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From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	akpm@osdl.org, Ingo Molnar <mingo@elte.hu>,
	Thomas Gleixner <tglx@linutronix.de>,
	john stultz <johnstul@us.ibm.com>
Subject: Re: [PATCH] add support for dynamic ticks and preempt rcu
Date: Tue, 29 Jan 2008 21:21:57 -0800	[thread overview]
Message-ID: <20080130052157.GJ12073@linux.vnet.ibm.com> (raw)
In-Reply-To: <Pine.LNX.4.58.0801291113350.20371@gandalf.stny.rr.com>

On Tue, Jan 29, 2008 at 11:18:12AM -0500, Steven Rostedt wrote:
> 
> [
>  Paul, you had your Signed-off-by in the RT patch, so I attached it here
>   too
> ]

Works for me!!!

> The PREEMPT-RCU can get stuck if a CPU goes idle and NO_HZ is set. The
> idle CPU will not progress the RCU through its grace period and a
> synchronize_rcu my get stuck. Without this patch I have a box that will
> not boot when PREEMPT_RCU and NO_HZ are set. That same box boots fine with
> this patch.
> 
> Note: This patch came directly from the -rt patch where it has been tested
> for several months.

For those who attended my lightening talk yesterday on changing RCU to
"let sleeping CPUs lie", this is the patch.

If your architecture calls rcu_irq_enter() or irq_enter() upon
NMI/SMI/MC/whatever handler entry and also calls rcu_irq_exit() or
irq_exit() upon NMI/SMI/MC/whatever handler exit, you are covered.

Alternatively, if none of your architecture's NMI/SMI/MC/whatever
handlers never invoke rcu_read_lock()/rcu_read_unlock() and friends,
you are also covered.

I believe that we are covered, but I cannot claim to fully understand
all 20+ architectures.  ;-)

						Thanx, Paul

> Signed-off-by: Steven Rostedt <srostedt@redhat.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
>  include/linux/hardirq.h    |   10 ++
>  include/linux/rcuclassic.h |    3
>  include/linux/rcupreempt.h |   22 ++++
>  kernel/rcupreempt.c        |  224 ++++++++++++++++++++++++++++++++++++++++++++-
>  kernel/softirq.c           |    1
>  kernel/time/tick-sched.c   |    3
>  6 files changed, 259 insertions(+), 4 deletions(-)
> 
> Index: linux-compile.git/kernel/rcupreempt.c
> ===================================================================
> --- linux-compile.git.orig/kernel/rcupreempt.c	2008-01-29 11:03:21.000000000 -0500
> +++ linux-compile.git/kernel/rcupreempt.c	2008-01-29 11:10:08.000000000 -0500
> @@ -23,6 +23,10 @@
>   *		to Suparna Bhattacharya for pushing me completely away
>   *		from atomic instructions on the read side.
>   *
> + *  - Added handling of Dynamic Ticks
> + *      Copyright 2007 - Paul E. Mckenney <paulmck@us.ibm.com>
> + *                     - Steven Rostedt <srostedt@redhat.com>
> + *
>   * Papers:  http://www.rdrop.com/users/paulmck/RCU
>   *
>   * Design Document: http://lwn.net/Articles/253651/
> @@ -409,6 +413,212 @@ static void __rcu_advance_callbacks(stru
>  	}
>  }
> 
> +#ifdef CONFIG_NO_HZ
> +
> +DEFINE_PER_CPU(long, dynticks_progress_counter) = 1;
> +static DEFINE_PER_CPU(long, rcu_dyntick_snapshot);
> +static DEFINE_PER_CPU(int, rcu_update_flag);
> +
> +/**
> + * rcu_irq_enter - Called from Hard irq handlers and NMI/SMI.
> + *
> + * If the CPU was idle with dynamic ticks active, this updates the
> + * dynticks_progress_counter to let the RCU handling know that the
> + * CPU is active.
> + */
> +void rcu_irq_enter(void)
> +{
> +	int cpu = smp_processor_id();
> +
> +	if (per_cpu(rcu_update_flag, cpu))
> +		per_cpu(rcu_update_flag, cpu)++;
> +
> +	/*
> +	 * Only update if we are coming from a stopped ticks mode
> +	 * (dynticks_progress_counter is even).
> +	 */
> +	if (!in_interrupt() &&
> +	    (per_cpu(dynticks_progress_counter, cpu) & 0x1) == 0) {
> +		/*
> +		 * The following might seem like we could have a race
> +		 * with NMI/SMIs. But this really isn't a problem.
> +		 * Here we do a read/modify/write, and the race happens
> +		 * when an NMI/SMI comes in after the read and before
> +		 * the write. But NMI/SMIs will increment this counter
> +		 * twice before returning, so the zero bit will not
> +		 * be corrupted by the NMI/SMI which is the most important
> +		 * part.
> +		 *
> +		 * The only thing is that we would bring back the counter
> +		 * to a postion that it was in during the NMI/SMI.
> +		 * But the zero bit would be set, so the rest of the
> +		 * counter would again be ignored.
> +		 *
> +		 * On return from the IRQ, the counter may have the zero
> +		 * bit be 0 and the counter the same as the return from
> +		 * the NMI/SMI. If the state machine was so unlucky to
> +		 * see that, it still doesn't matter, since all
> +		 * RCU read-side critical sections on this CPU would
> +		 * have already completed.
> +		 */
> +		per_cpu(dynticks_progress_counter, cpu)++;
> +		/*
> +		 * The following memory barrier ensures that any
> +		 * rcu_read_lock() primitives in the irq handler
> +		 * are seen by other CPUs to follow the above
> +		 * increment to dynticks_progress_counter. This is
> +		 * required in order for other CPUs to correctly
> +		 * determine when it is safe to advance the RCU
> +		 * grace-period state machine.
> +		 */
> +		smp_mb(); /* see above block comment. */
> +		/*
> +		 * Since we can't determine the dynamic tick mode from
> +		 * the dynticks_progress_counter after this routine,
> +		 * we use a second flag to acknowledge that we came
> +		 * from an idle state with ticks stopped.
> +		 */
> +		per_cpu(rcu_update_flag, cpu)++;
> +		/*
> +		 * If we take an NMI/SMI now, they will also increment
> +		 * the rcu_update_flag, and will not update the
> +		 * dynticks_progress_counter on exit. That is for
> +		 * this IRQ to do.
> +		 */
> +	}
> +}
> +
> +/**
> + * rcu_irq_exit - Called from exiting Hard irq context.
> + *
> + * If the CPU was idle with dynamic ticks active, update the
> + * dynticks_progress_counter to put let the RCU handling be
> + * aware that the CPU is going back to idle with no ticks.
> + */
> +void rcu_irq_exit(void)
> +{
> +	int cpu = smp_processor_id();
> +
> +	/*
> +	 * rcu_update_flag is set if we interrupted the CPU
> +	 * when it was idle with ticks stopped.
> +	 * Once this occurs, we keep track of interrupt nesting
> +	 * because a NMI/SMI could also come in, and we still
> +	 * only want the IRQ that started the increment of the
> +	 * dynticks_progress_counter to be the one that modifies
> +	 * it on exit.
> +	 */
> +	if (per_cpu(rcu_update_flag, cpu)) {
> +		if (--per_cpu(rcu_update_flag, cpu))
> +			return;
> +
> +		/* This must match the interrupt nesting */
> +		WARN_ON(in_interrupt());
> +
> +		/*
> +		 * If an NMI/SMI happens now we are still
> +		 * protected by the dynticks_progress_counter being odd.
> +		 */
> +
> +		/*
> +		 * The following memory barrier ensures that any
> +		 * rcu_read_unlock() primitives in the irq handler
> +		 * are seen by other CPUs to preceed the following
> +		 * increment to dynticks_progress_counter. This
> +		 * is required in order for other CPUs to determine
> +		 * when it is safe to advance the RCU grace-period
> +		 * state machine.
> +		 */
> +		smp_mb(); /* see above block comment. */
> +		per_cpu(dynticks_progress_counter, cpu)++;
> +		WARN_ON(per_cpu(dynticks_progress_counter, cpu) & 0x1);
> +	}
> +}
> +
> +static void dyntick_save_progress_counter(int cpu)
> +{
> +	per_cpu(rcu_dyntick_snapshot, cpu) =
> +		per_cpu(dynticks_progress_counter, cpu);
> +}
> +
> +static inline int
> +rcu_try_flip_waitack_needed(int cpu)
> +{
> +	long curr;
> +	long snap;
> +
> +	curr = per_cpu(dynticks_progress_counter, cpu);
> +	snap = per_cpu(rcu_dyntick_snapshot, cpu);
> +	smp_mb(); /* force ordering with cpu entering/leaving dynticks. */
> +
> +	/*
> +	 * If the CPU remained in dynticks mode for the entire time
> +	 * and didn't take any interrupts, NMIs, SMIs, or whatever,
> +	 * then it cannot be in the middle of an rcu_read_lock(), so
> +	 * the next rcu_read_lock() it executes must use the new value
> +	 * of the counter.  So we can safely pretend that this CPU
> +	 * already acknowledged the counter.
> +	 */
> +
> +	if ((curr == snap) && ((curr & 0x1) == 0))
> +		return 0;
> +
> +	/*
> +	 * If the CPU passed through or entered a dynticks idle phase with
> +	 * no active irq handlers, then, as above, we can safely pretend
> +	 * that this CPU already acknowledged the counter.
> +	 */
> +
> +	if ((curr - snap) > 2 || (snap & 0x1) == 0)
> +		return 0;
> +
> +	/* We need this CPU to explicitly acknowledge the counter flip. */
> +
> +	return 1;
> +}
> +
> +static inline int
> +rcu_try_flip_waitmb_needed(int cpu)
> +{
> +	long curr;
> +	long snap;
> +
> +	curr = per_cpu(dynticks_progress_counter, cpu);
> +	snap = per_cpu(rcu_dyntick_snapshot, cpu);
> +	smp_mb(); /* force ordering with cpu entering/leaving dynticks. */
> +
> +	/*
> +	 * If the CPU remained in dynticks mode for the entire time
> +	 * and didn't take any interrupts, NMIs, SMIs, or whatever,
> +	 * then it cannot have executed an RCU read-side critical section
> +	 * during that time, so there is no need for it to execute a
> +	 * memory barrier.
> +	 */
> +
> +	if ((curr == snap) && ((curr & 0x1) == 0))
> +		return 0;
> +
> +	/*
> +	 * If the CPU either entered or exited an outermost interrupt,
> +	 * SMI, NMI, or whatever handler, then we know that it executed
> +	 * a memory barrier when doing so.  So we don't need another one.
> +	 */
> +	if (curr != snap)
> +		return 0;
> +
> +	/* We need the CPU to execute a memory barrier. */
> +
> +	return 1;
> +}
> +
> +#else /* !CONFIG_NO_HZ */
> +
> +# define dyntick_save_progress_counter(cpu)	do { } while (0)
> +# define rcu_try_flip_waitack_needed(cpu)	(1)
> +# define rcu_try_flip_waitmb_needed(cpu)	(1)
> +
> +#endif /* CONFIG_NO_HZ */
> +
>  /*
>   * Get here when RCU is idle.  Decide whether we need to
>   * move out of idle state, and return non-zero if so.
> @@ -447,8 +657,10 @@ rcu_try_flip_idle(void)
> 
>  	/* Now ask each CPU for acknowledgement of the flip. */
> 
> -	for_each_cpu_mask(cpu, rcu_cpu_online_map)
> +	for_each_cpu_mask(cpu, rcu_cpu_online_map) {
>  		per_cpu(rcu_flip_flag, cpu) = rcu_flipped;
> +		dyntick_save_progress_counter(cpu);
> +	}
> 
>  	return 1;
>  }
> @@ -464,7 +676,8 @@ rcu_try_flip_waitack(void)
> 
>  	RCU_TRACE_ME(rcupreempt_trace_try_flip_a1);
>  	for_each_cpu_mask(cpu, rcu_cpu_online_map)
> -		if (per_cpu(rcu_flip_flag, cpu) != rcu_flip_seen) {
> +		if (rcu_try_flip_waitack_needed(cpu) &&
> +		    per_cpu(rcu_flip_flag, cpu) != rcu_flip_seen) {
>  			RCU_TRACE_ME(rcupreempt_trace_try_flip_ae1);
>  			return 0;
>  		}
> @@ -509,8 +722,10 @@ rcu_try_flip_waitzero(void)
>  	smp_mb();  /*  ^^^^^^^^^^^^ */
> 
>  	/* Call for a memory barrier from each CPU. */
> -	for_each_cpu_mask(cpu, rcu_cpu_online_map)
> +	for_each_cpu_mask(cpu, rcu_cpu_online_map) {
>  		per_cpu(rcu_mb_flag, cpu) = rcu_mb_needed;
> +		dyntick_save_progress_counter(cpu);
> +	}
> 
>  	RCU_TRACE_ME(rcupreempt_trace_try_flip_z2);
>  	return 1;
> @@ -528,7 +743,8 @@ rcu_try_flip_waitmb(void)
> 
>  	RCU_TRACE_ME(rcupreempt_trace_try_flip_m1);
>  	for_each_cpu_mask(cpu, rcu_cpu_online_map)
> -		if (per_cpu(rcu_mb_flag, cpu) != rcu_mb_done) {
> +		if (rcu_try_flip_waitmb_needed(cpu) &&
> +		    per_cpu(rcu_mb_flag, cpu) != rcu_mb_done) {
>  			RCU_TRACE_ME(rcupreempt_trace_try_flip_me1);
>  			return 0;
>  		}
> Index: linux-compile.git/include/linux/hardirq.h
> ===================================================================
> --- linux-compile.git.orig/include/linux/hardirq.h	2008-01-29 11:03:15.000000000 -0500
> +++ linux-compile.git/include/linux/hardirq.h	2008-01-29 11:04:55.000000000 -0500
> @@ -109,6 +109,14 @@ static inline void account_system_vtime(
>  }
>  #endif
> 
> +#if defined(CONFIG_PREEMPT_RCU) && defined(CONFIG_NO_HZ)
> +extern void rcu_irq_enter(void);
> +extern void rcu_irq_exit(void);
> +#else
> +# define rcu_irq_enter() do { } while (0)
> +# define rcu_irq_exit() do { } while (0)
> +#endif /* CONFIG_PREEMPT_RCU */
> +
>  /*
>   * It is safe to do non-atomic ops on ->hardirq_context,
>   * because NMI handlers may not preempt and the ops are
> @@ -117,6 +125,7 @@ static inline void account_system_vtime(
>   */
>  #define __irq_enter()					\
>  	do {						\
> +		rcu_irq_enter();			\
>  		account_system_vtime(current);		\
>  		add_preempt_count(HARDIRQ_OFFSET);	\
>  		trace_hardirq_enter();			\
> @@ -135,6 +144,7 @@ extern void irq_enter(void);
>  		trace_hardirq_exit();			\
>  		account_system_vtime(current);		\
>  		sub_preempt_count(HARDIRQ_OFFSET);	\
> +		rcu_irq_exit();				\
>  	} while (0)
> 
>  /*
> Index: linux-compile.git/include/linux/rcuclassic.h
> ===================================================================
> --- linux-compile.git.orig/include/linux/rcuclassic.h	2008-01-29 11:03:15.000000000 -0500
> +++ linux-compile.git/include/linux/rcuclassic.h	2008-01-29 11:04:55.000000000 -0500
> @@ -160,5 +160,8 @@ extern void rcu_restart_cpu(int cpu);
>  extern long rcu_batches_completed(void);
>  extern long rcu_batches_completed_bh(void);
> 
> +#define rcu_enter_nohz()	do { } while (0)
> +#define rcu_exit_nohz()		do { } while (0)
> +
>  #endif /* __KERNEL__ */
>  #endif /* __LINUX_RCUCLASSIC_H */
> Index: linux-compile.git/include/linux/rcupreempt.h
> ===================================================================
> --- linux-compile.git.orig/include/linux/rcupreempt.h	2008-01-29 11:03:15.000000000 -0500
> +++ linux-compile.git/include/linux/rcupreempt.h	2008-01-29 11:04:55.000000000 -0500
> @@ -82,5 +82,27 @@ extern struct rcupreempt_trace *rcupreem
> 
>  struct softirq_action;
> 
> +#ifdef CONFIG_NO_HZ
> +DECLARE_PER_CPU(long, dynticks_progress_counter);
> +
> +static inline void rcu_enter_nohz(void)
> +{
> +	__get_cpu_var(dynticks_progress_counter)++;
> +	WARN_ON(__get_cpu_var(dynticks_progress_counter) & 0x1);
> +	mb();
> +}
> +
> +static inline void rcu_exit_nohz(void)
> +{
> +	mb();
> +	__get_cpu_var(dynticks_progress_counter)++;
> +	WARN_ON(!(__get_cpu_var(dynticks_progress_counter) & 0x1));
> +}
> +
> +#else /* CONFIG_NO_HZ */
> +#define rcu_enter_nohz()	do { } while (0)
> +#define rcu_exit_nohz()		do { } while (0)
> +#endif /* CONFIG_NO_HZ */
> +
>  #endif /* __KERNEL__ */
>  #endif /* __LINUX_RCUPREEMPT_H */
> Index: linux-compile.git/kernel/softirq.c
> ===================================================================
> --- linux-compile.git.orig/kernel/softirq.c	2008-01-29 11:03:15.000000000 -0500
> +++ linux-compile.git/kernel/softirq.c	2008-01-29 11:04:55.000000000 -0500
> @@ -306,6 +306,7 @@ 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();
> +	rcu_irq_exit();
>  #endif
>  	preempt_enable_no_resched();
>  }
> Index: linux-compile.git/kernel/time/tick-sched.c
> ===================================================================
> --- linux-compile.git.orig/kernel/time/tick-sched.c	2008-01-29 11:03:15.000000000 -0500
> +++ linux-compile.git/kernel/time/tick-sched.c	2008-01-29 11:04:55.000000000 -0500
> @@ -254,6 +254,7 @@ void tick_nohz_stop_sched_tick(void)
>  			ts->idle_tick = ts->sched_timer.expires;
>  			ts->tick_stopped = 1;
>  			ts->idle_jiffies = last_jiffies;
> +			rcu_enter_nohz();
>  		}
> 
>  		/*
> @@ -342,6 +343,8 @@ void tick_nohz_restart_sched_tick(void)
>  	if (!ts->tick_stopped)
>  		return;
> 
> +	rcu_exit_nohz();
> +
>  	/* Update jiffies first */
>  	now = ktime_get();
> 
> 

  reply	other threads:[~2008-01-30  5:22 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-01-29 16:18 Steven Rostedt
2008-01-30  5:21 ` Paul E. McKenney [this message]
2008-02-29  4:43 ` [PATCH] fix boot-time hangs from PREEMPT_RCU and NO_HZ Paul E. McKenney
2008-02-29  5:01   ` Andrew Morton
2008-02-29 15:48     ` Ingo Molnar
2008-02-29 16:55       ` Paul E. McKenney
2008-02-29 16:57         ` Ingo Molnar
2008-02-29 17:08           ` Steven Rostedt
2008-02-02  6:26 [PATCH] add support for dynamic ticks and preempt rcu Steven Rostedt
2008-02-02  9:01 ` Ingo Molnar
2008-02-02 15:17   ` Steven Rostedt

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