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Date: Tue, 13 Oct 2009 22:49:46 -0400 From: Steven Rostedt <rostedt@...dmis.org> To: Lai Jiangshan <laijs@...fujitsu.com> Cc: 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, tglx@...utronix.de, 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 Wed, 2009-10-14 at 10:05 +0800, Lai Jiangshan wrote: > Paul E. McKenney wrote: > You haven't explain the tick_nohz_stop_sched_tick() in *irq_exit()*. > (tick_nohz_stop_sched_tick() calls rcu_enter_nohz()) > > void irq_exit(void) > { > .... > rcu_irq_exit(); /* This is OK, the opposite is in irq_enter() */ > if (idle_cpu(smp_processor_id()) && !in_interrupt() && !need_resched()) > tick_nohz_stop_sched_tick(0); /* where is the opposite ??? */ > .... > } > > This means if the interrupt -is- returning to dyntick-idle mode, > rcu_enter_nohz() is called again. > > Take this flow as example: > > cpu_idle(): > while(1) { > > tick_nohz_stop_sched_tick() > rcu_enter_nohz() ***** > ------->interrupt happen > irq_enter() > irq_exit() > tick_nohz_stop_sched_tick() > rcu_enter_nohz() ***** > <-------interrupt returns > tick_nohz_restart_sched_tick() > rcu_exit_nohz() ***** > > } /* while(1) */ > > > You can see that rcu_enter_nohz() is called twice and > rcu_exit_nohz() is only called once in this flow. > > It's because tick_nohz_stop_sched_tick()/tick_nohz_restart_sched_tick() > are not called in pairs, so rcu_enter_nohz() and rcu_exit_nohz() > are not called in pairs either. But isn't the tick_nohz_stop_sched_tick have several exits? void tick_nohz_stop_sched_tick(int inidle) { [..] if (!inidle && !ts->inidle) goto end; ts->inidle = 1; [..] if (!ts->tick_stopped) { [..] ts->tick_stopped = 1; ts->idle_jiffies = last_jiffies; rcu_enter_nohz(); } [..] So I'm not sure calling tick_nohz_stop_sched_tick twice equals calling rcu_enter_nohz twice. -- Steve > > > > So I do believe that rcu_enter_nohz() and rcu_exit_nohz() are in fact > > invoked in pairs. One strange thing about this is that the idle loop > > first invokes rcu_enter_nohz(), then invokes rcu_exit_nohz(), while > > an interrupt handler first invokes rcu_irq_enter() and then invokes > > rcu_irq_exit(). So the idle loop enters dyntick-idle mode and then > > leaves it, while an interrupt handler might leave dyntick-idle mode and > > then re-enter it. > > > > Or am I still missing something here? > > > > Thanx, Paul > > > > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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