linux-kernel - Re: [PATCH] softlockup: fix problem with long kernel pauses from kgdb

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Date:	Mon, 27 Jul 2009 16:25:32 -0500
From:	Jason Wessel <jason.wessel@...driver.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...e.hu>, lkml <linux-kernel@...r.kernel.org>,
	"Deng, Dongdong" <Dongdong.Deng@...driver.com>
Subject: Re: [PATCH] softlockup: fix problem with long kernel pauses from
 kgdb

Peter Zijlstra wrote:
> On Mon, 2009-07-27 at 15:03 -0500, Jason Wessel wrote:
>   
>> The fix is to simply invoke sched_clock_tick() to update "cpu sched
>> clock" on exit from kgdb_handle_exception.
>>     
>
> Is that a regular IRQ context, or is that NMI context?
>
>   

The patch text there is incorrect (which I can fix), because there were
3 other versions of this particular patch.  The version of this patch
that tried to use sched_clock_tick() from the exception context hung the
system because there was lock contention for CPUs that were not in the
running state.

In this version, the sched_clock_tick() added in the softlockup_tick(),
which did not result in a hard hang.  I am pretty sure that
softlockup_tick() runs as a part of the timer interrupt processing from
run_local_timers(), so there was no further need to turn off the interrupts.

Jason.

>> Signed-off-by: Dongdong Deng <Dongdong.Deng@...driver.com>
>> Signed-off-by: Jason Wessel <jason.wessel@...driver.com>
>> Cc: Ingo Molnar <mingo@...e.hu>
>> Cc: peterz@...radead.org
>> ---
>>  kernel/softlockup.c |    3 +++
>>  1 file changed, 3 insertions(+)
>>
>> --- a/kernel/softlockup.c
>> +++ b/kernel/softlockup.c
>> @@ -118,6 +118,9 @@ void softlockup_tick(void)
>>         }
>>  
>>         if (touch_timestamp == 0) {
>> +               /* If the time stamp was touched externally make sure the
>> +                * scheduler tick is up to date as well */
>> +               sched_clock_tick();
>>                 __touch_softlockup_watchdog();
>>                 return;
>>         }
>>
>>     
>
> Aside from the funny comment style (please fix) the fix does look
> sensible.
>   

No problem.

Thanks,
Jason.
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