Re: [ucounts] 59ec71575a: will-it-scale.per_process_ops -10.3% regression

[Eric W. Biederman]

Oliver Sang <oliver.sang@xxxxxxxxx> writes:

> Hi Eric,
>
> On Wed, Jan 12, 2022 at 01:08:23PM -0600, Eric W. Biederman wrote:
>> kernel test robot <oliver.sang@xxxxxxxxx> writes:
>> 
>> > Greeting,
>> >
>> > FYI, we noticed a -10.3% regression of will-it-scale.per_process_ops due to commit:
>> 
>> 
>> I am having a bit of a challenge reading this report.  Does the -10.3%
>> mean that will-it-scale.per_process_ops is performing 70873 operations
>> per second instead of 78995 aka -10.3% fewer operations per second?
>
> yes
>
>> 
>> The per-profile.self-cycles-pp.do_dec_rlimits_put_ucounts now takes +2.6
>> more cycles, and perf-profile.self.cycles.pp.inc_rlimit_get_ucounts now
>> takes +3.8 more cycles.
>> 
>> Which if I read this correctly is a regression report about two
>> functions taking 3 or 4 cycles more after the code was rearranged inside
>> of them.
>> 
>> Is that correct?  Is this a regression report saying those two functions
>> are taking a few cycles more which leads to
>> will-it-scale.per_process_ops not being able to perform as many
>> operations per second?
>
> yes
>
>> 
>> Given the change in the commit mentioned I think this is all down to a
>> bug fix causing the code to be rearranged and a compiler optimizations.
>> 
>> I don't see much room to do anything about this.
>
> got it. this is a bug fix. Thanks for explanation!

The big surprise is that we haven't seen performance regressions up
until this point as the ucount rlimit work was merged.

Unless we can spot something very silly that the compiler is doing
with code generation (and those 3 or 4 cycles suggests it is possible
the compiler is being silly) it is going to take some reorganizing
to improve the performance.

Eric