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Message-ID: <20221027173334.GB5600@paulmck-ThinkPad-P17-Gen-1>
Date: Thu, 27 Oct 2022 10:33:34 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: Zhen Lei <thunder.leizhen@...wei.com>
Cc: Frederic Weisbecker <frederic@...nel.org>,
Neeraj Upadhyay <quic_neeraju@...cinc.com>,
Josh Triplett <josh@...htriplett.org>,
Steven Rostedt <rostedt@...dmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Lai Jiangshan <jiangshanlai@...il.com>,
Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] rcu: Add RCU stall diagnosis information
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@...wei.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.
> +
> /* 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.
> 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".
> + 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. ;-)
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.
Thanx, Paul
> }
>
> /* Complain about starvation of grace-period kthread. */
> --
> 2.25.1
>
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