linux-kernel - Re: [mm] 2d146aa3aa: vm-scalability.throughput -36.4% regression

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Date:   Wed, 1 Sep 2021 19:23:24 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Michal Koutn?? <mkoutny@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        andi.kleen@...el.com, kernel test robot <oliver.sang@...el.com>,
        Roman Gushchin <guro@...com>, Michal Hocko <mhocko@...e.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Balbir Singh <bsingharora@...il.com>,
        Tejun Heo <tj@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        kernel test robot <lkp@...el.com>,
        "Huang, Ying" <ying.huang@...el.com>,
        Zhengjun Xing <zhengjun.xing@...ux.intel.com>
Subject: Re: [mm] 2d146aa3aa: vm-scalability.throughput -36.4% regression


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@...el.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