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Message-ID: <87tuibucit.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Thu, 23 Sep 2021 12:44:10 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>,
<linux-kernel@...r.kernel.org>, <oliver.sang@...el.com>,
<mhocko@...e.com>, <weixugc@...gle.com>, <osalvador@...e.de>,
<rientjes@...gle.com>, <dan.j.williams@...el.com>,
<david@...hat.com>, <gthelen@...gle.com>,
<yang.shi@...ux.alibaba.com>, <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/2] mm/migrate: optimize hotplug-time demotion order
updates
"Huang, Ying" <ying.huang@...el.com> writes:
> Dave Hansen <dave.hansen@...el.com> writes:
>
>> On 9/17/21 5:55 PM, Huang, Ying wrote:
>>>> @@ -3147,6 +3177,16 @@ static void __set_migration_target_nodes
>>>> int node;
>>>>
>>>> /*
>>>> + * The "migration path" array is heavily optimized
>>>> + * for reads. This is the write side which incurs a
>>>> + * very heavy synchronize_rcu(). Avoid this overhead
>>>> + * when nothing of consequence has changed since the
>>>> + * last write.
>>>> + */
>>>> + if (!node_demotion_topo_changed())
>>>> + return;
>>>> +
>>>> + /*
>>>> * Avoid any oddities like cycles that could occur
>>>> * from changes in the topology. This will leave
>>>> * a momentary gap when migration is disabled.
>>> Now synchronize_rcu() is called in disable_all_migrate_targets(), which
>>> is called for MEM_GOING_OFFLINE. Can we remove the synchronize_rcu()
>>> from disable_all_migrate_targets() and call it in
>>> __set_migration_target_nodes() before we update the node_demotion[]?
>>
>> I see what you are saying. This patch just targeted
>> __set_migration_target_nodes() which is called in for
>> MEM_ONLINE/OFFLINE. But, it missed MEM_GOING_OFFLINE's call to
>> disable_all_migrate_targets().
>>
>> I think I found something better than what I had in this patch, or the
>> tweak you suggested: The 'memory_notify->status_change_nid' field is
>> passed to all memory hotplug notifiers and tells us whether the node is
>> going online/offline. Instead of trying to track the changes, I think
>> we can simply rely on it to tell us when a node is going online/offline.
>>
>> This removes the need for the demotion code to track *any* state. I've
>> attached a totally untested patch to do this.
>
> Yes. This sounds good. I will try to test this patch on my side.
I have tested this patch, it works as expected for memory hotplug. I
have asked 0-Day guys to test the original test case, but 0-Day doesn't
work very well for now, we need to wait for a while for 0-Day test
result.
Best Regards,
Huang, Ying
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