Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   "Kleen, Andi" <andi.kleen@intel.com>
To:     "Tang, Feng" <feng.tang@intel.com>,
        Peter Zijlstra <peterz@infradead.org>
CC:     "Chen, Rong A" <rong.a.chen@intel.com>,
        Jiri Olsa <jolsa@redhat.com>, Ingo Molnar <mingo@kernel.org>,
        Vince Weaver <vincent.weaver@maine.edu>,
        Jiri Olsa <jolsa@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
        Ravi Bangoria <ravi.bangoria@linux.ibm.com>,
        Stephane Eranian <eranian@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        "lkp@lists.01.org" <lkp@lists.01.org>,
        "Huang, Ying" <ying.huang@intel.com>
Subject: RE: [LKP] Re: [perf/x86] 81ec3f3c4c: will-it-scale.per_process_ops
 -5.5% regression
Thread-Topic: [LKP] Re: [perf/x86] 81ec3f3c4c: will-it-scale.per_process_ops
 -5.5% regression
Thread-Index: AQHV6I1t87VLv9kiVEyQ9lHGZx1Mi6gl75j1
Date:   Fri, 21 Feb 2020 18:05:02 +0000
Message-ID: <E8ECBC65D0B2554DAD44EBE43059B3740F1EAC@ORSMSX110.amr.corp.intel.com>
References: <20200205123216.GO12867@shao2-debian>
 <20200205125804.GM14879@hirez.programming.kicks-ass.net>,<20200221080325.GA67807@shbuild999.sh.intel.com>
In-Reply-To: <20200221080325.GA67807@shbuild999.sh.intel.com>
Accept-Language: en-US
Content-Language: en-US
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 8BIT
MIME-Version: 1.0
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


>So likely, this commit changes the layout of the kernel text
>and data, 

It should be only data here. text changes all the time anyways,
but data tends to be more stable.

> 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",

I wonder if it's the effect Andrew predicted a long time ago from
using __read_mostly. If all the __read_mostlies are moved somewhere
else the remaining read/write variables will get more sensitive to false sharing.

A simple experiment would be to add a __cacheline_aligned to align it,
and then add

____cacheline_aligned char dummy[0]; 

at the end to pad it to 64bytes.

Or hopefully Jiri can figure it out from the C2C data.

>btw, we've seen similar case that an irrelevant commit changes
>the benchmark, like a hugetlb patch improves pagefault test on
>a platform that never uses hugetlb https://lkml.org/lkml/2020/1/14/150

Yes we've had similar problems with the data segment before.

-Andi