Message-ID: <53E045DD.4010505@cn.fujitsu.com>
Date:	Tue, 5 Aug 2014 10:47:57 +0800
From:	Chai Wen <chaiw.fnst@...fujitsu.com>
To:	Don Zickus <dzickus@...hat.com>
CC:	<linux-kernel@...r.kernel.org>, <mingo@...hat.com>
Subject: Re: [PATCH 2/2] softlockup: make detector be aware of task switch
 of processes hogging cpu

On 08/04/2014 10:31 PM, Don Zickus wrote:

> On Mon, Aug 04, 2014 at 03:36:19PM +0800, chai wen wrote:
>>
>> For now, soft lockup detector warns once for each case of process softlockup.
>> But the thread 'watchdog/n' may can not always get cpu at the time slot between
>> the task switch of two processes hogging that cpu.
>> This case is a false negative of "warn only once for a process", as there may be
>> a different process that is going to hog the cpu. Is is better for detector to
>> be aware of it. 
> 
> I am not sure I fully understand the problem resolved.
> 
>>>From the changelog I understood that two processes bouncing back and forth
> could hog the cpu and could create a 'false negative' (a situation not
> reported but should).


Hi Don

Thanks for your comment.
Perhaps 'task-switch' is wrong and is some misleading here, sorry for that.

Here I mean the very case that between a termination of an old cpu hogging
process and a starting getting cpu of a new process hogging cpu.
The case that two or more processes bouncing back and forth and the thread 'watchdog/n'
getting no chance to run is rather difficult to be supposed. And I think this situation
does not exist.

When I am reading the code of warning once about a case, I think maybe it is
not so reliable by judging a "soft_watchdog_warn". And I tried a simple test to see
if it could handle the cased I mentioned above. Please see the comment and detail of
the test below.

> 
> But looking at the patch below I was a little confused on the
> __touch_watchdog addition.  See below:
> 
>>
>> Signed-off-by: chai wen <chaiw.fnst@...fujitsu.com>
>> ---
>>  kernel/watchdog.c |   18 ++++++++++++++++--
>>  1 files changed, 16 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/watchdog.c b/kernel/watchdog.c
>> index 4c2e11c..908050c 100644
>> --- a/kernel/watchdog.c
>> +++ b/kernel/watchdog.c
>> @@ -42,6 +42,7 @@ static DEFINE_PER_CPU(bool, softlockup_touch_sync);
>>  static DEFINE_PER_CPU(bool, soft_watchdog_warn);
>>  static DEFINE_PER_CPU(unsigned long, hrtimer_interrupts);
>>  static DEFINE_PER_CPU(unsigned long, soft_lockup_hrtimer_cnt);
>> +static DEFINE_PER_CPU(pid_t, softlockup_warn_pid_saved);
>>  #ifdef CONFIG_HARDLOCKUP_DETECTOR
>>  static DEFINE_PER_CPU(bool, hard_watchdog_warn);
>>  static DEFINE_PER_CPU(bool, watchdog_nmi_touch);
>> @@ -317,6 +318,8 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
>>  	 */
>>  	duration = is_softlockup(touch_ts);
>>  	if (unlikely(duration)) {
>> +		pid_t pid = task_pid_nr(current);
>> +
>>  		/*
>>  		 * If a virtual machine is stopped by the host it can look to
>>  		 * the watchdog like a soft lockup, check to see if the host
>> @@ -326,8 +329,18 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
>>  			return HRTIMER_RESTART;
>>  
>>  		/* only warn once */
>> -		if (__this_cpu_read(soft_watchdog_warn) == true)
>> +		if (__this_cpu_read(soft_watchdog_warn) == true) {
>> +			/*
>> +			 * soft lockup detector should be aware of that there
>> +			 * may be a task-swicth of two different processes
>> +			 * hogging the cpu continously
>> +			 */
>> +			if (__this_cpu_read(softlockup_warn_pid_saved) != pid) {
>> +				__this_cpu_write(soft_watchdog_warn, false);
>> +				__touch_watchdog();
>> +			}
> 
> The above piece is what I am trying to understand.  Are you saying that
> when two different processes are hogging the cpu, undo the
> soft_watchdog_warn and allow the second pid to be reported?
> 


Yes, Indeed.

> Just trying to understand the problem and see if this is the right
> approach (because 3 or more processes could cause the same problem???).
> 


Only 2 processes is involved in this case as mentioned above, and it is a case about
a termination of an old process and a starting of a new process.

Here is my test about the case:

	stuck.c:
	#include <stdlib.h>
	#include <stdio.h>

	int main(int argc, char **argv)
	{
    		while(1);
    		exit(0);
	}

	# gcc -o stuck stuck.c
	# ./stuck &
	[1] 30309
	# ./stuck &
	[2] 30310
	# taskset -pc 3 30309
	pid 30309's current affinity list: 0-3
	pid 30309's new affinity list: 3
	# taskset -pc 3 30310
	pid 30310's current affinity list: 0-3
	pid 30310's new affinity list: 3

	Then change the schedule policy of 30309 and 30310 to be SCHED_FIFO with the MAX_RT_PRIO-1 priority.
	As the firstly changed to SCHED_FIFO process hogging cpu#3, and is reported after about ~20s.
	After it is killed or terminated, the process 30310 is going out and keeping hogging the cpu continuously
	But this process can not be always reported by the detector in this test.
	If removing the 'warn once' checking, pid change and rather big lockup duration could be found.

thanks
chai wen

> Cheers,
> Don
> 
>>  			return HRTIMER_RESTART;
>> +		}
>>  
>>  		if (softlockup_all_cpu_backtrace) {
>>  			/* Prevent multiple soft-lockup reports if one cpu is already
>> @@ -342,7 +355,8 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
>>  
>>  		printk(KERN_EMERG "BUG: soft lockup - CPU#%d stuck for %us! [%s:%d]\n",
>>  			smp_processor_id(), duration,
>> -			current->comm, task_pid_nr(current));
>> +			current->comm, pid);
>> +		__this_cpu_write(softlockup_warn_pid_saved, pid);
>>  		print_modules();
>>  		print_irqtrace_events(current);
>>  		if (regs)
>> -- 
>> 1.7.1
>>
> .
> 



-- 
Regards

Chai Wen
--
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