Skylake: intel_pstate: fix PCT_TO_HWP macro

Jason Gerard DeRose jason at system76.com
Tue Sep 29 14:27:52 UTC 2015


On 09/29/2015 03:02 AM, Zhang, Xiong Y wrote:
> I tried the latest 15.10 kernel with "fix PCT_TO_HWP macro" patch, it indeed have system hang on SKL-S machine.
> Since upstream is still debugging this issue and seems the fix couldn't catch up 15.10 kernel freeze, I suggest Canonical to disable HWP on SKL-S with the attachment patch.
> The attachment patch is really small and won't generate regression, it only disable HWP on SKL-S and let legacy intel p state driver take effect on SKL-S. HWP is still enabled on SKL-Y/U

So does this patch disabled HWP on the Skylake K SKUs also? (Not sure 
whether you refer to them as SKL-K or not.)

Also, can you provide more details on the nature of the hang? Is it a 
temporary hang, or a permanent hang? Does it cause a kernel panic? Is 
there any data from the kernel that can confirm when these hangs occur 
(in dmesg, or /sys, or whatever)?

If it's a temporary hang, I think I have perhaps experienced it tested 
SKL-S (if so, it was pretty easy to spot during high bitrate 1080p video 
playback, but that could have been an unrelated issue).

Overall I think the rational for this is sound as obviously fixing the 
pstate driver is most important for SKL-U and SKL-H.

But I'm still personally concerned about this as System76 will soon be 
launching a laptop whose base config uses SKL-S and can be equipped with 
SKL-K.

If these hangs are temporary and don't particularly impact the user 
experience, my personal vote would probably be to keep HWP enabled on 
all Skylake SKUs :D

> thanks
>> -----Original Message-----
>> From: Jason Gerard DeRose [mailto:jason at system76.com]
>> Sent: Saturday, September 26, 2015 12:10 AM
>> To: Wang, Xiaolong; Zhang, Xiong Y
>> Cc: Ubuntu Kernel Team
>> Subject: Re: Skylake: intel_pstate: fix PCT_TO_HWP macro
>>
>> Wang,
>>
>> On 09/24/2015 09:01 PM, Wang, Xiaolong wrote:
>>> Hi Jason,
>>> We tested
>>>     a SKL-S Beta machine
>>>       (Processor Stepping in BIOS is Q0, F/M/S is 6/94/1), and
>>>     a SKL-S Qual machine
>>>       (Processor Stepping in BIOS is R0/S0, F/M/S is 6/94/3)
>>> On the Beta machine, Processor Frequency are found tied at the highest
>>> possible value -- HWP does not work;
>>> On the Qual machine, it hangs just after HWP enabled on each core.
>>
>> Thanks for sharing these insights.
>>
>>> Upstream have reproduced the hang on a Beta machine (weird),
>>> but they say removing "quiet" from boot parameters can boot to OS.
>>> I tried removing "quiet" on my Qual machine, it didn't work.
>>
>> Yeah, I'd be kinda doubtful that "quiet" would have any effect either
>> way, but I guess weirder things have happened :)
>>
>>> As I know, upstream is still checking it but had not get the root cause
>>> of that hang, and there is not a clear schedule to get that solved.
>>
>> Thank for the info!
>> -Jason
>>
>>> -Wang, Xiaolong
>>>
>>> On 2015/9/25 9:25, Zhang, Xiong Y wrote:
>>>> Hi, Xiaolong:
>>>>      Could you help me answer this ?
>>>>
>>>> thanks
>>>>> -----Original Message-----
>>>>> From: Jason Gerard DeRose [mailto:jason at system76.com]
>>>>> Sent: Thursday, September 24, 2015 10:44 PM
>>>>> To: Zhang, Xiong Y; Ubuntu Kernel Team
>>>>> Subject: Re: Skylake: intel_pstate: fix PCT_TO_HWP macro
>>>>>
>>>>> Thus far I've been testing this on SKL-S systems (one laptop, one
>>>>> desktop) and haven't experienced any system hangs.
>>>>>
>>>>> Can you share any insight into the conditions under which these hangs
>>>>> occur, so I can try to reproduce it?
>>>>>
>>>>> Also, when you say that hangs occur on "some SKL-S machines", are there
>>>>> any common denominators among the problematic machines? Or put
>> another
>>>>> way, are there some SKL-S machines that are completely immune to these
>>>>> hangs, and if so, what's the difference?
>>>>>
>>>>> Finally, an idea on the timeline for the complete fix?
>>>>>
>>>>> Thanks!
>>>>>
>>>>> On 09/23/2015 06:27 PM, Zhang, Xiong Y wrote:
>>>>>> Yes, this patch will resolve p states on SKL-Y and SKL-U, but it also
>> generate
>>>>> system hang on some SKL-S machines. We are still waiting extra patches to
>> fix
>>>>> it.
>>>>>>
>>>>>> thanks
>>>>>>> -----Original Message-----
>>>>>>> From: kernel-team-bounces at lists.ubuntu.com
>>>>>>> [mailto:kernel-team-bounces at lists.ubuntu.com] On Behalf Of Jason
>> Gerard
>>>>>>> DeRose
>>>>>>> Sent: Thursday, September 24, 2015 2:52 AM
>>>>>>> To: Ubuntu Kernel Team
>>>>>>> Subject: Skylake: intel_pstate: fix PCT_TO_HWP macro
>>>>>>>
>>>>>>> I think commit 74da56ce5c6715630aed3ccc0fcb86a9210c1a56 is very
>>>>> worthy
>>>>>>> of cherry-picking before the kernel freeze.
>>>>>>>
>>>>>>>     From the bug I just filed:
>>>>>>> https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1499040
>>>>>>>
>>>>>>> The intel pstate driver is currently quite broken on on Skylake hardware.
>>>>>>>
>>>>>>> The symptom is that the pstate driver can scale the frequency up, but
>>>>>>> wont then scale it back down. So as soon as you hit a decent amount of
>>>>>>> CPU load, you get stuck fluctuating in the turbo frequency ranges, never
>>>>>>> even dropping as low as the base clock frequency. Obviously this has
>>>>>>> very negative consequences for battery life.
>>>>>>>
>>>>>>> Commit 74da56ce5c6715630aed3ccc0fcb86a9210c1a56 fixes this
>>>>> upstream,
>>>>>>> which is included in 4.3rc2:
>>>>>>>
>>>>>>>
>>>>>
>> https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/commit/?id=74d
>>>>>>> a56ce5c6715630aed3ccc0fcb86a9210c1a56
>>>>>>>
>>>>>>> I built a custom Ubuntu kernel with this commit and tested it on a
>>>>>>> laptop with a Skylake i7-6700 CPU. As measured by powerstat, this
>> commit
>>>>>>> dropped the idle power consumption by 11 watts (from 34 to 23 watts,
>>>>>>> screen backlight was off during the test).
>>>>>>>
>>>>>>> I also tested this custom kernel on Haswell and Ivy Bridge hardware to
>>>>>>> make sure it didn't introduce any behaviour changes in the pstate driver
>>>>>>> on older hardware. As far as I can tell, on this older hardware pstate
>>>>>>> is behaving exactly the same with and without this commit.
>>>>>>>
>>>>>>> Thanks!
>>>>>>>
>>>>>>> --
>>>>>>> kernel-team mailing list
>>>>>>> kernel-team at lists.ubuntu.com
>>>>>>> https://lists.ubuntu.com/mailman/listinfo/kernel-team




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