Skylake: intel_pstate: fix PCT_TO_HWP macro

Jason Gerard DeRose jason at system76.com
Thu Sep 24 14:44:14 UTC 2015


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!
>>
>> --
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