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Date: Fri, 28 Oct 2022 11:16:55 +0800
From: "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To: <paulmck@...nel.org>
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 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@...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.
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
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