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

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Date:   Wed, 9 Nov 2022 07:59:01 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Zhen Lei <thunder.leizhen@...wei.com>,
        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

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
> >