linux-kernel - Re: [PATCH v3] sched/debug: Use sched_debug_lock to serialize use of cgroup

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Date:   Fri, 2 Apr 2021 23:09:09 -0400
From:   Waiman Long <longman@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Bharata B Rao <bharata@...ux.vnet.ibm.com>,
        Phil Auld <pauld@...hat.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] sched/debug: Use sched_debug_lock to serialize use of
 cgroup_path[] only

On 4/2/21 4:40 PM, Steven Rostedt wrote:
> On Thu,  1 Apr 2021 14:10:30 -0400
> Waiman Long <longman@...hat.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