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Message-ID: <44dcd73b-0e5d-408b-8640-ca1203f62525@arm.com>
Date: Wed, 11 Oct 2023 08:44:27 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: Andrew Morton <akpm@...ux-foundation.org>,
David Hildenbrand <david@...hat.com>,
Matthew Wilcox <willy@...radead.org>,
Huang Ying <ying.huang@...el.com>,
Gao Xiang <xiang@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
Yang Shi <shy828301@...il.com>, Michal Hocko <mhocko@...e.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC PATCH v1 2/2] mm: swap: Swap-out small-sized THP without
splitting
On 10/10/2023 15:21, Ryan Roberts wrote:
> The upcoming anonymous small-sized THP feature enables performance
> improvements by allocating large folios for anonymous memory. However
> I've observed that on an arm64 system running a parallel workload (e.g.
> kernel compilation) across many cores, under high memory pressure, the
> speed regresses. This is due to bottlenecking on the increased number of
> TLBIs added due to all the extra folio splitting.
>
> Therefore, solve this regression by adding support for swapping out
> small-sized THP without needing to split the folio, just like is already
> done for PMD-sized THP. This change only applies when CONFIG_THP_SWAP is
> enabled, and when the swap backing store is a non-rotating block device
> - these are the same constraints as for the existing PMD-sized THP
> swap-out support.
>
> Note that no attempt is made to swap-in THP here - this is still done
> page-by-page, like for PMD-sized THP.
>
> The main change here is to improve the swap entry allocator so that it
> can allocate any power-of-2 number of contiguous entries between [4, (1
> << PMD_ORDER)]. This is done by allocating a cluster for each distinct
> order and allocating sequentially from it until the cluster is full.
> This ensures that we don't need to search the map and we get no
> fragmentation due to alignment padding for different orders in the
> cluster. If there is no current cluster for a given order, we attempt to
> allocate a free cluster from the list. If there are no free clusters, we
> fail the allocation and the caller falls back to splitting the folio and
> allocates individual entries (as per existing PMD-sized THP fallback).
>
> As far as I can tell, this should not cause any extra fragmentation
> concerns, given how similar it is to the existing PMD-sized THP
> allocation mechanism. There will be up to (PMD_ORDER-1) clusters in
> concurrent use though. In practice, the number of orders in use will be
> small though.
>
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> ---
> include/linux/swap.h | 7 ++++++
> mm/swapfile.c | 60 +++++++++++++++++++++++++++++++++-----------
> mm/vmscan.c | 10 +++++---
> 3 files changed, 59 insertions(+), 18 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index a073366a227c..fc55b760aeff 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -320,6 +320,13 @@ struct swap_info_struct {
> */
> struct work_struct discard_work; /* discard worker */
> struct swap_cluster_list discard_clusters; /* discard clusters list */
> + unsigned int large_next[PMD_ORDER]; /*
Oh dear, I've tripped over this twice in a week now... PMD_ORDER is not a
compile-time const on powerpc, so results in build fail. This would have to be
allocated at init time, I guess.
> + * next free offset within current
> + * allocation cluster for large
> + * folios, or UINT_MAX if no current
> + * cluster. Index is (order - 1).
> + * Only when cluster_info is used.
> + */
> struct plist_node avail_lists[]; /*
> * entries in swap_avail_heads, one
> * entry per node.
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index c668838fa660..f8093dedc866 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -987,8 +987,10 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
> return n_ret;
> }
>
> -static int swap_alloc_cluster(struct swap_info_struct *si, swp_entry_t *slot)
> +static int swap_alloc_large(struct swap_info_struct *si, swp_entry_t *slot,
> + unsigned int nr_pages)
> {
> + int order;
> unsigned long idx;
> struct swap_cluster_info *ci;
> unsigned long offset;
> @@ -1002,20 +1004,47 @@ static int swap_alloc_cluster(struct swap_info_struct *si, swp_entry_t *slot)
> return 0;
> }
>
> - if (cluster_list_empty(&si->free_clusters))
> - return 0;
> + VM_WARN_ON(nr_pages < 2);
> + VM_WARN_ON(nr_pages > SWAPFILE_CLUSTER);
> + VM_WARN_ON(!is_power_of_2(nr_pages));
>
> - idx = cluster_list_first(&si->free_clusters);
> - offset = idx * SWAPFILE_CLUSTER;
> - ci = lock_cluster(si, offset);
> - alloc_cluster(si, idx);
> - cluster_set_count_flag(ci, SWAPFILE_CLUSTER, 0);
> + order = ilog2(nr_pages);
> + offset = si->large_next[order - 1];
> +
> + if (offset == UINT_MAX) {
> + if (cluster_list_empty(&si->free_clusters))
> + return 0;
>
> - memset(si->swap_map + offset, SWAP_HAS_CACHE, SWAPFILE_CLUSTER);
> + idx = cluster_list_first(&si->free_clusters);
> + offset = idx * SWAPFILE_CLUSTER;
> +
> + ci = lock_cluster(si, offset);
> + alloc_cluster(si, idx);
> + cluster_set_count_flag(ci, SWAPFILE_CLUSTER, 0);
> +
> + /*
> + * If scan_swap_map_slots() can't find a free cluster, it will
> + * check si->swap_map directly. To make sure this standby
> + * cluster isn't taken by scan_swap_map_slots(), mark the swap
> + * entries bad (occupied). (same approach as discard).
> + */
> + memset(si->swap_map + offset + nr_pages, SWAP_MAP_BAD,
> + SWAPFILE_CLUSTER - nr_pages);
> + } else {
> + idx = offset / SWAPFILE_CLUSTER;
> + ci = lock_cluster(si, offset);
> + }
> +
> + memset(si->swap_map + offset, SWAP_HAS_CACHE, nr_pages);
> unlock_cluster(ci);
> - swap_range_alloc(si, offset, SWAPFILE_CLUSTER);
> + swap_range_alloc(si, offset, nr_pages);
> *slot = swp_entry(si->type, offset);
>
> + offset += nr_pages;
> + if (idx != offset / SWAPFILE_CLUSTER)
> + offset = UINT_MAX;
> + si->large_next[order - 1] = offset;
> +
> return 1;
> }
>
> @@ -1041,7 +1070,7 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size)
> int node;
>
> /* Only single cluster request supported */
> - WARN_ON_ONCE(n_goal > 1 && size == SWAPFILE_CLUSTER);
> + WARN_ON_ONCE(n_goal > 1 && size > 1);
>
> spin_lock(&swap_avail_lock);
>
> @@ -1078,14 +1107,14 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size)
> spin_unlock(&si->lock);
> goto nextsi;
> }
> - if (size == SWAPFILE_CLUSTER) {
> + if (size > 1) {
> if (si->flags & SWP_BLKDEV)
> - n_ret = swap_alloc_cluster(si, swp_entries);
> + n_ret = swap_alloc_large(si, swp_entries, size);
> } else
> n_ret = scan_swap_map_slots(si, SWAP_HAS_CACHE,
> n_goal, swp_entries);
> spin_unlock(&si->lock);
> - if (n_ret || size == SWAPFILE_CLUSTER)
> + if (n_ret || size > 1)
> goto check_out;
> cond_resched();
>
> @@ -2725,6 +2754,9 @@ static struct swap_info_struct *alloc_swap_info(void)
> spin_lock_init(&p->cont_lock);
> init_completion(&p->comp);
>
> + for (i = 0; i < ARRAY_SIZE(p->large_next); i++)
> + p->large_next[i] = UINT_MAX;
> +
> return p;
> }
>
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index c16e2b1ea8ae..5984d2ae4547 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -1212,11 +1212,13 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
> if (!can_split_folio(folio, NULL))
> goto activate_locked;
> /*
> - * Split folios without a PMD map right
> - * away. Chances are some or all of the
> - * tail pages can be freed without IO.
> + * Split PMD-mappable folios without a
> + * PMD map right away. Chances are some
> + * or all of the tail pages can be freed
> + * without IO.
> */
> - if (!folio_entire_mapcount(folio) &&
> + if (folio_test_pmd_mappable(folio) &&
> + !folio_entire_mapcount(folio) &&
> split_folio_to_list(folio,
> folio_list))
> goto activate_locked;
> --
> 2.25.1
>
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