linux-kernel - Re: [ucounts] 59ec71575a: will-it-scale.per_process

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Message-ID: <20220113021704.GA18396@xsang-OptiPlex-9020>
Date:   Thu, 13 Jan 2022 10:17:04 +0800
From:   Oliver Sang <oliver.sang@...el.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Alexey Gladkov <legion@...nel.org>,
        Gleb Fotengauer-Malinovskiy <glebfm@...linux.org>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        lkp@...el.com, ying.huang@...el.com, feng.tang@...el.com,
        zhengjun.xing@...ux.intel.com, fengwei.yin@...el.com
Subject: Re: [ucounts]  59ec71575a:  will-it-scale.per_process_ops -10.3%
 regression

Hi Eric,

On Wed, Jan 12, 2022 at 01:08:23PM -0600, Eric W. Biederman wrote:
> kernel test robot <oliver.sang@...el.com> 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!

> 
> Eric
>