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

Paul E. McKenney
2022-11-09T15:59:11Z

On Wed, Nov 09, 2022 at 04:26:21PM +0100, Frederic Weisbecker wrote:
> 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?

This is a good point. One possible benefit is putting the needed information
in one spot, for example, in cases where the soft/hard lockup timeouts are
significantly different than the RCU CPU stall warning timeout.

Thoughts?

Thanx, Paul

> 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
> >