linux-kernel - Re: [PATCH RFC tip/core/rcu 1/3] rcu: The Bloatwatch Edition, v7

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Date:	Tue, 27 Oct 2009 15:56:16 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	tglx@...utronix.de, paulmck@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, mingo@...e.hu, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, dvhltc@...ibm.com, niv@...ibm.com,
	peterz@...radead.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	avi@...hat.com, mtosatti@...hat.com, torvalds@...ux-foundation.org
Subject: Re: [PATCH RFC tip/core/rcu 1/3] rcu: The Bloatwatch Edition, v7

On Tue, 2009-10-27 at 15:26 +0800, Lai Jiangshan wrote:
> Steven Rostedt wrote:

> I found something weird about NO_HZ, maybe I misunderstood the codes.
> 
> see this flow:
> 
> cpu idle
>   enter nohz
>   cpu halt
> ---->interrupt happens
>      irq_enter()
>         we don't reprogram the clock device                         #1
>      irq_exit()
>        tick_nohz_stop_sched_tick(inidle = 0)
>          something disallow this cpu reenter nohz                   #2
>          we don't reprogram the clock device                        #3
> <----interrupt return
>    cpu halt again and wait interrupt for a long time than expected  #4
>    exit nohz
> 
> 
> #1 tick_nohz_kick_tick() is disabled in the current mainline kernel,
>    so we don't calls tick_nohz_restart(ts, now) when irq_enter()
> 
> static void tick_nohz_kick_tick(int cpu)
> {
> #if 0                                            <------------- here
> 	/* Switch back to 2.6.27 behaviour */
> 
> 	struct tick_sched *ts = &per_cpu(tick_cpu_sched, cpu);
> 	ktime_t delta, now;
> 
> 	if (!ts->tick_stopped)
> 		return;
> 
> 	/*
> 	 * Do not touch the tick device, when the next expiry is either
> 	 * already reached or less/equal than the tick period.
> 	 */
> 	now = ktime_get();
> 	delta =	ktime_sub(hrtimer_get_expires(&ts->sched_timer), now);
> 	if (delta.tv64 <= tick_period.tv64)
> 		return;
> 
> 	tick_nohz_restart(ts, now);               <----------- here
> #endif
> }
> 
> 
> #2 When rcu_needs_cpu() or  printk_needs_cpu()
>    returns true then tick_nohz_stop_sched_tick() will just return.
> 
> #3 And we don't reprogram the clock device when #2 happens
> 
> #4 So we may be in nohz for a long time than expected, but actually
>     we have some work to do. (rcu, printk... etc)
> 
> So I think, we need to reprogram the clock device and restart the tick
> when #2 happens, or there is something that I have misunderstood.


This looks like a different question than you asked before. And
something I don't know the answer to ;-)

-- Steve


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