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Date: Wed, 8 May 2024 15:49:20 +0800
From: Kairui Song <ryncsn@...il.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
Matthew Wilcox <willy@...radead.org>, Chris Li <chrisl@...nel.org>,
Barry Song <v-songbaohua@...o.com>, Ryan Roberts <ryan.roberts@....com>, Neil Brown <neilb@...e.de>,
Minchan Kim <minchan@...nel.org>, Hugh Dickins <hughd@...gle.com>,
David Hildenbrand <david@...hat.com>, Yosry Ahmed <yosryahmed@...gle.com>, linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 12/12] mm/swap: reduce swap cache search space
On Wed, May 8, 2024 at 3:27 PM Huang, Ying <ying.huang@...el.com> wrote:
>
> Kairui Song <ryncsn@...il.com> writes:
>
> > From: Kairui Song <kasong@...cent.com>
> >
> > Currently we use one swap_address_space for every 64M chunk to reduce lock
> > contention, this is like having a set of smaller swap files inside one
> > swap device. But when doing swap cache look up or insert, we are
> > still using the offset of the whole large swap device. This is OK for
> > correctness, as the offset (key) is unique.
> >
> > But Xarray is specially optimized for small indexes, it creates the
> > radix tree levels lazily to be just enough to fit the largest key
> > stored in one Xarray. So we are wasting tree nodes unnecessarily.
> >
> > For 64M chunk it should only take at most 3 levels to contain everything. it should only take at most 3 levels to contain everything.
> > But if we are using the offset from the whole swap device, the offset (key)
> > value will be way beyond 64M, and so will the tree level.
> >
> > Optimize this by using a new helper swap_cache_index to get a swap
> > entry's unique offset in its own 64M swap_address_space.
> >
> > I see a ~1% performance gain in benchmark and actual workload with
> > high memory pressure.
> >
> > Test with `time memhog 128G` inside a 8G memcg using 128G swap (ramdisk
> > with SWP_SYNCHRONOUS_IO dropped, tested 3 times, results are stable. The
> > test result is similar but the improvement is smaller if SWP_SYNCHRONOUS_IO
> > is enabled, as swap out path can never skip swap cache):
> >
> > Before:
> > 6.07user 250.74system 4:17.26elapsed 99%CPU (0avgtext+0avgdata 8373376maxresident)k
> > 0inputs+0outputs (55major+33555018minor)pagefaults 0swaps
> >
> > After (1.8% faster):
> > 6.08user 246.09system 4:12.58elapsed 99%CPU (0avgtext+0avgdata 8373248maxresident)k
> > 0inputs+0outputs (54major+33555027minor)pagefaults 0swaps
> >
> > Similar result with MySQL and sysbench using swap:
> > Before:
> > 94055.61 qps
> >
> > After (0.8% faster):
> > 94834.91 qps
> >
> > Radix tree slab usage is also very slightly lower.
> >
> > Signed-off-by: Kairui Song <kasong@...cent.com>
> > ---
> > mm/huge_memory.c | 2 +-
> > mm/memcontrol.c | 2 +-
> > mm/mincore.c | 2 +-
> > mm/shmem.c | 2 +-
> > mm/swap.h | 15 +++++++++++++++
> > mm/swap_state.c | 19 ++++++++++---------
> > mm/swapfile.c | 6 +++---
> > 7 files changed, 32 insertions(+), 16 deletions(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index d35d526ed48f..45829cc049d2 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2918,7 +2918,7 @@ static void __split_huge_page(struct page *page, struct list_head *list,
> > split_page_memcg(head, order, new_order);
> >
> > if (folio_test_anon(folio) && folio_test_swapcache(folio)) {
> > - offset = swp_offset(folio->swap);
> > + offset = swap_cache_index(folio->swap);
> > swap_cache = swap_address_space(folio->swap);
> > xa_lock(&swap_cache->i_pages);
> > }
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index d11536ef59ef..81b005c459cb 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -6165,7 +6165,7 @@ static struct page *mc_handle_swap_pte(struct vm_area_struct *vma,
> > * Because swap_cache_get_folio() updates some statistics counter,
> > * we call find_get_page() with swapper_space directly.
> > */
> > - page = find_get_page(swap_address_space(ent), swp_offset(ent));
> > + page = find_get_page(swap_address_space(ent), swap_cache_index(ent));
> > entry->val = ent.val;
> >
> > return page;
> > diff --git a/mm/mincore.c b/mm/mincore.c
> > index dad3622cc963..e31cf1bde614 100644
> > --- a/mm/mincore.c
> > +++ b/mm/mincore.c
> > @@ -139,7 +139,7 @@ static int mincore_pte_range(pmd_t *pmd, unsigned long addr, unsigned long end,
> > } else {
> > #ifdef CONFIG_SWAP
> > *vec = mincore_page(swap_address_space(entry),
> > - swp_offset(entry));
> > + swap_cache_index(entry));
> > #else
> > WARN_ON(1);
> > *vec = 1;
> > diff --git a/mm/shmem.c b/mm/shmem.c
> > index fa2a0ed97507..326315c12feb 100644
> > --- a/mm/shmem.c
> > +++ b/mm/shmem.c
> > @@ -1756,7 +1756,7 @@ static int shmem_replace_folio(struct folio **foliop, gfp_t gfp,
> >
> > old = *foliop;
> > entry = old->swap;
> > - swap_index = swp_offset(entry);
> > + swap_index = swap_cache_index(entry);
> > swap_mapping = swap_address_space(entry);
> >
> > /*
> > diff --git a/mm/swap.h b/mm/swap.h
> > index 82023ab93205..93e3e1b58a7f 100644
> > --- a/mm/swap.h
> > +++ b/mm/swap.h
> > @@ -27,6 +27,7 @@ void __swap_writepage(struct folio *folio, struct writeback_control *wbc);
> > /* One swap address space for each 64M swap space */
> > #define SWAP_ADDRESS_SPACE_SHIFT 14
> > #define SWAP_ADDRESS_SPACE_PAGES (1 << SWAP_ADDRESS_SPACE_SHIFT)
> > +#define SWAP_ADDRESS_SPACE_MASK (BIT(SWAP_ADDRESS_SPACE_SHIFT) - 1)
>
> #define SWAP_ADDRESS_SPACE_MASK (SWAP_ADDRESS_SPACE_PAGES - 1)
> ?
>
> We can use BIT() in SWAP_ADDRESS_SPACE_PAGES definition.
>
I'll just use (SWAP_ADDRESS_SPACE_PAGES - 1) then, I was trying to
make the changes minimal, but prefered the BIT macro, a trivial
change.
> > extern struct address_space *swapper_spaces[];
> > #define swap_address_space(entry) \
> > (&swapper_spaces[swp_type(entry)][swp_offset(entry) \
> > @@ -40,6 +41,15 @@ static inline loff_t swap_dev_pos(swp_entry_t entry)
> > return ((loff_t)swp_offset(entry)) << PAGE_SHIFT;
> > }
> >
> > +/*
> > + * Return the swap cache index of the swap entry.
> > + */
> > +static inline pgoff_t swap_cache_index(swp_entry_t entry)
> > +{
> > + BUILD_BUG_ON((SWP_OFFSET_MASK | SWAP_ADDRESS_SPACE_MASK) != SWP_OFFSET_MASK);
> > + return swp_offset(entry) & SWAP_ADDRESS_SPACE_MASK;
> > +}
> > +
> > void show_swap_cache_info(void);
> > bool add_to_swap(struct folio *folio);
> > void *get_shadow_from_swap_cache(swp_entry_t entry);
> > @@ -86,6 +96,11 @@ static inline struct address_space *swap_address_space(swp_entry_t entry)
> > return NULL;
> > }
> >
> > +static inline pgoff_t swap_cache_index(swp_entry_t entry)
> > +{
> > + return 0;
> > +}
> > +
> > static inline void show_swap_cache_info(void)
> > {
> > }
> > diff --git a/mm/swap_state.c b/mm/swap_state.c
> > index 642c30d8376c..09415d4c7843 100644
> > --- a/mm/swap_state.c
> > +++ b/mm/swap_state.c
> > @@ -72,7 +72,7 @@ void show_swap_cache_info(void)
> > void *get_shadow_from_swap_cache(swp_entry_t entry)
> > {
> > struct address_space *address_space = swap_address_space(entry);
> > - pgoff_t idx = swp_offset(entry);
> > + pgoff_t idx = swap_cache_index(entry);
> > void *shadow;
> >
> > shadow = xa_load(&address_space->i_pages, idx);
> > @@ -89,7 +89,7 @@ int add_to_swap_cache(struct folio *folio, swp_entry_t entry,
> > gfp_t gfp, void **shadowp)
> > {
> > struct address_space *address_space = swap_address_space(entry);
> > - pgoff_t idx = swp_offset(entry);
> > + pgoff_t idx = swap_cache_index(entry);
> > XA_STATE_ORDER(xas, &address_space->i_pages, idx, folio_order(folio));
> > unsigned long i, nr = folio_nr_pages(folio);
> > void *old;
> > @@ -144,7 +144,7 @@ void __delete_from_swap_cache(struct folio *folio,
> > struct address_space *address_space = swap_address_space(entry);
> > int i;
> > long nr = folio_nr_pages(folio);
> > - pgoff_t idx = swp_offset(entry);
> > + pgoff_t idx = swap_cache_index(entry);
> > XA_STATE(xas, &address_space->i_pages, idx);
> >
> > xas_set_update(&xas, workingset_update_node);
> > @@ -248,18 +248,19 @@ void delete_from_swap_cache(struct folio *folio)
> > void clear_shadow_from_swap_cache(int type, unsigned long begin,
> > unsigned long end)
> > {
> > - unsigned long curr = begin;
> > + unsigned long curr = begin, offset;
>
> Better to rename "offset" as "index" to avoid confusion?
Good idea.
> > void *old;
> >
> > for (;;) {
> > + offset = curr & SWAP_ADDRESS_SPACE_MASK;
> > swp_entry_t entry = swp_entry(type, curr);
> > struct address_space *address_space = swap_address_space(entry);
> > - XA_STATE(xas, &address_space->i_pages, curr);
> > + XA_STATE(xas, &address_space->i_pages, offset);
> >
> > xas_set_update(&xas, workingset_update_node);
> >
> > xa_lock_irq(&address_space->i_pages);
> > - xas_for_each(&xas, old, end) {
> > + xas_for_each(&xas, old, offset + min(end - curr, SWAP_ADDRESS_SPACE_PAGES)) {
>
> Is there a bug in the original code? It doesn't check SWAP_ADDRESS_SPACE_PAGES.
That's OK, if the (end - curr) goes above SWAP_ADDRESS_SPACE_PAGES, it
means all content in current address_space needs to be purged.
xas_for_each will stop after it iterated all content in the current
address space. This is a bit hackish though.
>
> And should it be changed to
>
> xas_for_each(&xas, old, min(offset + end - curr, SWAP_ADDRESS_SPACE_PAGES))
It should be equivalent, as described above, but yeah, this looks
cleaner. I'll use your suggested code.
> ?
>
> > if (!xa_is_value(old))
> > continue;
> > xas_store(&xas, NULL);
> > @@ -350,7 +351,7 @@ struct folio *swap_cache_get_folio(swp_entry_t entry,
> > {
> > struct folio *folio;
> >
> > - folio = filemap_get_folio(swap_address_space(entry), swp_offset(entry));
> > + folio = filemap_get_folio(swap_address_space(entry), swap_cache_index(entry));
> > if (!IS_ERR(folio)) {
> > bool vma_ra = swap_use_vma_readahead();
> > bool readahead;
> > @@ -420,7 +421,7 @@ struct folio *filemap_get_incore_folio(struct address_space *mapping,
> > si = get_swap_device(swp);
> > if (!si)
> > return ERR_PTR(-ENOENT);
> > - index = swp_offset(swp);
> > + index = swap_cache_index(swp);
> > folio = filemap_get_folio(swap_address_space(swp), index);
> > put_swap_device(si);
> > return folio;
> > @@ -447,7 +448,7 @@ struct folio *__read_swap_cache_async(swp_entry_t entry, gfp_t gfp_mask,
> > * that would confuse statistics.
> > */
> > folio = filemap_get_folio(swap_address_space(entry),
> > - swp_offset(entry));
> > + swap_cache_index(entry));
> > if (!IS_ERR(folio))
> > goto got_folio;
> >
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 0b0ae6e8c764..4f0e8b2ac8aa 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -142,7 +142,7 @@ static int __try_to_reclaim_swap(struct swap_info_struct *si,
> > struct folio *folio;
> > int ret = 0;
> >
> > - folio = filemap_get_folio(swap_address_space(entry), offset);
> > + folio = filemap_get_folio(swap_address_space(entry), swap_cache_index(entry));
> > if (IS_ERR(folio))
> > return 0;
> > /*
> > @@ -2158,7 +2158,7 @@ static int try_to_unuse(unsigned int type)
> > (i = find_next_to_unuse(si, i)) != 0) {
> >
> > entry = swp_entry(type, i);
> > - folio = filemap_get_folio(swap_address_space(entry), i);
> > + folio = filemap_get_folio(swap_address_space(entry), swap_cache_index(entry));
> > if (IS_ERR(folio))
> > continue;
> >
> > @@ -3476,7 +3476,7 @@ EXPORT_SYMBOL_GPL(swapcache_mapping);
> >
> > pgoff_t __folio_swap_cache_index(struct folio *folio)
> > {
> > - return swp_offset(folio->swap);
> > + return swap_cache_index(folio->swap);
> > }
> > EXPORT_SYMBOL_GPL(__folio_swap_cache_index);
Thanks for the suggestions!
>
> --
> Best Regards,
> Huang, Ying
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