linux-kernel - Re: [LKP] Re: [perf/x86] 81ec3f3c4c: will-it-scale.per_process

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Date:   Fri, 21 Feb 2020 11:58:00 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Feng Tang <feng.tang@...el.com>
Cc:     kernel test robot <rong.a.chen@...el.com>,
        Jiri Olsa <jolsa@...hat.com>, Ingo Molnar <mingo@...nel.org>,
        Vince Weaver <vincent.weaver@...ne.edu>,
        Jiri Olsa <jolsa@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Ravi Bangoria <ravi.bangoria@...ux.ibm.com>,
        Stephane Eranian <eranian@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        andi.kleen@...el.com, ying.huang@...el.com
Subject: Re: [LKP] Re: [perf/x86] 81ec3f3c4c: will-it-scale.per_process_ops
 -5.5% regression

On Fri, Feb 21, 2020 at 04:03:25PM +0800, Feng Tang wrote:

> We checked more on this. We run 14 times test for it, and the
> results are consistent about the 5.5% degradation, and we
> run the same test on several other platforms, whose test results
> are also consistent, though there are no such -5.5% seen.

> So likely, this commit changes the layout of the kernel text
> and data, which may trigger some cacheline level change. From
> the system map of the 2 kernels, a big trunk of symbol's address
> changes which follow the global "pmu",
> 
> 5.0-rc6-systemap:
> 
> ffffffff8221d000 d pmu
> ffffffff8221d100 d pmc_reserve_mutex
> ffffffff8221d120 d amd_f15_PMC53
> ffffffff8221d160 d amd_f15_PMC50
> 
> 5.0-rc6+pmu-change-systemap:
> 
> ffffffff8221d000 d pmu
> ffffffff8221d120 d pmc_reserve_mutex
> ffffffff8221d140 d amd_f15_PMC53
> ffffffff8221d180 d amd_f15_PMC50
> 
> But we can hardly identify which exact symbol is responsible
> for the change, as too many symbols are offseted. 

*groan*, that's horrible.

Thanks for digging into this.