Re: [PATCH 3/4] mm: workingset: add vmstat counter for shadow nodes

Mel Gorman
2018-10-16T08:49:29Z

On Tue, Oct 09, 2018 at 03:08:45PM -0700, Andrew Morton wrote:
> On Tue, 9 Oct 2018 14:47:32 -0400 Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> > --- a/mm/workingset.c
> > +++ b/mm/workingset.c
> > @@ -378,11 +378,17 @@ void workingset_update_node(struct xa_node *node)
> > * as node->private_list is protected by the i_pages lock.
> > */
> > if (node->count && node->count == node->nr_values) {
> > - if (list_empty(&node->private_list))
> > + if (list_empty(&node->private_list)) {
> > list_lru_add(&shadow_nodes, &node->private_list);
> > + __inc_lruvec_page_state(virt_to_page(node),
> > + WORKINGSET_NODES);
> > + }
> > } else {
> > - if (!list_empty(&node->private_list))
> > + if (!list_empty(&node->private_list)) {
> > list_lru_del(&shadow_nodes, &node->private_list);
> > + __dec_lruvec_page_state(virt_to_page(node),
> > + WORKINGSET_NODES);
> > + }
> > }
> > }
>
> A bit worried that we're depending on the caller's caller to have
> disabled interrupts to avoid subtle and rare errors.
>
> Can we do this?
>
> --- a/mm/workingset.c~mm-workingset-add-vmstat-counter-for-shadow-nodes-fix
> +++ a/mm/workingset.c
> @@ -377,6 +377,8 @@ void workingset_update_node(struct radix
> * already where they should be. The list_empty() test is safe
> * as node->private_list is protected by the i_pages lock.
> */
> + WARN_ON_ONCE(!irqs_disabled()); /* For __inc_lruvec_page_state */
> +
> if (node->count && node->count == node->exceptional) {
> if (list_empty(&node->private_list)) {
> list_lru_add(&shadow_nodes, &node->private_list);

Note that for whatever reason, I've observed that irqs_disabled() is
actually quite an expensive call. I'm not saying the warning is a bad
idea but it should not be sprinkled around unnecessary and may be more
suitable as a debug option.

--
Mel Gorman
SUSE Labs