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

From: Paul E. McKenney
Date: Thu Nov 03 2022 - 13:28:39 EST

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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@xxxxxxxxxx>

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?

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. ;-)

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
>

Next message: Yury Norov: "Re: [PATCH v3 2/2] x86: Fix /proc/cpuinfo cpumask warning"
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In reply to: Zhen Lei: "[PATCH v3 4/4] rcu: Add RCU stall diagnosis information"
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