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Date:   Tue, 18 Feb 2020 09:40:23 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Wei Yang <richardw.yang@linux.intel.com>
Cc:     akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, rientjes@google.com
Subject: Re: [PATCH] mm/vmscan.c: remove cpu online notification for now
Message-ID: <20200218084023.GC21113@dhcp22.suse.cz>
References: <20200218004354.24996-1-richardw.yang@linux.intel.com>
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On Tue 18-02-20 08:43:54, Wei Yang wrote:
> The cpu online notification is used to adjust kswapd cpu affinity when a
> NUMA node gains a new CPU.
> 
> Since currently we don't see a real runtime configuration like this,
> let's drop this online notification for now.

This deserves much more explanation IMHO. What would you say about the
following.
"
kswapd kernel thread starts either with a CPU affinity set to the full
cpu mask of its target node or without any affinity at all if the node
is CPUless. There is a cpu hotplug callback (kswapd_cpu_online) that
implements an elaborate way to update this mask when a cpu is onlined.

It is not really clear whether there is any actual benefit from this
scheme. Completely CPU-less NUMA nodes rarely gain a new CPU during
runtime. Drop the code for that reason. If there is a real usecase then
we can resurrect and simplify the code.
"
> 
> Suggested-by: Michal Hocko <mhocko@kernel.org>
> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>

Acked-by: Michal Hocko <mhocko@suse.com>

> 
> ---
> v3:
>   * remove the cpu online notification suggested by Michal
> v2:
>   * rephrase the changelog
> ---
>  mm/vmscan.c | 27 +--------------------------
>  1 file changed, 1 insertion(+), 26 deletions(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index 665f33258cd7..a4fdf3dc8887 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -4023,27 +4023,6 @@ unsigned long shrink_all_memory(unsigned long nr_to_reclaim)
>  }
>  #endif /* CONFIG_HIBERNATION */
>  
> -/* It's optimal to keep kswapds on the same CPUs as their memory, but
> -   not required for correctness.  So if the last cpu in a node goes
> -   away, we get changed to run anywhere: as the first one comes back,
> -   restore their cpu bindings. */
> -static int kswapd_cpu_online(unsigned int cpu)
> -{
> -	int nid;
> -
> -	for_each_node_state(nid, N_MEMORY) {
> -		pg_data_t *pgdat = NODE_DATA(nid);
> -		const struct cpumask *mask;
> -
> -		mask = cpumask_of_node(pgdat->node_id);
> -
> -		if (cpumask_any_and(cpu_online_mask, mask) < nr_cpu_ids)
> -			/* One of our CPUs online: restore mask */
> -			set_cpus_allowed_ptr(pgdat->kswapd, mask);
> -	}
> -	return 0;
> -}
> -
>  /*
>   * This kswapd start function will be called by init and node-hot-add.
>   * On node-hot-add, kswapd will moved to proper cpus if cpus are hot-added.
> @@ -4083,15 +4062,11 @@ void kswapd_stop(int nid)
>  
>  static int __init kswapd_init(void)
>  {
> -	int nid, ret;
> +	int nid;
>  
>  	swap_setup();
>  	for_each_node_state(nid, N_MEMORY)
>   		kswapd_run(nid);
> -	ret = cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
> -					"mm/vmscan:online", kswapd_cpu_online,
> -					NULL);
> -	WARN_ON(ret < 0);
>  	return 0;
>  }
>  
> -- 
> 2.17.1

-- 
Michal Hocko
SUSE Labs