Commit 262ef8e5 authored by Mateusz Guzik's avatar Mateusz Guzik Committed by Andrew Morton
Browse files

fork: stop ignoring NUMA while handling cached thread stacks 

1. the numa parameter was straight up ignored.
2. nothing was done to check if the to-be-cached/allocated stack matches
   the local node

The id remains ignored on free in case of memoryless nodes.

Note the current caching is already bad as the cache keeps overflowing
and a different solution is needed for the long run, to be worked
out(tm).

Stats collected over a kernel build with the patch with the following
topology:
  NUMA node(s):              2
  NUMA node0 CPU(s):         0-11
  NUMA node1 CPU(s):         12-23

caller's node vs stack backing pages on free:
matching:	50083 (70%)
mismatched:	21492 (30%)

caching efficiency:
cached:		32651 (65.2%)
dropped:	17432 (34.8%)

Link: https://lkml.kernel.org/r/20251120054015.3019419-1-mjguzik@gmail.com


Signed-off-by: default avatarMateusz Guzik <mjguzik@gmail.com>
Reviewed-by: default avatarLinus Walleij <linus.walleij@linaro.org>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Linus Waleij <linus.walleij@linaro.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Kees Cook <kees@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 94984bfe

kernel/fork.c

+53 −10
Original line number Diff line number Diff line
@@ -208,9 +208,55 @@ struct vm_stack { 
	struct vm_struct *stack_vm_area;

};



static struct vm_struct *alloc_thread_stack_node_from_cache(struct task_struct *tsk, int node)

{

	struct vm_struct *vm_area;

	unsigned int i;



	/*

	 * If the node has memory, we are guaranteed the stacks are backed by local pages.

	 * Otherwise the pages are arbitrary.

	 *

	 * Note that depending on cpuset it is possible we will get migrated to a different

	 * node immediately after allocating here, so this does *not* guarantee locality for

	 * arbitrary callers.

	 */

	scoped_guard(preempt) {

		if (node != NUMA_NO_NODE && numa_node_id() != node)

			return NULL;



		for (i = 0; i < NR_CACHED_STACKS; i++) {

			vm_area = this_cpu_xchg(cached_stacks[i], NULL);

			if (vm_area)

				return vm_area;

		}

	}



	return NULL;

}



static bool try_release_thread_stack_to_cache(struct vm_struct *vm_area)

{

	unsigned int i;

	int nid;



	/*

	 * Don't cache stacks if any of the pages don't match the local domain, unless

	 * there is no local memory to begin with.

	 *

	 * Note that lack of local memory does not automatically mean it makes no difference

	 * performance-wise which other domain backs the stack. In this case we are merely

	 * trying to avoid constantly going to vmalloc.

	 */

	scoped_guard(preempt) {

		nid = numa_node_id();

		if (node_state(nid, N_MEMORY)) {

			for (i = 0; i < vm_area->nr_pages; i++) {

				struct page *page = vm_area->pages[i];

				if (page_to_nid(page) != nid)

					return false;

			}

		}



		for (i = 0; i < NR_CACHED_STACKS; i++) {

			struct vm_struct *tmp = NULL;
@@ -218,6 +264,7 @@ static bool try_release_thread_stack_to_cache(struct vm_struct *vm_area) 
			if (this_cpu_try_cmpxchg(cached_stacks[i], &tmp, vm_area))

				return true;

		}

	}

	return false;

}
@@ -283,13 +330,9 @@ static int alloc_thread_stack_node(struct task_struct *tsk, int node) 
{

	struct vm_struct *vm_area;

	void *stack;

	int i;



	for (i = 0; i < NR_CACHED_STACKS; i++) {

		vm_area = this_cpu_xchg(cached_stacks[i], NULL);

		if (!vm_area)

			continue;



	vm_area = alloc_thread_stack_node_from_cache(tsk, node);

	if (vm_area) {

		if (memcg_charge_kernel_stack(vm_area)) {

			vfree(vm_area->addr);

			return -ENOMEM;