linux-kernel - Re: [mm/migrate] 9eeb73028c: stress-ng.memhotplug.ops_per

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Date:   Sun, 5 Sep 2021 20:57:55 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Huang, Ying" <ying.huang@...el.com>,
        kernel test robot <oliver.sang@...el.com>,
        Rui Zhang <rui.zhang@...el.com>, Chen Yu <yu.c.chen@...el.com>,
        Len Brown <len.brown@...el.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        0day robot <lkp@...el.com>, Yang Shi <shy828301@...il.com>,
        Zi Yan <ziy@...dia.com>, Michal Hocko <mhocko@...e.com>,
        Wei Xu <weixugc@...gle.com>,
        Oscar Salvador <osalvador@...e.de>,
        David Rientjes <rientjes@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Greg Thelen <gthelen@...gle.com>,
        Keith Busch <kbusch@...nel.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        feng.tang@...el.com, zhengjun.xing@...ux.intel.com,
        dave.hansen@...ux.intel.com, linux-mm@...ck.org,
        mm-commits@...r.kernel.org, torvalds@...ux-foundation.org
Subject: Re: [mm/migrate] 9eeb73028c: stress-ng.memhotplug.ops_per_sec -53.8%
 regression

On 9/5/21 6:53 PM, Huang, Ying wrote:
>> in testcase: stress-ng
>> on test machine: 96 threads 2 sockets Intel(R) Xeon(R) Gold 6252 CPU @ 2.10GHz with 192G memory
>> with following parameters:
>>
>> 	nr_threads: 10%
>> 	disk: 1HDD
>> 	testtime: 60s
>> 	fs: ext4
>> 	class: os
>> 	test: memhotplug
>> 	cpufreq_governor: performance
>> 	ucode: 0x5003006
>>
> Because we added some operations during online/offline CPU, it's
> expected that the performance of online/offline CPU will decrease.  In
> most cases, the performance of CPU hotplug isn't a big problem.  But
> then I remembers that the performance of the CPU hotplug may influence
> suspend/resume performance :-(
> 
> It appears that it is easy and reasonable to enclose the added
> operations inside #ifdef CONFIG_NUMA.  Is this sufficient to restore the
> performance of suspend/resume?

It's "memhotplug", not CPUs, right?

I didn't do was to actively go out and look for changes that would
affect the migration order.  The code just does regenerates and writes
the order blindly when it sees any memory hotplug event.  I have the
feeling the synchronize_rcu()s are what's killing us.

It would be pretty easy to go and generate the order, but only do the
update and the RCU bits when the order changes from what was there.

I guess we have a motivation now.