Re: SMTC support status in latest git head.

["Kevin D. Kissell" <kevink@paralogos.com>]

I took a quick look last night, and the only thing that looked vaguely 
dangerous in changes since the timer changes I alluded to earlier was 
the global naming cleanup of irq-related function names that David 
Howell submitted.  The diff didn't look dangerous in itself, but some of 
the definitions are nested subtly for SMTC to maximize the amount of 
common code, and I could imagine something getting lost in translation 
there.  If that were really the problem, it would of course affect much 
more than just the timer subsystem, but early in the boot process, 
timers are pretty much the only interrupts that have to be handled 
correctly.

I'm travelling today, but will take a look at timekeeping_notify() 
tomorrow or the next day...

/K.

On 12/28/10 12:19 AM, Anoop P A wrote:
> Hi,
>
> I had a glance into the code diff without notice of any suspect-able
> code .
> Tracing the hang showed that it is getting hanged in timekeeping_notify
> function.
>
> Thanks,
> Anoop
>
> PS: I may not be available until Thursday
>
> On Mon, 2010-12-27 at 22:49 +0530, Anoop P A wrote:
>> Hi Kevin,
>>
>> It is very unlikely that the patch you pointed has any impact on the the
>> hang I am seeing. The patch you have mentioned got into kernel around
>> 2.6.32 timeframe. I am able to boot both 2.6.32 and  2.6.33 kernel ( +
>> stackframe patch) .
>>
>> Hi Stuart,
>>
>> I haven't got much time to spend on this today.
>>
>> I had got 2.6.36-stable(+ stack frame patch) booting last day and I have
>> observed hang issue with 2.6.37-rc1 ( Same as rc6 and current git head)
>>
>> So probably some patches in 2.6.37 branch introduced this hang.
>>
>> Hopefully I will get some free slot tomorrow so that I can look into
>> code diff .
>>
>> Thanks
>> Anoop
>>
>> On Mon, 2010-12-27 at 09:49 -0600, STUART VENTERS wrote:
>>> Kevin,
>>>
>>> Outstanding, sometimes it's better to be lucky than good.
>>>
>>>
>>> Anoop,
>>>
>>> Maybe we can get lucky again.
>>>
>>> If you can isolate the .33 works/.37 works_not bug to a specific pair of versions,
>>>     I'll be happy to do another diff.
>>>
>>>
>>> Hope you'll have had a good Christmas as well.
>>>    We've had snow in Alabama since Christmas eve!
>>>
>>>
>>> Regards,
>>>
>>> Stuart
>>>
>>>
>>> -----Original Message-----
>>> From: Kevin D. Kissell [mailto:kevink@paralogos.com]
>>> Sent: Friday, December 24, 2010 5:34 PM
>>> To: Anoop P A
>>> Cc: STUART VENTERS; Anoop P.A.; linux-mips@linux-mips.org
>>> Subject: Re: SMTC support status in latest git head.
>>>
>>>
>>> Ah, well, at least we have a stackframe.h fix that preserves David's
>>> performance tweak for the deeper pipelined processors.  In looking for
>>> this, I did notice that someone did some modification to the SMTC clock
>>> tick logic that I was skeptical had ever been tested.  If you've still
>>> got that kernel binary handy, you might check to see if it boots with
>>> maxtcs=1 maxvpes=1, maxtcs=2 maxvpes=1, and/or maxtcs=2 maxvpes=2.
>>>
>>> Oh, yes, and Merry Christmas one and all!
>>>
>>>               Regards,
>>>
>>>               Kevin K.
>>>
>>> On 12/24/10 8:02 AM, Anoop P A wrote:
>>>> On Fri, 2010-12-24 at 06:53 -0800, Kevin D. Kissell wrote:
>>>>> Excellent!  Now, does the attached patch (relative to 2.6.37.11) also
>>>>> fix things, while preserving the other fixes and performance enhancements?
>>>>>
>>>> I have tested that patch with 2.6.37 branch it well passes calibration
>>>> loop but hangs after switching to mips closource
>>>>
>>>> TC 6 going on-line as CPU 6
>>>> Brought up 7 CPUs
>>>> bio: create slab<bio-0>   at 0
>>>> SCSI subsystem initialized
>>>> Switching to clocksource MIPS
>>>>
>>>> I Presume this is a different issue as restoring older file didn't help
>>>> much to get rid of this hang.
>>>>
>>>> diff --git a/arch/mips/include/asm/stackframe.h
>>>> b/arch/mips/include/asm/stackframe.h
>>>> index 58730c5..7fc9f10 100644
>>>> --- a/arch/mips/include/asm/stackframe.h
>>>> +++ b/arch/mips/include/asm/stackframe.h
>>>> @@ -195,9 +195,9 @@
>>>>    		 * to cover the pipeline delay.
>>>>    		 */
>>>>    		.set	mips32
>>>> -		mfc0	v1, CP0_TCSTATUS
>>>> +		mfc0	v0, CP0_TCSTATUS
>>>>    		.set	mips0
>>>> -		LONG_S	v1, PT_TCSTATUS(sp)
>>>> +		LONG_S	v0, PT_TCSTATUS(sp)
>>>>    #endif /* CONFIG_MIPS_MT_SMTC */
>>>>    		LONG_S	$4, PT_R4(sp)
>>>>    		LONG_S	$5, PT_R5(sp)
>>>>
>>>>
>>>>> /K.
>>>>>
>>>>> On 12/24/10 6:39 AM, Anoop P A wrote:
>>>>>> Hi Kevin, Stuart ,
>>>>>>
>>>>>> Woohooo You guys spotted !.
>>>>>>
>>>>>>     http://git.linux-mips.org/?p=linux.git;a=commit;h=d5ec6e3c seems to be
>>>>>> the culprit
>>>>>>
>>>>>> Once I restored previous version of stackframe.h 2.6.33-stable started
>>>>>> booting !.
>>>>>>
>>>>>> Thanks,
>>>>>> Anoop
>>>>>>
>>>>>> On Fri, 2010-12-24 at 04:32 -0800, Kevin D. Kissell wrote:
>>>>>>> Thank you, Stuart!  I've spotted some definite breakage to SMTC between
>>>>>>> those versions.  In arch/mips/include/asm/stackframe.h, someone moved
>>>>>>> the store of the Status register value in SAVE_SOME (line 169 or 204,
>>>>>>> depending on the version) from two instructions after the mfc0 to a
>>>>>>> point after the #ifdef for SMTC, presumably to get better pipelining of
>>>>>>> the register access.  Unfortunately, the v1 register is also used in the
>>>>>>> SMTC-specific fragment to save TCStatus, so the Status value gets
>>>>>>> clobbered before it gets stored.  This will eventually result in the
>>>>>>> Status register getting a TCStatus value, which has some bits on common,
>>>>>>> but isn't identical and sooner or later Bad Things will happen.
>>>>>>>
>>>>>>> I'm a little surprised this wasn't caught by visual inspection of the patch.
>>>>>>>
>>>>>>> Possible solutions would include reverting the store of the CP0_STATUS
>>>>>>> value to the block above the #ifdef, or, to retain whatever performance
>>>>>>> advantage was obtained by moving the store downward, to use v0/$2
>>>>>>> instead of v1/$3, as the staging register for the TCStatus value.  I'd
>>>>>>> lean toward the second option, but I'm not in a position to test and
>>>>>>> submit a patch just now.
>>>>>>>
>>>>>>>                 Regards,
>>>>>>>
>>>>>>>                 Kevin K.
>>>>>>>
>>>>>>> On 12/23/10 1:09 PM, STUART VENTERS wrote:
>>>>>>>> Kevin,
>>>>>>>>
>>>>>>>> I'm not sure if it's useful,
>>>>>>>>        but finally I got the time to look at the two kernel versions Anoop pointed out.
>>>>>>>>         works   2.6.32-stable with patch 804
>>>>>>>>         works_not 2.6.33-stable
>>>>>>>>
>>>>>>>> greping for files with CONFIG_MIPS_MT_SMTC
>>>>>>>>        and looking for timer interrupt related stuff found the following differences:
>>>>>>>>
>>>>>>>>
>>>>>>>> arch/mips/include/asm/irq.h
>>>>>>>> arch/mips/kernel/irq.c
>>>>>>>>       do_IRQ
>>>>>>>>
>>>>>>>> arch/mips/include/asm/stackframe.h
>>>>>>>>       SAVE_SOME SAVE_TEMP get/set_saved_sp
>>>>>>>>
>>>>>>>> arch/mips/include/asm/time.h
>>>>>>>>       clocksource_set_clock
>>>>>>>>
>>>>>>>> arch/mips/kernel/process.c
>>>>>>>>       cpu_idle
>>>>>>>>
>>>>>>>> arch/mips/kernel/smtc.c
>>>>>>>>       __irq_entry
>>>>>>>>       ipi_decode
>>>>>>>>           SMTC_CLOCK_TICK
>>>>>>>>
>>>>>>>>
>>>>>>>> Enclosed are the two subsets of files for a more expert look.
>>>>>>>>
>>>>>>>> I'll try to look in more detail after Christmas.
>>>>>>>>
>>>>>>>>
>>>>>>>> Cheers,
>>>>>>>>
>>>>>>>> Stuart
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>