linux-kernel - Re: [PATCH 7/9] mm, page_alloc: remove stop_machine from build_all

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Date:   Thu, 20 Jul 2017 09:24:58 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        Johannes Weiner <hannes@...xchg.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 7/9] mm, page_alloc: remove stop_machine from
 build_all_zonelists

On 07/14/2017 10:00 AM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
> 
> build_all_zonelists has been (ab)using stop_machine to make sure that
> zonelists do not change while somebody is looking at them. This is
> is just a gross hack because a) it complicates the context from which
> we can call build_all_zonelists (see 3f906ba23689 ("mm/memory-hotplug:
> switch locking to a percpu rwsem")) and b) is is not really necessary
> especially after "mm, page_alloc: simplify zonelist initialization".
> 
> Updates of the zonelists happen very seldom, basically only when a zone
> becomes populated during memory online or when it loses all the memory
> during offline. A racing iteration over zonelists could either miss a
> zone or try to work on one zone twice. Both of these are something we
> can live with occasionally because there will always be at least one
> zone visible so we are not likely to fail allocation too easily for
> example.
> 
> Signed-off-by: Michal Hocko <mhocko@...e.com>

Some stress testing of this would still be worth, IMHO.

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  mm/page_alloc.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 78bd62418380..217889ecd13f 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5066,8 +5066,7 @@ static DEFINE_PER_CPU(struct per_cpu_nodestat, boot_nodestats);
>   */
>  DEFINE_MUTEX(zonelists_mutex);
>  
> -/* return values int ....just for stop_machine() */
> -static int __build_all_zonelists(void *data)
> +static void __build_all_zonelists(void *data)
>  {
>  	int nid;
>  	int cpu;
> @@ -5103,8 +5102,6 @@ static int __build_all_zonelists(void *data)
>  			set_cpu_numa_mem(cpu, local_memory_node(cpu_to_node(cpu)));
>  #endif
>  	}
> -
> -	return 0;
>  }
>  
>  static noinline void __init
> @@ -5147,9 +5144,7 @@ void __ref build_all_zonelists(pg_data_t *pgdat)
>  	if (system_state == SYSTEM_BOOTING) {
>  		build_all_zonelists_init();
>  	} else {
> -		/* we have to stop all cpus to guarantee there is no user
> -		   of zonelist */
> -		stop_machine_cpuslocked(__build_all_zonelists, pgdat, NULL);
> +		__build_all_zonelists(pgdat);
>  		/* cpuset refresh routine should be here */
>  	}
>  	vm_total_pages = nr_free_pagecache_pages();
>