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Message-ID: <20081003050205.GA6340@linux.vnet.ibm.com>
Date: Thu, 2 Oct 2008 22:02:05 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
akpm@...ux-foundation.org, laijs@...fujitsu.com,
manfred@...orfullife.com, dipankar@...ibm.com, niv@...ibm.com,
dvhltc@...ibm.com, josht@...ux.vnet.ibm.com,
ltt-dev@...ts.casi.polymtl.ca
Subject: Re: [PATCH tip/master] RCU-based detection of stalled CPUs for
Classic RCU
On Fri, Oct 03, 2008 at 12:41:27AM -0400, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@...ux.vnet.ibm.com) wrote:
> > Hello!
> >
> > This patch adds stalled-CPU detection to Classic RCU. This capability
> > is enabled by a new config variable CONFIG_RCU_CPU_STALL_DETECTOR, which
> > defaults disabled. This is a debugging feature to detect infinite loops
> > in kernel code, not something that non-kernel-hackers would be expected
> > to care about. This feature can detect looping CPUs in !PREEMPT builds
> > and looping CPUs with preemption disabled in PREEMPT builds. This is
> > essentially a port of this functionality from the treercu patch, replacing
> > the stall debug patch that is already in tip/core/rcu (commit 67182ae1c4).
> >
> > The changes from the patch in tip/core/rcu include making the config
> > variable name match that in treercu, changing from seconds to jiffies to
> > avoid spurious warnings, and printing a boot message when this feature
> > is enabled.
> >
>
> Hi Paul,
>
> Thanks for the previous explanations. Out of curiosity, what can this
> patch do that the nmi watchdog can't do ?
Hello, Mathieu,
Operate on architectures that don't have NMIs. Possibly allow shorter
timeouts than the NMI watchdog. Operate on a per-CPU basis (or does
the NMI watchdog do that these days?).
Thanx, Paul
> Mathieu
>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > ---
> >
> > include/linux/rcuclassic.h | 12 ++-
> > kernel/rcuclassic.c | 166 +++++++++++++++++++++++----------------------
> > lib/Kconfig.debug | 2
> > 3 files changed, 96 insertions(+), 84 deletions(-)
> >
> > diff --git a/include/linux/rcuclassic.h b/include/linux/rcuclassic.h
> > index 29bf528..2d72d20 100644
> > --- a/include/linux/rcuclassic.h
> > +++ b/include/linux/rcuclassic.h
> > @@ -40,15 +40,21 @@
> > #include <linux/cpumask.h>
> > #include <linux/seqlock.h>
> >
> > +#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
> > +#define RCU_SECONDS_TILL_STALL_CHECK 3 * HZ /* for rcp->jiffies_stall */
> > +#define RCU_SECONDS_TILL_STALL_RECHECK 30 * HZ /* for rcp->jiffies_stall */
> > +#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
> >
> > /* Global control variables for rcupdate callback mechanism. */
> > struct rcu_ctrlblk {
> > long cur; /* Current batch number. */
> > long completed; /* Number of the last completed batch */
> > long pending; /* Number of the last pending batch */
> > -#ifdef CONFIG_DEBUG_RCU_STALL
> > - unsigned long gp_check; /* Time grace period should end, in seconds. */
> > -#endif /* #ifdef CONFIG_DEBUG_RCU_STALL */
> > +#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
> > + unsigned long gp_start; /* Time at which GP started in jiffies. */
> > + unsigned long jiffies_stall;
> > + /* Time at which to check for CPU stalls. */
> > +#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
> >
> > int signaled;
> >
> > diff --git a/kernel/rcuclassic.c b/kernel/rcuclassic.c
> > index ed15128..eae2fb6 100644
> > --- a/kernel/rcuclassic.c
> > +++ b/kernel/rcuclassic.c
> > @@ -164,6 +164,87 @@ static void __call_rcu(struct rcu_head *head, struct rcu_ctrlblk *rcp,
> > }
> > }
> >
> > +#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
> > +
> > +static void record_gp_stall_check_time(struct rcu_ctrlblk *rcp)
> > +{
> > + rcp->gp_start = jiffies;
> > + rcp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_CHECK;
> > +}
> > +
> > +static void print_other_cpu_stall(struct rcu_ctrlblk *rcp)
> > +{
> > + int cpu;
> > + long delta;
> > + unsigned long flags;
> > +
> > + /* Only let one CPU complain about others per time interval. */
> > +
> > + spin_lock_irqsave(&rcp->lock, flags);
> > + delta = jiffies - rcp->jiffies_stall;
> > + if (delta < 2 || rcp->cur != rcp->completed) {
> > + spin_unlock_irqrestore(&rcp->lock, flags);
> > + return;
> > + }
> > + rcp->jiffies_stall = jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
> > + spin_unlock_irqrestore(&rcp->lock, flags);
> > +
> > + /* OK, time to rat on our buddy... */
> > +
> > + printk(KERN_ERR "RCU detected CPU stalls:");
> > + for_each_possible_cpu(cpu) {
> > + if (cpu_isset(cpu, rcp->cpumask))
> > + printk(" %d", cpu);
> > + }
> > + printk(" (detected by %d, t=%ld jiffies)\n",
> > + smp_processor_id(), (long)(jiffies - rcp->gp_start));
> > +}
> > +
> > +static void print_cpu_stall(struct rcu_ctrlblk *rcp)
> > +{
> > + unsigned long flags;
> > +
> > + printk(KERN_ERR "RCU detected CPU %d stall (t=%lu/%lu jiffies)\n",
> > + smp_processor_id(), jiffies,
> > + jiffies - rcp->gp_start);
> > + dump_stack();
> > + spin_lock_irqsave(&rcp->lock, flags);
> > + if ((long)(jiffies - rcp->jiffies_stall) >= 0)
> > + rcp->jiffies_stall =
> > + jiffies + RCU_SECONDS_TILL_STALL_RECHECK;
> > + spin_unlock_irqrestore(&rcp->lock, flags);
> > + set_need_resched(); /* kick ourselves to get things going. */
> > +}
> > +
> > +static void check_cpu_stall(struct rcu_ctrlblk *rcp)
> > +{
> > + long delta;
> > +
> > + delta = jiffies - rcp->jiffies_stall;
> > + if (cpu_isset(smp_processor_id(), rcp->cpumask) && delta >= 0) {
> > +
> > + /* We haven't checked in, so go dump stack. */
> > + print_cpu_stall(rcp);
> > +
> > + } else if (rcp->cur != rcp->completed && delta >= 2) {
> > +
> > + /* They had two seconds to dump stack, so complain. */
> > + print_other_cpu_stall(rcp);
> > + }
> > +}
> > +
> > +#else /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
> > +
> > +static void record_gp_stall_check_time(struct rcu_ctrlblk *rcp)
> > +{
> > +}
> > +
> > +static void check_cpu_stall(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
> > +{
> > +}
> > +
> > +#endif /* #else #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
> > +
> > /**
> > * call_rcu - Queue an RCU callback for invocation after a grace period.
> > * @head: structure to be used for queueing the RCU updates.
> > @@ -293,84 +374,6 @@ static void rcu_do_batch(struct rcu_data *rdp)
> > * period (if necessary).
> > */
> >
> > -#ifdef CONFIG_DEBUG_RCU_STALL
> > -
> > -static inline void record_gp_check_time(struct rcu_ctrlblk *rcp)
> > -{
> > - rcp->gp_check = get_seconds() + 3;
> > -}
> > -
> > -static void print_other_cpu_stall(struct rcu_ctrlblk *rcp)
> > -{
> > - int cpu;
> > - long delta;
> > - unsigned long flags;
> > -
> > - /* Only let one CPU complain about others per time interval. */
> > -
> > - spin_lock_irqsave(&rcp->lock, flags);
> > - delta = get_seconds() - rcp->gp_check;
> > - if (delta < 2L || cpus_empty(rcp->cpumask)) {
> > - spin_unlock(&rcp->lock);
> > - return;
> > - }
> > - rcp->gp_check = get_seconds() + 30;
> > - spin_unlock_irqrestore(&rcp->lock, flags);
> > -
> > - /* OK, time to rat on our buddy... */
> > -
> > - printk(KERN_ERR "RCU detected CPU stalls:");
> > - for_each_cpu_mask(cpu, rcp->cpumask)
> > - printk(" %d", cpu);
> > - printk(" (detected by %d, t=%lu/%lu)\n",
> > - smp_processor_id(), get_seconds(), rcp->gp_check);
> > -}
> > -
> > -static void print_cpu_stall(struct rcu_ctrlblk *rcp)
> > -{
> > - unsigned long flags;
> > -
> > - printk(KERN_ERR "RCU detected CPU %d stall (t=%lu/%lu)\n",
> > - smp_processor_id(), get_seconds(), rcp->gp_check);
> > - dump_stack();
> > - spin_lock_irqsave(&rcp->lock, flags);
> > - if ((long)(get_seconds() - rcp->gp_check) >= 0L)
> > - rcp->gp_check = get_seconds() + 30;
> > - spin_unlock_irqrestore(&rcp->lock, flags);
> > -}
> > -
> > -static void check_cpu_stall(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
> > -{
> > - long delta;
> > -
> > - delta = get_seconds() - rcp->gp_check;
> > - if (cpu_isset(smp_processor_id(), rcp->cpumask) && delta >= 0L) {
> > -
> > - /* We haven't checked in, so go dump stack. */
> > -
> > - print_cpu_stall(rcp);
> > -
> > - } else {
> > - if (!cpus_empty(rcp->cpumask) && delta >= 2L) {
> > - /* They had two seconds to dump stack, so complain. */
> > - print_other_cpu_stall(rcp);
> > - }
> > - }
> > -}
> > -
> > -#else /* #ifdef CONFIG_DEBUG_RCU_STALL */
> > -
> > -static inline void record_gp_check_time(struct rcu_ctrlblk *rcp)
> > -{
> > -}
> > -
> > -static inline void
> > -check_cpu_stall(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
> > -{
> > -}
> > -
> > -#endif /* #else #ifdef CONFIG_DEBUG_RCU_STALL */
> > -
> > /*
> > * Register a new batch of callbacks, and start it up if there is currently no
> > * active batch and the batch to be registered has not already occurred.
> > @@ -381,7 +384,7 @@ static void rcu_start_batch(struct rcu_ctrlblk *rcp)
> > if (rcp->cur != rcp->pending &&
> > rcp->completed == rcp->cur) {
> > rcp->cur++;
> > - record_gp_check_time(rcp);
> > + record_gp_stall_check_time(rcp);
> >
> > /*
> > * Accessing nohz_cpu_mask before incrementing rcp->cur needs a
> > @@ -603,7 +606,7 @@ static void rcu_process_callbacks(struct softirq_action *unused)
> > static int __rcu_pending(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
> > {
> > /* Check for CPU stalls, if enabled. */
> > - check_cpu_stall(rcp, rdp);
> > + check_cpu_stall(rcp);
> >
> > if (rdp->nxtlist) {
> > long completed_snap = ACCESS_ONCE(rcp->completed);
> > @@ -769,6 +772,9 @@ static struct notifier_block __cpuinitdata rcu_nb = {
> > */
> > void __init __rcu_init(void)
> > {
> > +#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
> > + printk(KERN_INFO "RCU-based detection of stalled CPUs is enabled.\n");
> > +#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
> > rcu_cpu_notify(&rcu_nb, CPU_UP_PREPARE,
> > (void *)(long)smp_processor_id());
> > /* Register notifier for non-boot CPUs */
> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > index 4e921a8..e0e0582 100644
> > --- a/lib/Kconfig.debug
> > +++ b/lib/Kconfig.debug
> > @@ -616,7 +616,7 @@ config RCU_TORTURE_TEST_RUNNABLE
> > Say N here if you want the RCU torture tests to start only
> > after being manually enabled via /proc.
> >
> > -config RCU_CPU_STALL
> > +config RCU_CPU_STALL_DETECTOR
> > bool "Check for stalled CPUs delaying RCU grace periods"
> > depends on CLASSIC_RCU
> > default n
>
> --
> Mathieu Desnoyers
> OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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