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Subject: Re: [mm] 2d146aa3aa: vm-scalability.throughput -36.4% regression
To:     Feng Tang <feng.tang@intel.com>
Cc:     Michal Koutn?? <mkoutny@suse.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        andi.kleen@intel.com, kernel test robot <oliver.sang@intel.com>,
        Roman Gushchin <guro@fb.com>, Michal Hocko <mhocko@suse.com>,
        Shakeel Butt <shakeelb@google.com>,
        Balbir Singh <bsingharora@gmail.com>,
        Tejun Heo <tj@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>, lkp@lists.01.org,
        kernel test robot <lkp@intel.com>,
        "Huang, Ying" <ying.huang@intel.com>,
        Zhengjun Xing <zhengjun.xing@linux.intel.com>
References: <20210816032855.GB72770@shbuild999.sh.intel.com>
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 <20210902013558.GA97410@shbuild999.sh.intel.com>
From:   Andi Kleen <ak@linux.intel.com>
Message-ID: <e8d087a4-a286-3561-66ef-1e9cfb38605f@linux.intel.com>
Date:   Wed, 1 Sep 2021 19:23:24 -0700
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On 9/1/2021 6:35 PM, Feng Tang wrote:
> On Wed, Sep 01, 2021 at 08:12:24AM -0700, Andi Kleen wrote:
>> Feng Tang <feng.tang@intel.com> writes:
>>> Yes, the tests I did is no matter where the 128B padding is added, the
>>> performance can be restored and even improved.
>> I wonder if we can find some cold, rarely accessed, data to put into the
>> padding to not waste it. Perhaps some name strings? Or the destroy
>> support, which doesn't sound like its commonly used.
> Yes, I tried to move 'destroy_work', 'destroy_rwork' and 'parent' over
> before the 'refcnt' together with some padding, it restored the performance
> to about 10~15% regression. (debug patch pasted below)
>
> But I'm not sure if we should use it, before we can fully explain the
> regression.

Narrowing it down to a single prefetcher seems good enough to me. The 
behavior of the prefetchers is fairly complicated and hard to predict, 
so I doubt you'll ever get a 100% step by step explanation.


-Andi