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

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Date:	Wed, 14 Oct 2009 08:37:18 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	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,
	tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.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

Paul E. McKenney wrote:
>> It's an old issue.
>> It's not only about RCUTINY, it's also about other rcu implementations:
>>
>> rcu_enter_nohz()/rcu_exit_nohz() are not called in pairs.
>>
>> irq_exit() calls tick_nohz_stop_sched_tick() which calls rcu_enter_nohz(),
>> where is the corresponding rcu_exit_nohz()?
>> (or tick_nohz_restart_sched_tick())?
> 
> The tick_nohz_restart_sched_tick() function is called from the various
> per-architecture cpu_idle() functions (or default_idle() or whatever
> name that the architecture uses).  For example, in:
> 
> 	arch/x86/kernel/process_64.c
> 
> the cpu_idle() function invokes tick_nohz_restart_sched_tick() just
> before invoking schedule() to exit the idle loop.
> 
> And, as you say, tick_nohz_restart_sched_tick() invokes rcu_exit_nohz().
> 
> 							Thanx, Paul
> 
> 

These tick_nohz_restart_sched_tick() which are called from the various
per-architecture cpu_idle() functions are not the opposite of
the tick_nohz_stop_sched_tick() in *irq_exit()*. So I figure that 
rcu_enter_nohz()/rcu_exit_nohz() are not called in pairs.
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