Re: [PATCH v2 3/3] rcu: Add RCU stall diagnosis information

[Leizhen (ThunderTown)]

On 2022/10/28 1:33, Paul E. McKenney wrote:
> On Sat, Oct 22, 2022 at 08:45:25PM +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.
>>
>> Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
>> ---
>>  kernel/rcu/tree.c       | 16 ++++++++++++++++
>>  kernel/rcu/tree.h       | 11 +++++++++++
>>  kernel/rcu/tree_stall.h | 28 ++++++++++++++++++++++++++++
>>  3 files changed, 55 insertions(+)
>>
>> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
>> index 6bb8e72bc8151ef..56c49a3117e7a81 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 (rdp->snap_record.gp_seq != rdp->gp_seq) {
>> +			u64 *cpustat;
>> +			struct rcu_snap_record *r;
>> +
>> +			cpustat = kcpustat_cpu(rdp->cpu).cpustat;
>> +
>> +			r = &rdp->snap_record;
>> +			r->cputime_irq     = cpustat[CPUTIME_IRQ];
>> +			r->cputime_softirq = cpustat[CPUTIME_SOFTIRQ];
>> +			r->cputime_system  = cpustat[CPUTIME_SYSTEM];
>> +			r->nr_hardirqs = kstat_cpu_irqs_sum(rdp->cpu);
>> +			r->nr_softirqs = kstat_cpu_softirqs_sum(rdp->cpu);
>> +			r->nr_csw = nr_context_switches_cpu(rdp->cpu);
>> +			r->gp_seq = rdp->gp_seq;
> 
> This needs to be optional.  Yes, it is normally rarely executed, but
> people who don't want the additional information should not pay the
> price for it.
> 
>> +		}
>>  	}
>>  
>>  	return 0;
>> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
>> index d4a97e40ea9c3e2..fb3121d15cca6f8 100644
>> --- a/kernel/rcu/tree.h
>> +++ b/kernel/rcu/tree.h
>> @@ -158,6 +158,16 @@ union rcu_noqs {
>>  	u16 s; /* Set of bits, aggregate OR here. */
>>  };
>>  
>> +struct rcu_snap_record {
>> +	unsigned long	gp_seq;
>> +	u64		cputime_irq;
>> +	u64		cputime_softirq;
>> +	u64		cputime_system;
>> +	unsigned int	nr_hardirqs;
>> +	unsigned int	nr_softirqs;
>> +	unsigned long long nr_csw;
>> +};
> 
> Please add a comment saying what this is and what it is used for.

OK, I will do it.

> 
>> +
>>  /* Per-CPU data for read-copy update. */
>>  struct rcu_data {
>>  	/* 1) quiescent-state and grace-period handling : */
>> @@ -262,6 +272,7 @@ 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;
> 
> And here as well, please.

OK

> 
>>  	int cpu;
>>  };
>> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
>> index 5653560573e22d6..f8c9d0284d116a8 100644
>> --- a/kernel/rcu/tree_stall.h
>> +++ b/kernel/rcu/tree_stall.h
>> @@ -428,6 +428,32 @@ 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 *r;
> 
> Let's please follow convention and call it "rsrp" rather than just "r".

OK

> 
>> +	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
>> +
>> +	r = &rdp->snap_record;
>> +	if (r->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: %8d %10d %12lld\n",
>> +		kstat_cpu_irqs_sum(cpu) - r->nr_hardirqs,
>> +		kstat_cpu_softirqs_sum(cpu) - r->nr_softirqs,
>> +		nr_context_switches_cpu(cpu) - r->nr_csw);
>> +	pr_err("cputime: %8lld %10lld %12lld   ==> %lld(ms)\n",
>> +		div_u64(cpustat[CPUTIME_IRQ] - r->cputime_irq, NSEC_PER_MSEC),
>> +		div_u64(cpustat[CPUTIME_SOFTIRQ] - r->cputime_softirq, NSEC_PER_MSEC),
>> +		div_u64(cpustat[CPUTIME_SYSTEM] - r->cputime_system, NSEC_PER_MSEC),
>> +		jiffies64_to_msecs(half_timeout));
>> +}
>> +
>>  /*
>>   * Print out diagnostic information for the specified stalled CPU.
>>   *
>> @@ -484,6 +510,8 @@ 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);
> 
> Again, please make this conditional.  One way to do that is with a
> Kconfig option.  Another is with a kernel boot parameter, as is done
> wtih module_param() elsewhere in tree_stall.h.  Or if the parsing needs
> to be fancy (it shouldn't) using kernel_param_ops as is done in tree.c.
> Distros tend to like kernel boot parameters, while people dealing with
> large numbers of devices tend to like Kconfig options.  Choose wisely.  ;-)

OK. I will add both Kconfig option and boot parameter, let the user
choose freely.

> 
> Please make this initially default-off.  If enough people enable it for
> long enough, we can later switch it to default-on and maybe even later
> to unconditional.  But let's start carefully.

Right. You're so far-sighted.

> 
> 							Thanx, Paul
> 
>>  }
>>  
>>  /* Complain about starvation of grace-period kthread.  */
>> -- 
>> 2.25.1
>>
> .
> 

-- 
Regards,
  Zhen Lei