Re: [PATCH 07/19] mm/mpol: Add MPOL_MF_NOOP

[Andrea Arcangeli <aarcange@redhat.com>]

On Tue, Jul 31, 2012 at 09:12:11PM +0200, Peter Zijlstra wrote:
> From: Lee Schermerhorn <lee.schermerhorn@hp.com>
> 
> This patch augments the MPOL_MF_LAZY feature by adding a "NOOP"
> policy to mbind().  When the NOOP policy is used with the 'MOVE
> and 'LAZY flags, mbind() [check_range()] will walk the specified
> range and unmap eligible pages so that they will be migrated on
> next touch.
> 
> This allows an application to prepare for a new phase of operation
> where different regions of shared storage will be assigned to
> worker threads, w/o changing policy.  Note that we could just use
> "default" policy in this case.  However, this also allows an
> application to request that pages be migrated, only if necessary,
> to follow any arbitrary policy that might currently apply to a
> range of pages, without knowing the policy, or without specifying
> multiple mbind()s for ranges with different policies.

This is a new kapi change. I could hardly understand the above so I
wonder how long it will take before userland programmers will be
familiar with MPOL_NOOP to actually use it in most apps? Could you
just enable/disable your logics using a sysfs knob instead?

enabling/disabling sched-numa is something an admin can easily do with
a sysfs control, patching and rebuilding a proprietary app using mbind
calls, no way, especially if the app is proprietary.

> 
> Signed-off-by: Lee Schermerhorn <lee.schermerhorn@hp.com>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
>  include/linux/mempolicy.h |    1 +
>  mm/mempolicy.c            |    8 ++++----
>  2 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/mempolicy.h b/include/linux/mempolicy.h
> index 87fabfa..668311a 100644
> --- a/include/linux/mempolicy.h
> +++ b/include/linux/mempolicy.h
> @@ -21,6 +21,7 @@ enum {
>  	MPOL_BIND,
>  	MPOL_INTERLEAVE,
>  	MPOL_LOCAL,
> +	MPOL_NOOP,		/* retain existing policy for range */
>  	MPOL_MAX,	/* always last member of enum */
>  };
>  
> diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> index 4fba5f2..251ef31 100644
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -251,10 +251,10 @@ static struct mempolicy *mpol_new(unsigned short mode, unsigned short flags,
>  	pr_debug("setting mode %d flags %d nodes[0] %lx\n",
>  		 mode, flags, nodes ? nodes_addr(*nodes)[0] : -1);
>  
> -	if (mode == MPOL_DEFAULT) {
> +	if (mode == MPOL_DEFAULT || mode == MPOL_NOOP) {
>  		if (nodes && !nodes_empty(*nodes))
>  			return ERR_PTR(-EINVAL);
> -		return NULL;	/* simply delete any existing policy */
> +		return NULL;
>  	}
>  	VM_BUG_ON(!nodes);
>  
> @@ -1069,7 +1069,7 @@ static long do_mbind(unsigned long start, unsigned long len,
>  	if (start & ~PAGE_MASK)
>  		return -EINVAL;
>  
> -	if (mode == MPOL_DEFAULT)
> +	if (mode == MPOL_DEFAULT || mode == MPOL_NOOP)
>  		flags &= ~MPOL_MF_STRICT;
>  
>  	len = (len + PAGE_SIZE - 1) & PAGE_MASK;
> @@ -1121,7 +1121,7 @@ static long do_mbind(unsigned long start, unsigned long len,
>  			  flags | MPOL_MF_INVERT, &pagelist);
>  
>  	err = PTR_ERR(vma);	/* maybe ... */
> -	if (!IS_ERR(vma))
> +	if (!IS_ERR(vma) && mode != MPOL_NOOP)
>  		err = mbind_range(mm, start, end, new);
>  
>  	if (!err) {
> -- 
> 1.7.2.3
> 
> 
> 
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