linux-kernel - Re: [PATCH] sched: do not stop ticks when cpu is not idle

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Date:	Fri, 18 Jul 2008 12:24:46 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	eric miao <eric.y.miao@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Jack Ren <jack.ren@...vell.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>
Subject: Re: [PATCH] sched: do not stop ticks when cpu is not idle


* eric miao <eric.y.miao@...il.com> wrote:

> Issue: the sched tick would be stopped in some race conditions.

> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -4027,7 +4027,8 @@ need_resched_nonpreemptible:
>  		rq->nr_switches++;
>  		rq->curr = next;
>  		++*switch_count;
> -
> +		if (rq->curr != rq->idle)
> +			tick_nohz_restart_sched_tick();
>  		context_switch(rq, prev, next); /* unlocks the rq */

applied to tip/sched/urgent, thanks Eric.

Thomas, Peter, Dmitry, do you concur with the analysis? (commit below) 

It looks a bit ugly to me in the middle of schedule() - is there no wait 
to solve this within kernel/time/*.c ?

	Ingo

-------------->
commit ca1b5a8a9abb3db57562a838f41cdba842f13fe8
Author: eric miao <eric.y.miao@...il.com>
Date:   Fri Jul 18 14:41:29 2008 +0800

    sched: do not stop ticks when cpu is not idle
    
    Issue: the sched tick would be stopped in some race conditions.
    
    One of issues caused by that is:
    
    Since there is no timer ticks any more from then, the jiffies update will be
    up to other interrupt to happen. The jiffies will not be updated for a long
    time, until next interrupt happens.  That will cause APIs like
    wait_for_completion_timeout(&complete, timeout) to return timeout by mistake,
    since it is using a old jiffies as start time.
    
    Please see comments (1)~(6) inline for how the ticks are stopped
    by mistake when cpu is not idle:
    
    void cpu_idle(void)
    {
    ...
    	while (1) {
    		void (*idle)(void) = pm_idle;
    		if (!idle)
    			idle = default_idle;
    		leds_event(led_idle_start);
    		tick_nohz_stop_sched_tick();
    		while (!need_resched())
    			idle();
    		leds_event(led_idle_end);
    		tick_nohz_restart_sched_tick();
    			(1) ticks are retarted before switch to other tasks
    		preempt_enable_no_resched();
    		schedule();
    		preempt_disable();
    	}
    }
    
    asmlinkage void __sched schedule(void)
    {
    	...
    	...
    need_resched:
    	(6) the idle task will be scheduled out again and switch to next task,
    	 with ticks stopped in (5). So the next task will be running with tick stopped.
    	preempt_disable();
    	cpu = smp_processor_id();
    	rq = cpu_rq(cpu);
    	rcu_qsctr_inc(cpu);
    	prev = rq->curr;
    	switch_count = &prev->nivcsw;
    
    	release_kernel_lock(prev);
    need_resched_nonpreemptible:
    
    	schedule_debug(prev);
    
    	hrtick_clear(rq);
    
    	/*
    	 * Do the rq-clock update outside the rq lock:
    	 */
    	local_irq_disable();
    	__update_rq_clock(rq);
    	spin_lock(&rq->lock);
    	clear_tsk_need_resched(prev); (2) resched flag is clear from idle task
    
    	....
    
    		context_switch(rq, prev, next); /* unlocks the rq */
    		(3) IRQ will be enabled at end of context_swtich( ).
    	...
    	preempt_enable_no_resched();
    	if (unlikely(test_thread_flag(TIF_NEED_RESCHED)))
    			(4) the idle task is scheduled back. If an interrupt happen here,
    				 The irq_exit( ) will be called at end of the irq handler.
    		goto need_resched;
    }
    
    void irq_exit(void)
    {
    ...
    	/* Make sure that timer wheel updates are propagated */
    	if (!in_interrupt() && idle_cpu(smp_processor_id()) && !need_resched())
    		tick_nohz_stop_sched_tick();
    		(5) The ticks will be stopped again since current
    			task is idle task and its resched flag is clear in (2).
    	rcu_irq_exit();
    	preempt_enable_no_resched();
    }
    
    Signed-off-by: Jack Ren <jack.ren@...vell.com>
    Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 kernel/sched.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index 1ee18db..e0e0162 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4446,7 +4446,8 @@ need_resched_nonpreemptible:
 		rq->nr_switches++;
 		rq->curr = next;
 		++*switch_count;
-
+		if (rq->curr != rq->idle)
+			tick_nohz_restart_sched_tick();
 		context_switch(rq, prev, next); /* unlocks the rq */
 		/*
 		 * the context switch might have flipped the stack from under
--
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