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

Frederic Weisbecker
2022-11-09T15:26:31Z

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
>