linux-kernel - Re: [RFC PATCH] sched/debug: Use terse backtrace for idly sleeping threads.

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Message-ID: <54fc7ab8-2995-e864-7f74-c4434d23622c@i-love.sakura.ne.jp>
Date:   Fri, 20 Jul 2018 22:27:17 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tejun Heo <tj@...nel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] sched/debug: Use terse backtrace for idly sleeping
 threads.

On 2018/07/19 22:46, Peter Zijlstra wrote:
> On Thu, Jul 19, 2018 at 10:37:23PM +0900, Tetsuo Handa wrote:
>> This patch can be applied before proposing abovementioned changes.
>> Since there are many kernel threads whose backtrace is boring due to idly
>> waiting for an event inside the main loop, this patch introduces a kernel
>> config option (which allows SysRq-t to use one-liner backtrace for threads
>> idly waiting for an event) and simple helpers (which allow current thread
>> to declare that current thread is about to start/end idly waiting).
> 
>> diff --git a/drivers/base/devtmpfs.c b/drivers/base/devtmpfs.c
>> index f776807..6b8c8bd 100644
>> --- a/drivers/base/devtmpfs.c
>> +++ b/drivers/base/devtmpfs.c
>> @@ -406,7 +406,9 @@ static int devtmpfsd(void *p)
>>  		}
>>  		__set_current_state(TASK_INTERRUPTIBLE);
>>  		spin_unlock(&req_lock);
>> +		start_idle_sleeping();
>>  		schedule();
>> +		end_idle_sleeping();
>>  	}
>>  	return 0;
>>  out:
> 
> So I _really_ hate the idea of sprinking that all around the kernel like
> this.
> 

Does that comment mean the idea of "using one-liner backtrace for threads
idly waiting for an event" itself is OK?

Since there already is schedule_idle() function, introducing idly_schedule()
etc. is very confusing. What I'm trying to do is to tell debug function that
"I'm currently in neutral situation and hence dumping my backtrace will not
give you interesting result". Since such section needs to be carefully
annotated with comments, I think that lockdep-like annotation fits better
than introducing wrapped functions.