Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH v3] sched/debug: Use sched_debug_lock to serialize use of
 cgroup_path[] only
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Bharata B Rao <bharata@linux.vnet.ibm.com>,
        Phil Auld <pauld@redhat.com>,
        Daniel Thompson <daniel.thompson@linaro.org>,
        linux-kernel@vger.kernel.org
References: <20210401181030.7689-1-longman@redhat.com>
 <20210402164014.53c84f05@gandalf.local.home>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <b5d7e93f-c594-7678-eb8d-275ddd9cd8ce@redhat.com>
Date:   Fri, 2 Apr 2021 23:09:09 -0400
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101
 Thunderbird/78.8.0
MIME-Version: 1.0
In-Reply-To: <20210402164014.53c84f05@gandalf.local.home>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
Content-Language: en-US
Precedence: bulk

On 4/2/21 4:40 PM, Steven Rostedt wrote:
> On Thu,  1 Apr 2021 14:10:30 -0400
> Waiman Long <longman@redhat.com> wrote:
>
>> The handling of sysrq key can be activated by echoing the key to
>> /proc/sysrq-trigger or via the magic key sequence typed into a terminal
>> that is connected to the system in some way (serial, USB or other mean).
>> In the former case, the handling is done in a user context. In the
>> latter case, it is likely to be in an interrupt context.
>>
>> There should be no more than one instance of sysrq key processing via
>> a terminal, but multiple instances of /proc/sysrq-trigger is possible.
>>
>> Currently in print_cpu() of kernel/sched/debug.c, sched_debug_lock is
>> taken with interrupt disabled for the whole duration of the calls to
>> print_*_stats() and print_rq() which could last for the quite some time
>> if the information dump happens on the serial console.
>>
>> If the system has many cpus and the sched_debug_lock is somehow busy
>> (e.g. parallel sysrq-t), the system may hit a hard lockup panic
>> depending on the actually serial console implementation of the
>> system. For instance,
> Wouldn't placing strategically located "touch_nmi_watchdog()"s around fix
> this?
>
> -- Steve
>
The main problem with sched_debug_lock is that under certain 
circumstances, a lock waiter may wait a long time to acquire the lock 
(in seconds). We can't insert touch_nmi_watchdog() while the cpu is 
waiting for the spinlock.

Cheers,
Longman