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

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Date:   Thu, 10 Nov 2022 12:35:02 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Leizhen (ThunderTown)" <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

On Thu, Nov 10, 2022 at 03:29:04PM +0800, Leizhen (ThunderTown) wrote:
> 
> 
> On 2022/11/10 1:03, Frederic Weisbecker wrote:
> > 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.
> 
> But not all ARCHs support hardlockup, such as s390. Maybe arm64.
> 
> config HARDLOCKUP_DETECTOR
>         bool "Detect Hard Lockups"
>         depends on DEBUG_KERNEL && !S390
>         depends on HAVE_HARDLOCKUP_DETECTOR_PERF || HAVE_HARDLOCKUP_DETECTOR_ARCH

Ah fair point indeed.

Thanks!