Re: [RFC PATCH 6.1.y 0/1] cpufreq: amd-pstate: Enable CPU boost in passive and guided modes

[Mario Limonciello <mario.limonciello@amd.com>]

On 10/28/2024 16:33, Nabil S. Alramli wrote:
> Hi Mario,
> 
> Thank you for taking a look at my patch.
> 
> What do you think about the following for the commit message in the next
> revision of the PATCH, and omitting the cover letter since most of it is
> incorporated here?
> 
> ***********************************************************************
> 
> cpufreq: amd-pstate: Enable CPU boost in passive and guided modes
> 
> The CPU frequency cannot be boosted when using the amd_pstate driver in
> passive or guided mode. This is fixed here.

No need to say things like "I did this" or "this patch does that".
Just drop last sentence.

> 
> For example, on a host that has AMD EPYC 7662 64-Core processor without
> this patch running at full CPU load:
"On a host that has an AMD EPYC 7662 processor while running with 
amd-pstate configured for passive mode on full CPU load the processor 
only reaches 2.0 GHz."

> 
> $ for i in $(cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq); \
>    do ni=$(echo "scale=1; $i/1000000" | bc -l); echo "$ni GHz"; done | \
>    sort | uniq -c
> 
>      128 2.0 GHz
> 
> And with this patch:

On later kernels the CPU can reach 3.3GHz.

> 
> $ for i in $(cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq); \
>    do ni=$(echo "scale=1; $i/1000000" | bc -l); echo "$ni GHz"; done | \
>    sort | uniq -c
> 
>      128 3.3 GHz
> 
> The CPU frequency is dependent on a setting called highest_perf which is
> the multiplier used to compute it. The highest_perf value comes from
> cppc_init_perf when the driver is built-in and from pstate_init_perf when
> it is a loaded module. Both of these calls have the following condition:
> 
>          highest_perf = amd_get_highest_perf();
>          if (highest_perf > __cppc_highest_perf_)
>                  highest_perf = __cppc_highest_perf;
> 
> Where again __cppc_highest_perf is either the return from
> cppc_get_perf_caps in the built-in case or AMD_CPPC_HIGHEST_PERF in the
> module case. Both of these functions actually return the nominal value,
> Whereas the call to amd_get_highest_perf returns the correct boost
> value, so the condition tests true and highest_perf always ends up being
> the nominal value, therefore never having the ability to boost CPU
> frequency.
> 
> Since amd_get_highest_perf already returns the boost value we should
> just eliminate this check.
> 
> The issue was introduced in v6.1 via commit bedadcfb011f ("cpufreq:
> amd-pstate: Fix initial highest_perf value"), and exists in stable kernels

"In stable 6.1" kernels.

> 
> In v6.6.51, a large change, commit 1ec40a175a48 ("cpufreq: amd-pstate:
> Enable amd-pstate preferred core support"), was introduced which
> significantly refactored the code. This commit cannot be ported back on
> its own, and would require reviewing and cherry picking at least a few
> dozen of commits in cpufreq, amd-pstate, ACPI, CPPC.
> 
I'd just say "this has been fixed in 6.6.y and newer but due to 
refactoring that change isn't feasible to bring back to 6.1.y"

> This means kernels v6.1 up until v6.6.51 are affected by this
> significant performance issue, and cannot be easily remediated. This
> patch simplifies the fix to a single commit.

Again no need to say "this patch".

> 
> ***********************************************************************
> 
> On 10/28/2024 4:07 PM, Mario Limonciello wrote:
>> On 10/24/2024 22:23, Yuan, Perry wrote:
>>> [AMD Official Use Only - AMD Internal Distribution Only]
>>>
>>>> -----Original Message-----
>>>> From: Nabil S. Alramli <dev@nalramli.com>
>>>> Sent: Friday, October 25, 2024 9:05 AM
>>>> To: stable@vger.kernel.org
>>>> Cc: nalramli@fastly.com; jdamato@fastly.com; khubert@fastly.com;
>>>> Yuan, Perry
>>>> <Perry.Yuan@amd.com>; Meng, Li (Jassmine) <Li.Meng@amd.com>; Huang, Ray
>>>> <Ray.Huang@amd.com>; rafael@kernel.org; viresh.kumar@linaro.org; linux-
>>>> pm@vger.kernel.org; linux-kernel@vger.kernel.org; Nabil S. Alramli
>>>> <dev@nalramli.com>
>>>> Subject: [RFC PATCH 6.1.y 0/1] cpufreq: amd-pstate: Enable CPU boost
>>>> in passive
>>>> and guided modes
>>>>
>>>> Greetings,
>>>>
>>>> This is a RFC for a maintenance patch to an issue in the amd_pstate
>>>> driver where
>>>> CPU frequency cannot be boosted in passive or guided modes. Without
>>>> this patch,
>>>> AMD machines using stable kernels are unable to get their CPU
>>>> frequency boosted,
>>>> which is a significant performance issue.
>>>>
>>>> For example, on a host that has AMD EPYC 7662 64-Core processor
>>>> without this
>>>> patch running at full CPU load:
>>>>
>>>> $ for i in $(cat
>>>> /sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq); \
>>>>     do ni=$(echo "scale=1; $i/1000000" | bc -l); echo "$ni GHz"; done | \
>>>>     sort | uniq -c
>>>>
>>>>       128 2.0 GHz
>>>>
>>>> And with this patch:
>>>>
>>>> $ for i in $(cat
>>>> /sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq); \
>>>>     do ni=$(echo "scale=1; $i/1000000" | bc -l); echo "$ni GHz"; done | \
>>>>     sort | uniq -c
>>>>
>>>>       128 3.3 GHz
>>>>
>>>> I am not sure what the correct process is for submitting patches
>>>> which affect only
>>>> stable trees but not the current code base, and do not apply to the
>>>> current tree. As
>>>> such, I am submitting this directly to stable@, but please let me
>>>> know if I should be
>>>> submitting this elsewhere.
>>>>
>>>> The issue was introduced in v6.1 via commit bedadcfb011f ("cpufreq:
>>>> amd-pstate: Fix initial highest_perf value"), and exists in stable
>>>> kernels up until
>>>> v6.6.51.
>>>>
>>>> In v6.6.51, a large change, commit 1ec40a175a48 ("cpufreq: amd-pstate:
>>>> Enable amd-pstate preferred core support"), was introduced which
>>>> significantly
>>>> refactored the code. This commit cannot be ported back on its own,
>>>> and would
>>>> require reviewing and cherry picking at least a few dozen of commits
>>>> in cpufreq,
>>>> amd-pstate, ACPI, CPPC.
>>>>
>>>> This means kernels v6.1 up until v6.6.51 are affected by this
>>>> significant
>>>> performance issue, and cannot be easily remediated.
>>>>
>>>> Thank you for your attention and I look forward to your response in
>>>> regards to what
>>>> the best way to proceed is for getting this important performance fix
>>>> merged.
>>>>
>>>> Best Regards,
>>>>
>>>> Nabil S. Alramli (1):
>>>>     cpufreq: amd-pstate: Enable CPU boost in passive and guided modes
>>>>
>>>>    drivers/cpufreq/amd-pstate.c | 8 ++------
>>>>    1 file changed, 2 insertions(+), 6 deletions(-)
>>>>
>>>> -- 
>>>> 2.35.1
>>>
>>> Add Mario and Gautham for any help.
>>>
>>> Perry.
>>>
>>
>> If doing a patch that is only for 6.1.y then I think that some more of
>> this information from the cover letter needs to push into the patch itself.
>>
>> But looking over the patch and considering how much we've changed this
>> in the newer kernels I think it is a sensible localized change for 6.1.y.
>>
>> As this is fixed in 6.6.51 via a more complete backport patch please
>> only tag 6.1 in your "Cc: stable@vger.kernel.org" from the patch.
>>