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

[Frederic Weisbecker <frederic@kernel.org>]

On Wed, Nov 09, 2022 at 07:59:01AM -0800, Paul E. McKenney wrote:
> 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.

Arguably, the hardlockup/softlockup detectors usually trigger after RCU stall,
unless all CPUs are caught into a hardlockup, in which case only the hardlockup
detector has a chance.

Anyway I would say that in this case just lower the delay for the lockup
detectors to consider the situation is a lockup?

Thanks.


> 
> 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
> > >