Re: v3.13.5 intel_pstate: cpufreq: __cpufreq_add_dev: ->get() failed

[Dirk Brandewie <dirk.brandewie@gmail.com>]

Hi Patrick,

Sorry for the slow response you caught me taking a few days off :-)

On 03/07/2014 07:49 AM, Patrik Lundquist wrote:
> Hi,
>
> booting 3.13.5 on a dual socket Ivy Bridge-EP resulted in this error:
>
> [    0.194139] smpboot: CPU0: Intel(R) Xeon(R) CPU E5-2687W v2 @
> 3.40GHz (fam: 06, model: 3e, stepping: 04)
> ...
> [    0.246755] x86: Booting SMP configuration:
> [    0.250935] .... node  #0, CPUs:        #1  #2  #3  #4  #5  #6  #7
> [    0.357648] .... node  #1, CPUs:    #8  #9 #10 #11 #12 #13 #14 #15
> [    0.553293] x86: Booted up 2 nodes, 16 CPUs
> [    0.557666] smpboot: Total of 16 processors activated (108850.19 BogoMIPS)
> ...
> [    5.210204] Intel P-state driver initializing.
> [    5.232407] Intel pstate controlling: cpu 0
> [    5.253628] Intel pstate controlling: cpu 1
> [    5.274899] cpufreq: __cpufreq_add_dev: ->get() failed
> [    5.294856] Intel pstate controlling: cpu 2
> [    5.313553] Intel pstate controlling: cpu 3
> [    5.332526] Intel pstate controlling: cpu 4
> [    5.352347] Intel pstate controlling: cpu 5
> [    5.372112] Intel pstate controlling: cpu 6
> [    5.391097] Intel pstate controlling: cpu 7
> [    5.410272] Intel pstate controlling: cpu 8
> [    5.429092] Intel pstate controlling: cpu 9
> [    5.447714] Intel pstate controlling: cpu 10
> [    5.465872] Intel pstate controlling: cpu 11
> [    5.482942] Intel pstate controlling: cpu 12
> [    5.498414] Intel pstate controlling: cpu 13
> [    5.513586] Intel pstate controlling: cpu 14
> [    5.529200] Intel pstate controlling: cpu 15
>
> CPU 1 is alive and well but missing the cpufreq driver. The system is
> running fine otherwise.

This is a regression introduced by commit
da60ce9f2fa cpufreq: call cpufreq_driver->get() after calling ->init()


A return of zero from cpufreq_driver->get() is a warning at best for
intel_pstate at init time.  In fact zero is a valid return value AFAICT.
I should be doing something rational in any case.

>
> Looking closer at the problem gives that intel_pstate_init_cpu() is
> successful but intel_pstate_get(), which is called right after by
> cpufreq, fails.
>
> Since all_cpu_data[1] is initialized it gives that sample->freq must
> be zero. So the bug should be in intel_pstate_calc_busy() which
> incorrectly sets sample->freq to zero.
>
> I guess cpu->pstate.max_pstate == 4000000 since that's what
> cpuinfo_max_freq and scaling_max_freq is on the other cores.
>
> So the error is likely that core_pct is calculated to 0 in
> intel_pstate.c:intel_pstate_calc_busy():
>
>      core_pct = div64_u64(int_tofp(sample->aperf * 100),
>                   sample->mperf);
>

The truncation from the integer math is the likely culprit.

>
>
> Might be fixed by this commit but should be backported in that case:
>
> commit fcb6a15c2e7e76d493e6f91ea889ab40e1c643a4
> Author: Dirk Brandewie <dirk.j.brandewie@intel.com>
> Date:   Mon Feb 3 08:55:31 2014 -0800
>
>      intel_pstate: Take core C0 time into account for core busy calculation
>

This commit and the follow-on to fix a performance regression it introduced
are on my list to get into stable.

If you could file a bugzilla and add me to the CC list it would help me out
when I update stable.
>
>
> My options to explore the problem further by backporting patches and
> continuous reboots are a bit limited at the moment.
>
> Regards,
> Patrik
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