Re: [edk2-devel] [RFC][PATCH v1] UefiCpuPkg/MpInitLib DXE: Reduce AP status check interval

["Laszlo Ersek" <lersek@redhat.com>]

On 03/16/20 02:37, Ni, Ray wrote:
>> The use case is valid, IMO. And the commit message is helpful.
>>
>> But I really think this constant should be PCD. Here's why I think a
>> platform might want to control it:
>>
>> - The best (finest) possible resolution for timer events is platform
>> dependent, IIUC. The duration of the "idle tick" is platform-specific.
>> And, it likely makes no sense to set AP_CHECK_INTERVAL to a duration
>> that's around, or under, what the arch timer resolution allows for.
>>
>> - In the other direction, CheckAndUpdateApsStatus() contains a loop that
>> counts up to CpuMpData->CpuCount. In a very large system (hundreds or
>> maybe thousands of APs) this function may have non-negligible cost.
>>
>> I suggest introducing a PCD for this (measured in msecs) and using 100
>> msecs as the default.
> 
> Laszlo,
> Adding a PCD means platform integrators need to consider which value to set.
> Most of the time, they may just use the default PCD value.
> Then, why not we add the PCD later when a real case is met?

The patch changes existent behavior; it is not for a newly introduced
feature.

Because most platforms are not in the edk2 tree, we don't know what
platforms could be regressed by increasing the polling frequency
tenfold. (And remember that the polling action has O(n) cost, where "n"
is the logical processor count.)

Under your suggestion, the expression "real case is met" amounts to
"someone reports a regression" (possibly after the next stable tag,
even). I don't think that's a good idea.

In particular, the patch is motivated by RegisterCpuFeaturesLib -- the
CpuFeaturesInitialize() function -- on some platform(s) that Hao uses.
But there are platforms that don't use RegisterCpuFeaturesLib, and still
use MpInitLib.

Thanks,
Laszlo