Subject: Re: [PATCH 5/5] mm: migrate: Add vm.accel_page_copy in sysfs to
 control whether to use multi-threaded to accelerate page copy.
To: Zi Yan <zi.yan@sent.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
References: <20161122162530.2370-1-zi.yan@sent.com>
 <20161122162530.2370-6-zi.yan@sent.com>
Cc: akpm@linux-foundation.org, minchan@kernel.org, vbabka@suse.cz,
        mgorman@techsingularity.net, kirill.shutemov@linux.intel.com,
        n-horiguchi@ah.jp.nec.com, Zi Yan <zi.yan@cs.rutgers.edu>,
        Zi Yan <ziy@nvidia.com>
From: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Date: Thu, 24 Nov 2016 15:39:12 +0530
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On 11/22/2016 09:55 PM, Zi Yan wrote:
> From: Zi Yan <zi.yan@cs.rutgers.edu>
> 
> From: Zi Yan <ziy@nvidia.com>
> 
> Since base page migration did not gain any speedup from
> multi-threaded methods, we only accelerate the huge page case.
> 
> Signed-off-by: Zi Yan <ziy@nvidia.com>
> Signed-off-by: Zi Yan <zi.yan@cs.rutgers.edu>
> ---
>  kernel/sysctl.c | 11 +++++++++++
>  mm/migrate.c    |  6 ++++++
>  2 files changed, 17 insertions(+)
> 
> diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> index d54ce12..6c79444 100644
> --- a/kernel/sysctl.c
> +++ b/kernel/sysctl.c
> @@ -98,6 +98,8 @@
>  #if defined(CONFIG_SYSCTL)
> 
> 
> +extern int accel_page_copy;

Hmm, accel_mthread_copy because this is achieved by a multi threaded
copy mechanism.

> +
>  /* External variables not in a header file. */
>  extern int suid_dumpable;
>  #ifdef CONFIG_COREDUMP
> @@ -1361,6 +1363,15 @@ static struct ctl_table vm_table[] = {
>  		.proc_handler   = &hugetlb_mempolicy_sysctl_handler,
>  	},
>  #endif
> +	{
> +		.procname	= "accel_page_copy",
> +		.data		= &accel_page_copy,
> +		.maxlen		= sizeof(accel_page_copy),
> +		.mode		= 0644,
> +		.proc_handler	= proc_dointvec,
> +		.extra1		= &zero,
> +		.extra2		= &one,
> +	},
>  	 {
>  		.procname	= "hugetlb_shm_group",
>  		.data		= &sysctl_hugetlb_shm_group,
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 244ece6..e64b490 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -48,6 +48,8 @@
> 
>  #include "internal.h"
> 
> +int accel_page_copy = 1;
> +

So its enabled by default.

>  /*
>   * migrate_prep() needs to be called before we start compiling a list of pages
>   * to be migrated using isolate_lru_page(). If scheduling work on other CPUs is
> @@ -651,6 +653,10 @@ static void copy_huge_page(struct page *dst, struct page *src,
>  		nr_pages = hpage_nr_pages(src);
>  	}
> 
> +	/* Try to accelerate page migration if it is not specified in mode  */
> +	if (accel_page_copy)
> +		mode |= MIGRATE_MT;

So even if none of the system calls requested for a multi threaded copy,
this setting will override every thing and make it multi threaded.