[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <31c1dfb1-623f-4a9e-7f2c-48b83dd4490d@huawei.com>
Date: Wed, 9 Nov 2022 16:46:01 +0800
From: "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To: <paulmck@...nel.org>
CC: Frederic Weisbecker <frederic@...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] rcu: Illustrate the stall information of
CONFIG_RCU_CPU_STALL_CPUTIME=y
On 2022/11/9 10:09, Leizhen (ThunderTown) wrote:
>
>
> On 2022/11/9 4:46, Paul E. McKenney wrote:
>> On Mon, Nov 07, 2022 at 11:29:35PM +0800, Zhen Lei wrote:
>>> Describes how to quickly determine the RCU stall fault type based on the
>>> extra output information during CONFIG_RCU_CPU_STALL_CPUTIME=y.
>>>
>>> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
>>
>> Hearing no objections, I queued the following for further review.
>>
>> This commit might of course need to change based on your ongoing
>> discussion with Robert. I that case, please feel free to send me a
>> replacment patch or to send me an incremental patch that I can fold into
>> this patch. Either way works.
>
> I'll issue incremental patches on the basis of your adjustment! This will
> make it clearer and save your time in reviewing.
I found that Patch 4/4 had one line of description that needed to be changed,
so I had to switch to method 1.
>
> Thanks for your help. I really admire your verbal skills. Your improved
> description is much better than mine.
>
>>
>> Thanx, Paul
>>
>> ------------------------------------------------------------------------
>>
>> commit b05c2a06ff8a1267b7e8dc812e3944119535d6b6
>> Author: Zhen Lei <thunder.leizhen@...wei.com>
>> Date: Mon Nov 7 23:29:35 2022 +0800
>>
>> doc: Document CONFIG_RCU_CPU_STALL_CPUTIME=y stall information
>>
>> This commit doucments how to quickly determine the bug causing a given
>> RCU CPU stall fault warning based on the output information provided
>> by CONFIG_RCU_CPU_STALL_CPUTIME=y.
>>
>> [ paulmck: Apply wordsmithing. ]
>>
>> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
>> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
>>
>> diff --git a/Documentation/RCU/stallwarn.rst b/Documentation/RCU/stallwarn.rst
>> index dfa4db8c0931e..bd8cf6c640984 100644
>> --- a/Documentation/RCU/stallwarn.rst
>> +++ b/Documentation/RCU/stallwarn.rst
>> @@ -390,3 +390,82 @@ for example, "P3421".
>>
>> It is entirely possible to see stall warnings from normal and from
>> expedited grace periods at about the same time during the same run.
>> +
>> +RCU_CPU_STALL_CPUTIME
>> +=====================
>> +
>> +In kernels built with CONFIG_RCU_CPU_STALL_CPUTIME=y or booted with
>> +rcupdate.rcu_cpu_stall_cputime=1, the following additional information
>> +is supplied with each RCU CPU stall warning::
>> +
>> +rcu: hardirqs softirqs csw/system
>> +rcu: number: 624 45 0
>> +rcu: cputime: 69 1 2425 ==> 2500(ms)
>> +
>> +These statistics are collected during the second half of the rcu stall
>> +timeout. The values in row "number:" are the number of hard interrupts,
>> +number of soft interrupts, and number of context switches on the stalled
>> +CPU. The first three values in row "cputime:" indicate the CPU time in
>> +milliseconds consumed by hard interrupts, soft interrupts, and tasks
>> +on the stalled CPU. The last number is the measurement interval, again
>> +in milliseconds. Because user-mode tasks normally do not cause RCU CPU
>> +stalls, these tasks are typically kernel tasks, which is why only the
>> +system CPU time are considered.
>> +
>> +The following describes four typical scenarios:
>> +
>> +1. A CPU looping with interrupts disabled.::
>> +
>> + rcu: hardirqs softirqs csw/system
>> + rcu: number: 0 0 0
>> + 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().
>> .
>>
>
--
Regards,
Zhen Lei
Powered by blists - more mailing lists