Re: [LKP] Re: [perf/x86] 81ec3f3c4c: will-it-scale.per_process_ops -5.5% regression

From: Peter Zijlstra
Date: Fri Feb 21 2020 - 05:58:16 EST

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.