Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <10f7d339-c451-8284-ccd8-ce9a0fe14f48@linux.alibaba.com>
Date:   Wed, 3 Aug 2022 17:25:49 +0800
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:102.0)
 Gecko/20100101 Thunderbird/102.0.3
Subject: Re: [PATCH] sched/debug: avoid executing show_state and causing rcu
 stall warning
To:     Ingo Molnar <mingo@kernel.org>
Cc:     mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        bristot@redhat.com, vschneid@redhat.com,
        linux-kernel@vger.kernel.org
References: <1659489525-82994-1-git-send-email-liusong@linux.alibaba.com>
 <Yuo0U2aWUZRLBAsA@gmail.com>
 <c4058896-947d-c802-ac57-bf03eb986378@linux.alibaba.com>
 <Yuo7eYzQnzqx4u7F@gmail.com>
From:   Liu Song <liusong@linux.alibaba.com>
In-Reply-To: <Yuo7eYzQnzqx4u7F@gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

> * Liu Song <liusong@linux.alibaba.com> wrote:
>
>>> * Liu Song <liusong@linux.alibaba.com> wrote:
>>>
>>>> From: Liu Song <liusong@linux.alibaba.com>
>>>>
>>>> If the number of CPUs is large, "sysrq_sched_debug_show" will execute for
>>>> a long time. Every time I execute "echo t > /proc/sysrq-trigger" on my
>>>> 128-core machine, the rcu stall warning will be triggered. Moreover,
>>>> sysrq_sched_debug_show does not need to be protected by rcu_read_lock,
>>>     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>>>> and no rcu stall warning will appear after adjustment.
>>>>
>>> That doesn't mean it doesn't have to be protected by *any* lock - which
>>> your patch implements AFAICS.
>>>
>>> There's a couple of lines such as:
>>>
>>>           for_each_online_cpu(cpu) {
>> Hi,
>>
>> Here I refer to the implementation of "sysrq_timer_list_show", and I don't
>> see any lock.
>>
>> Maybe there is a problem with the implementation of "sysrq_timer_list_show".
> But we are talking about sysrq_sched_debug_show(), which your patch tries
> to relax the RCU locking of.

Hi,

I'm not sure for_each_online_cpu && print_cpu must need a lock to 
protect, so I refer to other codes

under kernel that reference the implementation. It looks like some 
places use "get_online_cpus" to prevent

cpu hotplug, but many places don't have obvious protection, so I'm also 
confused if protection is necessarily

required.


Thanks

>
> Thanks,
>
> 	Ingo