Re: [PATCH] mm: Only re-generate demotion targets when a numa node changes its N_CPU state

From: Baolin Wang <baolin.wang@linux.alibaba.com>
To: Oscar Salvador <osalvador@suse.de>, "Huang, Ying" <ying.huang@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Abhishek Goel <huntbag@linux.vnet.ibm.com>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: Only re-generate demotion targets when a numa node changes its N_CPU state
Date: Thu, 10 Mar 2022 14:24:38 +0800	[thread overview]
Message-ID: <be249da5-1f10-165a-02a4-a2342cd18692@linux.alibaba.com> (raw)
In-Reply-To: <YimRJS9XFE8nswkd@localhost.localdomain>

Hi Oscar,

On 3/10/2022 1:48 PM, Oscar Salvador wrote:
> On Thu, Mar 10, 2022 at 08:39:53AM +0800, Huang, Ying wrote:
>> We need to call set_migration_target_nodes() during system boot
>> somewhere, either here or in init_mm_internals().
> 
> Hi Huang Ying,
> 
> vmstat_cpu_online() already gets called during boot:
> 
> static struct cpuhp_step cpuhp_hp_states[] = {
> ...
> 	#ifdef CONFIG_SMP
> 	[CPUHP_CREATE_THREADS]= {
> 		.name			= "threads:prepare",
> 		.startup.single		= smpboot_create_threads,
> 		.teardown.single	= NULL,
> 		.cant_stop		= true,
> 	},
> ...
> 
> smpboot_create_threads
>   __smpboot_create_thread
>    smpboot_thread_fn
>     ht->thread_fn()
>      cpuhp_thread_fun
>       cpuhp_invoke_callback
>        vmstat_cpu_online
> 
> That for every CPU that is brought up during boot.
> 
> So unless I am missing something, I would say we are already covered
> there, right?

I've tested your patch, unfortunately it can not work. It already set 
the node with N_CPU state in init_cpu_node_state() earlier, so in 
vmstat_cpu_online() ,the sentence 'if(!node_state(cpu_to_node(cpu), 
N_CPU))' is always false, which means it will not call 
set_migration_target_nodes().

Like Huang Ying said, we should call set_migration_target_nodes() in 
migrate_on_reclaim_init() or init_mm_internals().