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

["Paul E. McKenney" <paulmck@kernel.org>]

On Fri, Oct 28, 2022 at 11:16:55AM +0800, Leizhen (ThunderTown) wrote:
> 
> 
> 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@huawei.com>
> >> ---
> >>  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.

Works for me!  ;-)

							Thanx, Paul