Re: OpenBMC kernel and CONFIG_PREEMPT

[Zev Weiss <zweiss@equinix.com>]

On Fri, Apr 07, 2023 at 01:12:21PM PDT, William Kennington wrote:
>I just looked again, AFAICT it was never enabled on aspeed or npcm7xx, just
>the new 8xx part. Hopefully someone else with aspeed chips will confirm
>from their actual `.config`
>

Yeah, I'm pretty sure that's correct -- my aspeed builds already show:

    $ zcat /proc/config.gz | grep PREEMPT
    CONFIG_PREEMPT_NONE_BUILD=y
    CONFIG_PREEMPT_NONE=y
    # CONFIG_PREEMPT_VOLUNTARY is not set
    # CONFIG_PREEMPT is not set

Incidentally, out of curiosity I tried booting kernels built with
CONFIG_PREEMPT=y and didn't see any major changes in boot performance on
either ast2500 or ast2400.


Zev


>On Fri, Apr 7, 2023 at 1:05 PM William Kennington <wak@google.com> wrote:
>
>> It's interesting because I think it has a bigger impact on the 8xx chip
>> than the 7xx chip. AFAIK I was indirectly comparing this and it seems like
>> it didn't make a huge difference for us on our older BMCs. We have it
>> disabled on our 5.15 configs, but used it in 5.10. I don't think we noticed
>> a meaningful difference when porting forward. Probably you need a certain
>> amount of startup services trying to steal time from each other.
>>
>> On Fri, Apr 7, 2023 at 12:10 PM Patrick Williams <patrick@stwcx.xyz>
>> wrote:
>>
>>> Joel,
>>>
>>> Would you mind looking at this commit[1]?  Nuvoton seems to have a lot
>>> of data that suggests that disabling CONFIG_PREEMPT leads to
>>> significantly better performance.  The boot time is almost 50% faster.
>>>
>>> I'm surprised that:
>>>
>>>     1. CONFIG_PREEMPT really results in that worse of performance.
>>>     2. The Nuvoton chip is special from any other ARM variant in this
>>>        regard.
>>>
>>> Should we disable this across the board on all our platforms?
>>>
>>> 1.
>>> https://github.com/openbmc/openbmc/commit/52ec37dd0b8776ce7f43e65e0be578c3606340b7
>>> --
>>> Patrick Williams
>>>
>>