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

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Date:   Mon, 24 Feb 2020 21:20:14 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        ying.huang@...el.com
Cc:     Jiri Olsa <jolsa@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        kernel test robot <rong.a.chen@...el.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>,
        "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, "Huang, Ying" <ying.huang@...el.com>
Subject: Re: [LKP] Re: [perf/x86] 81ec3f3c4c: will-it-scale.per_process_ops
 -5.5% regression

On Mon, Feb 24, 2020 at 10:19:15AM +0800, Feng Tang wrote:
> > 
> > > No, it's not the biggest, I tried another machine 'Xeon Phi(TM) CPU 7295',
> > > which has 72C/288T, and the regression is not seen. This is the part
> > > confusing me :)
> > 
> > Hmm.
> > 
> > Humor me - what  happens if you turn off SMT on that Cascade Lake
> > system?  Maybe it's about the thread ID bit in the L1? Although again,
> > I'd have expected things to get _worse_ if it's the two fields that
> > are now in the same cachline thanks to alignment.
> 
> I'll try it and report back.

I added "nosmt=force" on the 2S 4 nodes 96C/192T machine, and tested
both 96 and 192 processes, and the regression still exists.

Also for Ying's suggestion about separate 'sigpending' to another cache
line than '__refcount', it can not heal the regression either.

Thanks,
Feng