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

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Date:   Sun, 4 Apr 2021 21:29:43 -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 4/4/21 9:27 PM, Waiman Long wrote:
> On 4/4/21 12:02 PM, Steven Rostedt wrote:
>> On Fri, 2 Apr 2021 23:09:09 -0400
>> Waiman Long <longman@...hat.com> wrote:
>>
>>> 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.
>> The problem I have with the patch is that it seems to be a hack (as it
>> doesn't fix the issue in all cases). Since sched_debug_lock is
>> "special", perhaps we can add wrappers to take it, and instead of doing
>> the spin_lock_irqsave(), do a trylock loop. Add lockdep annotation to
>> tell lockdep that this is not a try lock (so that it can still detect
>> deadlocks).
>>
>> Then have the strategically placed touch_nmi_watchdog() also increment
>> a counter. Then in that trylock loop, if it sees the counter get
>> incremented, it knows that forward progress is being made by the lock
>> holder, and it too can call touch_nmi_watchdog().
>
> Thanks for the suggestion, but it also sound complicated.
>
> I think we can fix this lockup problem if we are willing to lose some 
> information in case of contention. As you have suggested, a trylock 
> will be used to acquire sched_debug_lock. If succeeded, all is good. 
> Otherwise, a shorter stack buffer will be used for cgroup path. The 
> path may be truncated in this case. If we detect that the full length 
> of the buffer is used, we assume truncation and add, e.g. "...", to 
> indicate there is more to the actual path.
>
> Do you think this is an acceptable comprise? 

Actually, I don't really need to disable interrupt under this situation 
as deadlock can't happen.

Cheers,
Longman