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Date: Sun, 2 Oct 2011 13:45:58 +0200 From: Frederic Weisbecker <fweisbec@...il.com> To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> Cc: "Kirill A. Shutemov" <kirill@...temov.name>, linux-kernel@...r.kernel.org, Dipankar Sarma <dipankar@...ibm.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, Lai Jiangshan <laijs@...fujitsu.com> Subject: Re: linux-next-20110923: warning kernel/rcutree.c:1833 On Sat, Oct 01, 2011 at 08:23:16PM -0700, Paul E. McKenney wrote: > On Sat, Oct 01, 2011 at 10:07:14AM -0700, Paul E. McKenney wrote: > > On Sat, Oct 01, 2011 at 02:24:45PM +0200, Frederic Weisbecker wrote: > > > On Fri, Sep 30, 2011 at 12:24:38PM -0700, Paul E. McKenney wrote: > > > > And here is a first cut, probably totally broken, but a start. > > > > > > > > With this change, I am wondering about tick_nohz_stop_sched_tick()'s > > > > invocation of rcu_idle_enter() -- this now needs to be called regardless > > > > of whether or not tick_nohz_stop_sched_tick() actually stops the tick. > > > > Except that if tick_nohz_stop_sched_tick() is invoked with inidle==0, > > > > it looks like we should -not- call rcu_idle_enter(). > > > > > > Because of the new check in rcu_check_callbacks()? Yeah. > > > > > > If you think it's fine to call rcu_enter_nohz() unconditionally > > > everytime we enter the idle loop then yeah. I just don't know > > > the overhead it adds, as it adds an unconditional tiny piece of > > > code before we can finally save the power. > > > > > > Either entering idle involves extended quiescent state as in this > > > patch, or you separate both and then rcu_enter_nohz() is only > > > called when the tick is stopped. > > > > > > If you choose to merge both, you indeed need to call rcu_idle_enter() > > > and rcu_idle_exit() whether the tick is stopped or not. > > > > > > > I eventually just left the rcu_idle_enter() calls in their current > > > > places due to paranoia about messing up and ending up with unbalanced > > > > rcu_idle_enter() and rcu_idle_exit() calls. Any thoughts on how to > > > > make this work better? > > > > > > Yeah something like this (untested): > > > > > > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c > > > index d5097c4..ad3ecad 100644 > > > --- a/kernel/time/tick-sched.c > > > +++ b/kernel/time/tick-sched.c > > > @@ -273,9 +273,12 @@ void tick_nohz_stop_sched_tick(int inidle) > > > * updated. Thus, it must not be called in the event we are called from > > > * irq_exit() with the prior state different than idle. > > > */ > > > - if (!inidle && !ts->inidle) > > > + if (inidle) > > > + rcu_idle_enter(); > > > + else if (!ts->inidle) > > > goto end; > > > > > > + > > > /* > > > * Set ts->inidle unconditionally. Even if the system did not > > > * switch to NOHZ mode the cpu frequency governers rely on the > > > @@ -409,7 +412,6 @@ void tick_nohz_stop_sched_tick(int inidle) > > > ts->idle_tick = hrtimer_get_expires(&ts->sched_timer); > > > ts->tick_stopped = 1; > > > ts->idle_jiffies = last_jiffies; > > > - rcu_enter_nohz(); > > > } > > > > > > ts->idle_sleeps++; > > > @@ -505,6 +507,9 @@ void tick_nohz_restart_sched_tick(void) > > > ktime_t now; > > > > > > local_irq_disable(); > > > + > > > + rcu_idle_exit(); > > > + > > > if (ts->idle_active || (ts->inidle && ts->tick_stopped)) > > > now = ktime_get(); > > > > > > @@ -519,8 +524,6 @@ void tick_nohz_restart_sched_tick(void) > > > > > > ts->inidle = 0; > > > > > > - rcu_exit_nohz(); > > > - > > > /* Update jiffies first */ > > > select_nohz_load_balancer(0); > > > tick_do_update_jiffies64(now); > > > > > > > > > > > > > > > More things about your patch below: > > > > > > > --- a/kernel/rcutiny.c > > > > +++ b/kernel/rcutiny.c > > > > @@ -54,31 +54,47 @@ static void __call_rcu(struct rcu_head *head, > > > > > > > > #include "rcutiny_plugin.h" > > > > > > > > -#ifdef CONFIG_NO_HZ > > > > - > > > > static long rcu_dynticks_nesting = 1; > > > > > > > > /* > > > > - * Enter dynticks-idle mode, which is an extended quiescent state > > > > - * if we have fully entered that mode (i.e., if the new value of > > > > - * dynticks_nesting is zero). > > > > + * Enter idle, which is an extended quiescent state if we have fully > > > > + * entered that mode (i.e., if the new value of dynticks_nesting is zero). > > > > */ > > > > -void rcu_enter_nohz(void) > > > > +void rcu_idle_enter(void) > > > > { > > > > if (--rcu_dynticks_nesting == 0) > > > > rcu_sched_qs(0); /* implies rcu_bh_qsctr_inc(0) */ > > > > } > > > > > > > > /* > > > > - * Exit dynticks-idle mode, so that we are no longer in an extended > > > > - * quiescent state. > > > > + * Exit idle, so that we are no longer in an extended quiescent state. > > > > */ > > > > -void rcu_exit_nohz(void) > > > > +void rcu_idle_exit(void) > > > > { > > > > rcu_dynticks_nesting++; > > > > } > > > > > > > > -#endif /* #ifdef CONFIG_NO_HZ */ > > > > +#ifdef CONFIG_PROVE_RCU > > > > + > > > > +/* > > > > + * Test whether the current CPU is idle. > > > > + */ > > > > > > Is idle from an RCU point of view yeah. > > > > Good point -- I now say "Test whether RCU thinks that the current CPU is idle." > > > > > > +int rcu_is_cpu_idle(void) > > > > +{ > > > > + return !rcu_dynticks_nesting; > > > > +} > > > > + > > > > +#endif /* #ifdef CONFIG_PROVE_RCU */ > > > > + > > > > +/* > > > > + * Test whether the current CPU was interrupted from idle. Nested > > > > + * interrupts don't count, we must be running at the first interrupt > > > > + * level. > > > > + */ > > > > +int rcu_is_cpu_rrupt_from_idle(void) > > > > +{ > > > > + return rcu_dynticks_nesting <= 0; > > > > +} > > > > > > > > /* > > > > * Helper function for rcu_sched_qs() and rcu_bh_qs(). > > > > @@ -131,10 +147,7 @@ void rcu_bh_qs(int cpu) > > > > */ > > > > void rcu_check_callbacks(int cpu, int user) > > > > { > > > > - if (user || > > > > - (idle_cpu(cpu) && > > > > - !in_softirq() && > > > > - hardirq_count() <= (1 << HARDIRQ_SHIFT))) > > > > + if (user || rcu_is_cpu_rrupt_from_idle()) > > > > rcu_sched_qs(cpu); > > > > > > It wasn't obvious to me in the first shot. This might need a comment > > > that tells rcu_check_callbacks() is called from an interrupt > > > and thus need to handle that first level in the check. > > > > OK, I added "This function must be called from hardirq context". > > > > > Other than that, looks good overall. > > > > Keeping fingers firmly crossed for the testing... > > And it appears sane in testing thus far. I have consolidated to one > patch and pushed to https://github.com/paulmckrcu/linux branch rcu/dev. > > Testing continues. Great. I'll start the rebase then. Thanks. -- 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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