On Tue, 16 Nov 2021 12:12:37 +0530 "Aneesh Kumar K.V" <aneesh.kumar@xxxxxxxxxxxxx> wrote:
> This syscall can be used to set a home node for the MPOL_BIND
> and MPOL_PREFERRED_MANY memory policy. Users should use this
> syscall after setting up a memory policy for the specified range
> as shown below.
>
> mbind(p, nr_pages * page_size, MPOL_BIND, new_nodes->maskp,
> new_nodes->size + 1, 0);
> sys_set_mempolicy_home_node((unsigned long)p, nr_pages * page_size,
> home_node, 0);
>
> The syscall allows specifying a home node/preferred node from which kernel
> will fulfill memory allocation requests first.
>
> For address range with MPOL_BIND memory policy, if nodemask specifies more
> than one node, page allocations will come from the node in the nodemask
> with sufficient free memory that is closest to the home node/preferred node.
>
> For MPOL_PREFERRED_MANY if the nodemask specifies more than one node,
> page allocation will come from the node in the nodemask with sufficient
> free memory that is closest to the home node/preferred node. If there is
> not enough memory in all the nodes specified in the nodemask, the allocation
> will be attempted from the closest numa node to the home node in the system.
>
> This helps applications to hint at a memory allocation preference node
> and fallback to _only_ a set of nodes if the memory is not available
> on the preferred node. Fallback allocation is attempted from the node which is
> nearest to the preferred node.
>
> This helps applications to have control on memory allocation numa nodes and
> avoids default fallback to slow memory NUMA nodes. For example a system with
> NUMA nodes 1,2 and 3 with DRAM memory and 10, 11 and 12 of slow memory
>
> new_nodes = numa_bitmask_alloc(nr_nodes);
>
> numa_bitmask_setbit(new_nodes, 1);
> numa_bitmask_setbit(new_nodes, 2);
> numa_bitmask_setbit(new_nodes, 3);
>
> p = mmap(NULL, nr_pages * page_size, protflag, mapflag, -1, 0);
> mbind(p, nr_pages * page_size, MPOL_BIND, new_nodes->maskp, new_nodes->size + 1, 0);
>
> sys_set_mempolicy_home_node(p, nr_pages * page_size, 2, 0);
>
> This will allocate from nodes closer to node 2 and will make sure kernel will
> only allocate from nodes 1, 2 and3. Memory will not be allocated from slow memory
> nodes 10, 11 and 12
>
> With MPOL_PREFERRED_MANY on the other hand will first try to allocate from the
> closest node to node 2 from the node list 1, 2 and 3. If those nodes don't have
> enough memory, kernel will allocate from slow memory node 10, 11 and 12 which
> ever is closer to node 2.
>
> ...
>
> @@ -1477,6 +1478,60 @@ static long kernel_mbind(unsigned long start, unsigned long len,
> return do_mbind(start, len, lmode, mode_flags, &nodes, flags);
> }
>
> +SYSCALL_DEFINE4(set_mempolicy_home_node, unsigned long, start, unsigned long, len,
> + unsigned long, home_node, unsigned long, flags)
> +{
> + struct mm_struct *mm = current->mm;
> + struct vm_area_struct *vma;
> + struct mempolicy *new;
> + unsigned long vmstart;
> + unsigned long vmend;
> + unsigned long end;
> + int err = -ENOENT;
> +
> + if (start & ~PAGE_MASK)
> + return -EINVAL;
> + /*
> + * flags is used for future extension if any.
> + */
> + if (flags != 0)
> + return -EINVAL;
> +
> + if (!node_online(home_node))
> + return -EINVAL;
What's the thinking here? The node can later be offlined and the
kernel takes no action to reset home nodes, so why not permit setting a
presently-offline node as the home node? Checking here seems rather
arbitrary?
