Re: [PATCH v3 4/4] rcu: Add RCU stall diagnosis information

["Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>]

On 2022/11/4 1:28, Paul E. McKenney wrote:
> On Thu, Nov 03, 2022 at 08:35:15PM +0800, Zhen Lei wrote:
>> In some extreme cases, such as the I/O pressure test, the CPU usage may
>> be 100%, causing RCU stall. In this case, the printed information about
>> current is not useful. Displays the number and usage of hard interrupts,
>> soft interrupts, and context switches that are generated within half of
>> the CPU stall timeout, can help us make a general judgment. In other
>> cases, we can preliminarily determine whether an infinite loop occurs
>> when local_irq, local_bh or preempt is disabled.
>>
>> For example:
>> rcu: INFO: rcu_preempt self-detected stall on CPU
>> rcu:     0-....: (1250 ticks this GP) <omitted>
>> rcu:          hardirqs   softirqs   csw/system
>> rcu:  number:      624         45            0
>> rcu: cputime:       69          1         2425   ==> 2500(ms)
>>
>> The example above shows that the number of hard and soft interrupts is
>> small, there is zero context switching, and the system takes up a lot of
>> time. We can quickly conclude that the current task is infinitely looped
>> with preempt_disable().
>>
>> The impact on system performance is negligible because snapshot is
>> recorded only one time after 1/2 CPU stall timeout.
>>
>> This enhanced debugging information is suppressed by default and can be
>> enabled by CONFIG_RCU_CPU_STALL_DEEP_DEBUG=y or
>> rcupdate.rcu_cpu_stall_deep_debug=1.
>>
>> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> 
> Much better, thank you!
> 
> How about rcu_cpu_stall_cputime instead of rcu_cpu_stall_deep_debug and
> RCU_CPU_STALL_CPUTIME instead of RCU_CPU_STALL_DEEP_DEBUG?

Very well. I'll update and post v5 today.

> 
> Just to forestall things like rcu_cpu_stall_deeper_debug, to say nothing
> of rcu_cpu_stall_deepest_debug or rcu_cpu_stall_abyssal_debug.  ;-)

Ha ha! Right.

> 
> 							Thanx, Paul
> 
>> ---
>>  .../admin-guide/kernel-parameters.txt         |  3 ++
>>  kernel/rcu/Kconfig.debug                      | 10 ++++++
>>  kernel/rcu/rcu.h                              |  1 +
>>  kernel/rcu/tree.c                             | 16 ++++++++++
>>  kernel/rcu/tree.h                             | 17 ++++++++++
>>  kernel/rcu/tree_stall.h                       | 31 +++++++++++++++++++
>>  kernel/rcu/update.c                           |  2 ++
>>  7 files changed, 80 insertions(+)
>>
>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>> index a465d5242774af8..fc619e9e8d056da 100644
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -5082,6 +5082,9 @@
>>  			rcupdate.rcu_cpu_stall_timeout to be used (after
>>  			conversion from seconds to milliseconds).
>>  
>> +	rcupdate.rcu_cpu_stall_deep_debug= [KNL]
>> +			Provide additional rcu stall debug information.
>> +
>>  	rcupdate.rcu_expedited= [KNL]
>>  			Use expedited grace-period primitives, for
>>  			example, synchronize_rcu_expedited() instead
>> diff --git a/kernel/rcu/Kconfig.debug b/kernel/rcu/Kconfig.debug
>> index 1b0c41d490f0588..7b18763c574cb13 100644
>> --- a/kernel/rcu/Kconfig.debug
>> +++ b/kernel/rcu/Kconfig.debug
>> @@ -95,6 +95,16 @@ config RCU_EXP_CPU_STALL_TIMEOUT
>>  	  says to use the RCU_CPU_STALL_TIMEOUT value converted from
>>  	  seconds to milliseconds.
>>  
>> +config RCU_CPU_STALL_DEEP_DEBUG
>> +	bool "Provide additional rcu stall debug information"
>> +	depends on RCU_STALL_COMMON
>> +	default n
>> +	help
>> +	  Statistics during the period from RCU_CPU_STALL_TIMEOUT/2 to
>> +	  RCU_CPU_STALL_TIMEOUT, such as the number of (hard interrupts, soft
>> +	  interrupts, task switches) and the cputime of (hard interrupts, soft
>> +	  interrupts, kerenl tasks) are added to the rcu stall report.
>> +
>>  config RCU_TRACE
>>  	bool "Enable tracing for RCU"
>>  	depends on DEBUG_KERNEL
>> diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
>> index be5979da07f5985..2445b0c58348400 100644
>> --- a/kernel/rcu/rcu.h
>> +++ b/kernel/rcu/rcu.h
>> @@ -224,6 +224,7 @@ extern int rcu_cpu_stall_ftrace_dump;
>>  extern int rcu_cpu_stall_suppress;
>>  extern int rcu_cpu_stall_timeout;
>>  extern int rcu_exp_cpu_stall_timeout;
>> +extern int rcu_cpu_stall_deep_debug;
>>  int rcu_jiffies_till_stall_check(void);
>>  int rcu_exp_jiffies_till_stall_check(void);
>>  
>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> index 93416afebd59c7a..a90d2e2e5bbd99e 100644
>> --- a/kernel/rcu/tree.c
>> +++ b/kernel/rcu/tree.c
>> @@ -931,6 +931,22 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
>>  			rdp->rcu_iw_gp_seq = rnp->gp_seq;
>>  			irq_work_queue_on(&rdp->rcu_iw, rdp->cpu);
>>  		}
>> +
>> +		if (rcu_cpu_stall_deep_debug && rdp->snap_record.gp_seq != rdp->gp_seq) {
>> +			u64 *cpustat;
>> +			struct rcu_snap_record *rsrp;
>> +
>> +			cpustat = kcpustat_cpu(rdp->cpu).cpustat;
>> +
>> +			rsrp = &rdp->snap_record;
>> +			rsrp->cputime_irq     = cpustat[CPUTIME_IRQ];
>> +			rsrp->cputime_softirq = cpustat[CPUTIME_SOFTIRQ];
>> +			rsrp->cputime_system  = cpustat[CPUTIME_SYSTEM];
>> +			rsrp->nr_hardirqs = kstat_cpu_irqs_sum(rdp->cpu);
>> +			rsrp->nr_softirqs = kstat_cpu_softirqs_sum(rdp->cpu);
>> +			rsrp->nr_csw = nr_context_switches_cpu(rdp->cpu);
>> +			rsrp->gp_seq = rdp->gp_seq;
>> +		}
>>  	}
>>  
>>  	return 0;
>> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
>> index d4a97e40ea9c3e2..44d49819a31f050 100644
>> --- a/kernel/rcu/tree.h
>> +++ b/kernel/rcu/tree.h
>> @@ -158,6 +158,22 @@ union rcu_noqs {
>>  	u16 s; /* Set of bits, aggregate OR here. */
>>  };
>>  
>> +/*
>> + * Record the snapshot of the core stats at 1/2 rcu stall timeout. The member
>> + * gp_seq is used to ensure that all members are updated only once during the
>> + * second half period. The snapshot is taken only if this gp_seq is not equal
>> + * to rdp->gp_seq.
>> + */
>> +struct rcu_snap_record {
>> +	unsigned long	gp_seq;		/* Track rdp->gp_seq counter */
>> +	u64		cputime_irq;	/* Accumulated cputime of hard irqs */
>> +	u64		cputime_softirq;/* Accumulated cputime of soft irqs */
>> +	u64		cputime_system; /* Accumulated cputime of kernel tasks */
>> +	unsigned long	nr_hardirqs;	/* Accumulated number of hard irqs */
>> +	unsigned int	nr_softirqs;	/* Accumulated number of soft irqs */
>> +	unsigned long long nr_csw;	/* Accumulated number of task switches */
>> +};
>> +
>>  /* Per-CPU data for read-copy update. */
>>  struct rcu_data {
>>  	/* 1) quiescent-state and grace-period handling : */
>> @@ -262,6 +278,7 @@ struct rcu_data {
>>  	short rcu_onl_gp_flags;		/* ->gp_flags at last online. */
>>  	unsigned long last_fqs_resched;	/* Time of last rcu_resched(). */
>>  	unsigned long last_sched_clock;	/* Jiffies of last rcu_sched_clock_irq(). */
>> +	struct rcu_snap_record snap_record; /* Snapshot of core stats at 1/2 rcu stall timeout */
>>  
>>  	int cpu;
>>  };
>> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
>> index 5653560573e22d6..5b258bcdb25ba04 100644
>> --- a/kernel/rcu/tree_stall.h
>> +++ b/kernel/rcu/tree_stall.h
>> @@ -428,6 +428,35 @@ static bool rcu_is_rcuc_kthread_starving(struct rcu_data *rdp, unsigned long *jp
>>  	return j > 2 * HZ;
>>  }
>>  
>> +static void print_cpu_stat_info(int cpu)
>> +{
>> +	u64 *cpustat;
>> +	unsigned long half_timeout;
>> +	struct rcu_snap_record *rsrp;
>> +	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
>> +
>> +	if (!rcu_cpu_stall_deep_debug)
>> +		return;
>> +
>> +	rsrp = &rdp->snap_record;
>> +	if (rsrp->gp_seq != rdp->gp_seq)
>> +		return;
>> +
>> +	cpustat = kcpustat_cpu(cpu).cpustat;
>> +	half_timeout = rcu_jiffies_till_stall_check() / 2;
>> +
>> +	pr_err("         hardirqs   softirqs   csw/system\n");
>> +	pr_err(" number: %8ld %10d %12lld\n",
>> +		kstat_cpu_irqs_sum(cpu) - rsrp->nr_hardirqs,
>> +		kstat_cpu_softirqs_sum(cpu) - rsrp->nr_softirqs,
>> +		nr_context_switches_cpu(cpu) - rsrp->nr_csw);
>> +	pr_err("cputime: %8lld %10lld %12lld   ==> %lld(ms)\n",
>> +		div_u64(cpustat[CPUTIME_IRQ] - rsrp->cputime_irq, NSEC_PER_MSEC),
>> +		div_u64(cpustat[CPUTIME_SOFTIRQ] - rsrp->cputime_softirq, NSEC_PER_MSEC),
>> +		div_u64(cpustat[CPUTIME_SYSTEM] - rsrp->cputime_system, NSEC_PER_MSEC),
>> +		jiffies64_to_msecs(half_timeout));
>> +}
>> +
>>  /*
>>   * Print out diagnostic information for the specified stalled CPU.
>>   *
>> @@ -484,6 +513,8 @@ static void print_cpu_stall_info(int cpu)
>>  	       data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
>>  	       rcuc_starved ? buf : "",
>>  	       falsepositive ? " (false positive?)" : "");
>> +
>> +	print_cpu_stat_info(cpu);
>>  }
>>  
>>  /* Complain about starvation of grace-period kthread.  */
>> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
>> index 738842c4886b235..9c68c88a700f185 100644
>> --- a/kernel/rcu/update.c
>> +++ b/kernel/rcu/update.c
>> @@ -508,6 +508,8 @@ int rcu_cpu_stall_timeout __read_mostly = CONFIG_RCU_CPU_STALL_TIMEOUT;
>>  module_param(rcu_cpu_stall_timeout, int, 0644);
>>  int rcu_exp_cpu_stall_timeout __read_mostly = CONFIG_RCU_EXP_CPU_STALL_TIMEOUT;
>>  module_param(rcu_exp_cpu_stall_timeout, int, 0644);
>> +int rcu_cpu_stall_deep_debug __read_mostly = IS_ENABLED(CONFIG_RCU_CPU_STALL_DEEP_DEBUG);
>> +module_param(rcu_cpu_stall_deep_debug, int, 0644);
>>  #endif /* #ifdef CONFIG_RCU_STALL_COMMON */
>>  
>>  // Suppress boot-time RCU CPU stall warnings and rcutorture writer stall
>> -- 
>> 2.25.1
>>
> .
> 

-- 
Regards,
  Zhen Lei