Re: [PATCH] mm: swap: determine swap device by using page nid

[Yang Shi]

On Thu, Apr 7, 2022 at 12:52 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
>
> [Cc Aaron who has introduced the per node swap changes]
>
> On Wed 06-04-22 19:09:53, Yang Shi wrote:
> > The swap devices are linked to per node priority lists, the swap device
> > closer to the node has higher priority on that node's priority list.
> > This is supposed to improve I/O latency, particularly for some fast
> > devices.  But the current code gets nid by calling numa_node_id() which
> > actually returns the nid that the reclaimer is running on instead of the
> > nid that the page belongs to.
> >
> > Pass the page's nid dow to get_swap_pages() in order to pick up the
> > right swap device.  But it doesn't work for the swap slots cache which
> > is per cpu.  We could skip swap slots cache if the current node is not
> > the page's node, but it may be overkilling. So keep using the current
> > node's swap slots cache.  The issue was found by visual code inspection
> > so it is not sure how much improvement could be achieved due to lack of
> > suitable testing device.  But anyway the current code does violate the
> > design.
>
> Do you have any perf numbers for this change?

No, it was found by visual code inspection and offline discussion with
Huang Ying.

>
> > Cc: Huang Ying <ying.huang@xxxxxxxxx>
> > Signed-off-by: Yang Shi <shy828301@xxxxxxxxx>
> > ---
> >  include/linux/swap.h | 3 ++-
> >  mm/swap_slots.c      | 7 ++++---
> >  mm/swapfile.c        | 5 ++---
> >  3 files changed, 8 insertions(+), 7 deletions(-)
> >
> > diff --git a/include/linux/swap.h b/include/linux/swap.h
> > index 27093b477c5f..e442cf6b61ea 100644
> > --- a/include/linux/swap.h
> > +++ b/include/linux/swap.h
> > @@ -497,7 +497,8 @@ extern void si_swapinfo(struct sysinfo *);
> >  extern swp_entry_t get_swap_page(struct page *page);
> >  extern void put_swap_page(struct page *page, swp_entry_t entry);
> >  extern swp_entry_t get_swap_page_of_type(int);
> > -extern int get_swap_pages(int n, swp_entry_t swp_entries[], int entry_size);
> > +extern int get_swap_pages(int n, swp_entry_t swp_entries[], int entry_size,
> > +                       int node);
> >  extern int add_swap_count_continuation(swp_entry_t, gfp_t);
> >  extern void swap_shmem_alloc(swp_entry_t);
> >  extern int swap_duplicate(swp_entry_t);
> > diff --git a/mm/swap_slots.c b/mm/swap_slots.c
> > index 2b5531840583..a1c5cf6a4302 100644
> > --- a/mm/swap_slots.c
> > +++ b/mm/swap_slots.c
> > @@ -264,7 +264,7 @@ static int refill_swap_slots_cache(struct swap_slots_cache *cache)
> >       cache->cur = 0;
> >       if (swap_slot_cache_active)
> >               cache->nr = get_swap_pages(SWAP_SLOTS_CACHE_SIZE,
> > -                                        cache->slots, 1);
> > +                                        cache->slots, 1, numa_node_id());
> >
> >       return cache->nr;
> >  }
> > @@ -305,12 +305,13 @@ swp_entry_t get_swap_page(struct page *page)
> >  {
> >       swp_entry_t entry;
> >       struct swap_slots_cache *cache;
> > +     int nid = page_to_nid(page);
> >
> >       entry.val = 0;
> >
> >       if (PageTransHuge(page)) {
> >               if (IS_ENABLED(CONFIG_THP_SWAP))
> > -                     get_swap_pages(1, &entry, HPAGE_PMD_NR);
> > +                     get_swap_pages(1, &entry, HPAGE_PMD_NR, nid);
> >               goto out;
> >       }
> >
> > @@ -342,7 +343,7 @@ swp_entry_t get_swap_page(struct page *page)
> >                       goto out;
> >       }
> >
> > -     get_swap_pages(1, &entry, 1);
> > +     get_swap_pages(1, &entry, 1, nid);
> >  out:
> >       if (mem_cgroup_try_charge_swap(page, entry)) {
> >               put_swap_page(page, entry);
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 63c61f8b2611..151fffe0fd60 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -1036,13 +1036,13 @@ static void swap_free_cluster(struct swap_info_struct *si, unsigned long idx)
> >       swap_range_free(si, offset, SWAPFILE_CLUSTER);
> >  }
> >
> > -int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size)
> > +int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size,
> > +                int node)
> >  {
> >       unsigned long size = swap_entry_size(entry_size);
> >       struct swap_info_struct *si, *next;
> >       long avail_pgs;
> >       int n_ret = 0;
> > -     int node;
> >
> >       /* Only single cluster request supported */
> >       WARN_ON_ONCE(n_goal > 1 && size == SWAPFILE_CLUSTER);
> > @@ -1060,7 +1060,6 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size)
> >       atomic_long_sub(n_goal * size, &nr_swap_pages);
> >
> >  start_over:
> > -     node = numa_node_id();
> >       plist_for_each_entry_safe(si, next, &swap_avail_heads[node], avail_lists[node]) {
> >               /* requeue si to after same-priority siblings */
> >               plist_requeue(&si->avail_lists[node], &swap_avail_heads[node]);
> > --
> > 2.26.3
>
> --
> Michal Hocko
> SUSE Labs