static int stop_machine(void)
 {
 	int i, ret = 0;
-	struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
-
-	/* One high-prio thread per cpu.  We'll do this one. */
-	sched_setscheduler(current, SCHED_FIFO, &param);
 
 	atomic_set(&stopmachine_thread_ack, 0);
 	stopmachine_num_threads = 0;
@@ -184,6 +180,10 @@ struct task_struct *__stop_machine_run(i
 
 	p = kthread_create(do_stop, &smdata, "kstopmachine");
 	if (!IS_ERR(p)) {
+		struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
+
+		/* One high-prio thread per cpu.  We'll do this one. */
+		sched_setscheduler(p, SCHED_FIFO, &param);
 		kthread_bind(p, cpu);
 		wake_up_process(p);
 		wait_for_completion(&smdata.done);
-
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