linux-kernel - Re: [PATCH v6 0/2] rcu: Add RCU stall diagnosis information

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Date:   Wed, 9 Nov 2022 16:26:21 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Zhen Lei <thunder.leizhen@...wei.com>
Cc:     "Paul E . McKenney" <paulmck@...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, Robert Elliott <elliott@....com>
Subject: Re: [PATCH v6 0/2] rcu: Add RCU stall diagnosis information

Hi Zhen Lei,

On Wed, Nov 09, 2022 at 05:37:36PM +0800, Zhen Lei wrote:
> v5 --> v6:
> 1. When there are more than two continuous RCU stallings, correctly handle the
>    value of the second and subsequent sampling periods. Update comments and
>    document.
>    Thanks to Elliott, Robert for the test.
> 2. Change "rcu stall" to "RCU stall".
> 
> v4 --> v5:
> 1. Resolve a git am conflict. No code change.
> 
> v3 --> v4:
> 1. Rename rcu_cpu_stall_deep_debug to rcu_cpu_stall_cputime.
> 
> v2 --> v3:
> 1. Fix the return type of kstat_cpu_irqs_sum()
> 2. Add Kconfig option CONFIG_RCU_CPU_STALL_DEEP_DEBUG and boot parameter
>    rcupdate.rcu_cpu_stall_deep_debug.
> 3. Add comments and normalize local variable name
> 
> 
> v1 --> v2:
> 1. Fixed a bug in the code. If the rcu stall is detected by another CPU,
>    kcpustat_this_cpu cannot be used.
> @@ -451,7 +451,7 @@ static void print_cpu_stat_info(int cpu)
>         if (r->gp_seq != rdp->gp_seq)
>                 return;
> 
> -       cpustat = kcpustat_this_cpu->cpustat;
> +       cpustat = kcpustat_cpu(cpu).cpustat;
> 2. Move the start point of statistics from rcu_stall_kick_kthreads() to
>    rcu_implicit_dynticks_qs(), removing the dependency on irq_work.
> 
> v1:
> 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.

That looks useful but I have to ask: what does it bring that the softlockup
and hardlockup watchdog can not already solve?

Thanks.

> 
> Zhen Lei (2):
>   rcu: Add RCU stall diagnosis information
>   doc: Document CONFIG_RCU_CPU_STALL_CPUTIME=y stall information
> 
>  Documentation/RCU/stallwarn.rst               | 88 +++++++++++++++++++
>  .../admin-guide/kernel-parameters.txt         |  6 ++
>  kernel/rcu/Kconfig.debug                      | 11 +++
>  kernel/rcu/rcu.h                              |  1 +
>  kernel/rcu/tree.c                             | 17 ++++
>  kernel/rcu/tree.h                             | 19 ++++
>  kernel/rcu/tree_stall.h                       | 29 ++++++
>  kernel/rcu/update.c                           |  2 +
>  8 files changed, 173 insertions(+)
> 
> -- 
> 2.25.1
>