Re: [PATCH 2/6] futex: Implement FUTEX2_NUMA

["Christoph Lameter (Ampere)" <cl@gentwo.org>]

On Fri, 25 Oct 2024, Peter Zijlstra wrote:

> Extend the futex2 interface to be numa aware.
>
> When FUTEX2_NUMA is specified for a futex, the user value is extended
> to two words (of the same size). The first is the user value we all
> know, the second one will be the node to place this futex on.
>
>   struct futex_numa_32 {
> 	u32 val;
> 	u32 node;
>   };
>
> When node is set to ~0, WAIT will set it to the current node_id such
> that WAKE knows where to find it. If userspace corrupts the node value
> between WAIT and WAKE, the futex will not be found and no wakeup will
> happen.
>
> When FUTEX2_NUMA is not set, the node is simply an extention of the
> hash, such that traditional futexes are still interleaved over the
> nodes.


Would it be possible to follow the NUMA memory policy set up for a task
when making these decisions? We may not need a separate FUTEX2_NUMA
option. There are supportive functions in mm/mempolicy.c that will yield
a node for the futex logic to use.

See f.e. linux/include/uapi/mempolicy.h for the types of memory policy
that can be set for a task in current->mempolicy.


MPOL_DEFAULT	get local memory / use system default policy
MPOL_INTERLEAVE interleaved over nodes
MPOL_BIND	use the node specified in the task policy.
MPOL_LOCAL	get_local_memory

etc.

You will get a page or objects with the correct node by calling
alloc_pages() or kmalloc without GFP_THISNODE.

If you just need the node to use then use

mempolicy_slab_node()

and assign that to the node of the futex.

The function will determine which node to use depending on the active
memory policy.