linux-kernel - Re: [PATCH v7 5/6] doc: Document CONFIG_RCU_CPU_STALL

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Date:   Thu, 17 Nov 2022 10:03:17 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     Frederic Weisbecker <frederic@...nel.org>
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 v7 5/6] doc: Document CONFIG_RCU_CPU_STALL_CPUTIME=y stall
 information



On 2022/11/17 6:55, Frederic Weisbecker wrote:
> On Fri, Nov 11, 2022 at 09:07:08PM +0800, Zhen Lei wrote:
>> +1. A CPU looping with interrupts disabled.::
>> +
>> +   rcu:          hardirqs   softirqs   csw/system
>> +   rcu:  number:        0          0            0
>> +65;6003;1c   rcu: cputime:        0          0            0   ==> 2500(ms)
>> +
>> +   Because interrupts have been disabled throughout the measurement
>> +   interval, there are no interrupts and no context switches.
>> +   Furthermore, because CPU time consumption was measured using interrupt
>> +   handlers, the system CPU consumption is misleadingly measured as zero.
>> +   This scenario will normally also have "(0 ticks this GP)" printed on
>> +   this CPU's summary line.
>> +
>> +2. A CPU looping with bottom halves disabled.
>> +
>> +   This is similar to the previous example, but with non-zero number of
>> +   and CPU time consumed by hard interrupts, along with non-zero CPU
>> +   time consumed by in-kernel execution.::
>> +
>> +   rcu:          hardirqs   softirqs   csw/system
>> +   rcu:  number:      624          0            0
>> +   rcu: cputime:       49          0         2446   ==> 2500(ms)
>> +
>> +   The fact that there are zero softirqs gives a hint that these were
>> +   disabled, perhaps via local_bh_disable().  It is of course possible
>> +   that there were no softirqs, perhaps because all events that would
>> +   result in softirq execution are confined to other CPUs.  In this case,
>> +   the diagnosis should continue as shown in the next example.
>> +
>> +3. A CPU looping with preemption disabled.
>> +
>> +   Here, only the number of context switches is zero.::
>> +
>> +   rcu:          hardirqs   softirqs   csw/system
>> +   rcu:  number:      624         45            0
>> +   rcu: cputime:       69          1         2425   ==> 2500(ms)
>> +
>> +   This situation hints that the stalled CPU was looping with preemption
>> +   disabled.
>> +
>> +4. No looping, but massive hard and soft interrupts.::
>> +
>> +   rcu:          hardirqs   softirqs   csw/system
>> +   rcu:  number:       xx         xx            0
>> +   rcu: cputime:       xx         xx            0   ==> 2500(ms)
>> +
>> +   Here, the number and CPU time of hard interrupts are all non-zero,
>> +   but the number of context switches and the in-kernel CPU time consumed
>> +   are zero. The number and cputime of soft interrupts will usually be
>> +   non-zero, but could be zero, for example, if the CPU was spinning
>> +   within a single hard interrupt handler.
>> +
>> +   If this type of RCU CPU stall warning can be reproduced, you can
>> +   narrow it down by looking at /proc/interrupts or by writing code to
>> +   trace each interrupt, for example, by referring to show_interrupts().
> 
> One last question I have. Usually all these informations can be deduced by
> just looking at the stacktrace that comes along an RCU stall report. So on
> which kind of situation the stacktrace is not enough?

Interrupt storm.

> 
> Thanks.
> .
> 

-- 
Regards,
  Zhen Lei