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Date:   Wed, 9 Mar 2022 13:02:49 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Oscar Salvador <osalvador@suse.de>
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        "Huang, Ying" <ying.huang@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
Message-Id: <20220309130249.60733272aafb3bd5f2e6b88c@linux-foundation.org>
In-Reply-To: <20220309144644.4278-1-osalvador@suse.de>
References: <20220309144644.4278-1-osalvador@suse.de>
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On Wed,  9 Mar 2022 15:46:44 +0100 Oscar Salvador <osalvador@suse.de> wrote:

> Abhishek reported that after patch [1], hotplug operations are
> taking ~double the expected time. [2]
> 
> The reason behind is that the CPU callbacks that migrate_on_reclaim_init()
> sets always call set_migration_target_nodes() whenever a CPU is brought
> up/down.
> But we only care about numa nodes going from having cpus to become
> cpuless, and vice versa, as that influences the demotion_target order.
> 
> We do already have two CPU callbacks (vmstat_cpu_online() and vmstat_cpu_dead())
> that check exactly that, so get rid of the CPU callbacks in
> migrate_on_reclaim_init() and only call set_migration_target_nodes() from
> vmstat_cpu_{dead,online}() whenever a numa node change its N_CPU state.
> 
> [1] https://lore.kernel.org/linux-mm/20210721063926.3024591-2-ying.huang@intel.com/
> [2] https://lore.kernel.org/linux-mm/eb438ddd-2919-73d4-bd9f-b7eecdd9577a@linux.vnet.ibm.com/
> 
> ...
>
>  extern bool numa_demotion_enabled;
> +#ifdef CONFIG_HOTPLUG_CPU
> +extern void set_migration_target_nodes(void);
> +#else
> +static inline void set_migration_target_nodes() {}

Compiler won't like that.  Please test (compile and runtime) with
CONFIG_HOTPLUG_CPU=n.

>
> ...
>