Re: [PATCH v4 4/6] mm: swap: Allow storage of all mTHP orders

Huang, Ying
2024-03-12T07:53:10Z

Ryan Roberts <ryan.roberts@arm.com> writes:

> Multi-size THP enables performance improvements by allocating large,
> pte-mapped 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 when the large folios are swapped out.
>
> Therefore, solve this regression by adding support for swapping out mTHP
> 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 (m)THP here - this is still done
> page-by-page, like for PMD-sized THP. But swapping-out mTHP is a
> prerequisite for swapping-in mTHP.
>
> 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 [1, (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 can fall back to splitting the folio
> and allocates individual entries (as per existing PMD-sized THP
> fallback).
>
> The per-order current clusters are maintained per-cpu using the existing
> infrastructure. This is done to avoid interleving pages from different
> tasks, which would prevent IO being batched. This is already done for
> the order-0 allocations so we follow the same pattern.
>
> As is done for order-0 per-cpu clusters, the scanner now can steal
> order-0 entries from any per-cpu-per-order reserved cluster. This
> ensures that when the swap file is getting full, space doesn't get tied
> up in the per-cpu reserves.
>
> This change only modifies swap to be able to accept any order mTHP. It
> doesn't change the callers to elide doing the actual split. That will be
> done in separate changes.
>
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
> include/linux/swap.h | 8 ++-
> mm/swapfile.c | 167 +++++++++++++++++++++++++------------------
> 2 files changed, 103 insertions(+), 72 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 0cb082bee717..39b5c18ccc6a 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -268,13 +268,19 @@ struct swap_cluster_info {
> */
> #define SWAP_NEXT_INVALID 0
>
> +#ifdef CONFIG_THP_SWAP
> +#define SWAP_NR_ORDERS (PMD_ORDER + 1)
> +#else
> +#define SWAP_NR_ORDERS 1
> +#endif
> +
> /*
> * We assign a cluster to each CPU, so each CPU can allocate swap entry from
> * its own cluster and swapout sequentially. The purpose is to optimize swapout
> * throughput.
> */
> struct percpu_cluster {
> - unsigned int next; /* Likely next allocation offset */
> + unsigned int next[SWAP_NR_ORDERS]; /* Likely next allocation offset */
> };
>
> struct swap_cluster_list {
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 3828d81aa6b8..61118a090796 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -551,10 +551,12 @@ static void free_cluster(struct swap_info_struct *si, unsigned long idx)
>
> /*
> * The cluster corresponding to page_nr will be used. The cluster will be
> - * removed from free cluster list and its usage counter will be increased.
> + * removed from free cluster list and its usage counter will be increased by
> + * count.
> */
> -static void inc_cluster_info_page(struct swap_info_struct *p,
> - struct swap_cluster_info *cluster_info, unsigned long page_nr)
> +static void add_cluster_info_page(struct swap_info_struct *p,
> + struct swap_cluster_info *cluster_info, unsigned long page_nr,
> + unsigned long count)
> {
> unsigned long idx = page_nr / SWAPFILE_CLUSTER;
>
> @@ -563,9 +565,19 @@ static void inc_cluster_info_page(struct swap_info_struct *p,
> if (cluster_is_free(&cluster_info[idx]))
> alloc_cluster(p, idx);
>
> - VM_BUG_ON(cluster_count(&cluster_info[idx]) >= SWAPFILE_CLUSTER);
> + VM_BUG_ON(cluster_count(&cluster_info[idx]) + count > SWAPFILE_CLUSTER);
> cluster_set_count(&cluster_info[idx],
> - cluster_count(&cluster_info[idx]) + 1);
> + cluster_count(&cluster_info[idx]) + count);
> +}
> +
> +/*
> + * The cluster corresponding to page_nr will be used. The cluster will be
> + * removed from free cluster list and its usage counter will be increased by 1.
> + */
> +static void inc_cluster_info_page(struct swap_info_struct *p,
> + struct swap_cluster_info *cluster_info, unsigned long page_nr)
> +{
> + add_cluster_info_page(p, cluster_info, page_nr, 1);
> }
>
> /*
> @@ -595,7 +607,7 @@ static void dec_cluster_info_page(struct swap_info_struct *p,
> */
> static bool
> scan_swap_map_ssd_cluster_conflict(struct swap_info_struct *si,
> - unsigned long offset)
> + unsigned long offset, int order)
> {
> struct percpu_cluster *percpu_cluster;
> bool conflict;
> @@ -609,24 +621,39 @@ scan_swap_map_ssd_cluster_conflict(struct swap_info_struct *si,
> return false;
>
> percpu_cluster = this_cpu_ptr(si->percpu_cluster);
> - percpu_cluster->next = SWAP_NEXT_INVALID;
> + percpu_cluster->next[order] = SWAP_NEXT_INVALID;
> + return true;
> +}
> +
> +static inline bool swap_range_empty(char *swap_map, unsigned int start,
> + unsigned int nr_pages)
> +{
> + unsigned int i;
> +
> + for (i = 0; i < nr_pages; i++) {
> + if (swap_map[start + i])
> + return false;
> + }
> +
> return true;
> }
>
> /*
> - * Try to get a swap entry from current cpu's swap entry pool (a cluster). This
> - * might involve allocating a new cluster for current CPU too.
> + * Try to get a swap entry (or size indicated by order) from current cpu's swap

IMO, it's not necessary to make mTHP a special case other than base
page. So, this can be changed to

* Try to get swap entries with specified order from current cpu's swap

> + * entry pool (a cluster). This might involve allocating a new cluster for
> + * current CPU too.
> */
> static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
> - unsigned long *offset, unsigned long *scan_base)
> + unsigned long *offset, unsigned long *scan_base, int order)
> {
> + unsigned int nr_pages = 1 << order;
> struct percpu_cluster *cluster;
> struct swap_cluster_info *ci;
> unsigned int tmp, max;
>
> new_cluster:
> cluster = this_cpu_ptr(si->percpu_cluster);
> - tmp = cluster->next;
> + tmp = cluster->next[order];
> if (tmp == SWAP_NEXT_INVALID) {
> if (!cluster_list_empty(&si->free_clusters)) {
> tmp = cluster_next(&si->free_clusters.head) *
> @@ -647,26 +674,27 @@ static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
>
> /*
> * Other CPUs can use our cluster if they can't find a free cluster,
> - * check if there is still free entry in the cluster
> + * check if there is still free entry in the cluster, maintaining
> + * natural alignment.
> */
> max = min_t(unsigned long, si->max, ALIGN(tmp + 1, SWAPFILE_CLUSTER));
> if (tmp < max) {
> ci = lock_cluster(si, tmp);
> while (tmp < max) {
> - if (!si->swap_map[tmp])
> + if (swap_range_empty(si->swap_map, tmp, nr_pages))
> break;
> - tmp++;
> + tmp += nr_pages;
> }
> unlock_cluster(ci);
> }
> if (tmp >= max) {
> - cluster->next = SWAP_NEXT_INVALID;
> + cluster->next[order] = SWAP_NEXT_INVALID;
> goto new_cluster;
> }
> *offset = tmp;
> *scan_base = tmp;
> - tmp += 1;
> - cluster->next = tmp < max ? tmp : SWAP_NEXT_INVALID;
> + tmp += nr_pages;
> + cluster->next[order] = tmp < max ? tmp : SWAP_NEXT_INVALID;
> return true;
> }
>
> @@ -796,13 +824,14 @@ static bool swap_offset_available_and_locked(struct swap_info_struct *si,
>
> static int scan_swap_map_slots(struct swap_info_struct *si,
> unsigned char usage, int nr,
> - swp_entry_t slots[])
> + swp_entry_t slots[], unsigned int nr_pages)

IMHO, it's better to use order as parameter directly. We can change the
parameter of get_swap_pages() too.

> {
> struct swap_cluster_info *ci;
> unsigned long offset;
> unsigned long scan_base;
> unsigned long last_in_cluster = 0;
> int latency_ration = LATENCY_LIMIT;
> + int order = ilog2(nr_pages);
> int n_ret = 0;
> bool scanned_many = false;
>
> @@ -817,6 +846,26 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
> * And we let swap pages go all over an SSD partition. Hugh
> */
>
> + if (nr_pages > 1) {
> + /*
> + * Should not even be attempting large allocations when huge
> + * page swap is disabled. Warn and fail the allocation.
> + */
> + if (!IS_ENABLED(CONFIG_THP_SWAP) ||
> + nr_pages > SWAPFILE_CLUSTER ||
> + !is_power_of_2(nr_pages)) {
> + VM_WARN_ON_ONCE(1);
> + return 0;
> + }
> +
> + /*
> + * Swapfile is not block device or not using clusters so unable
> + * to allocate large entries.
> + */
> + if (!(si->flags & SWP_BLKDEV) || !si->cluster_info)
> + return 0;
> + }
> +
> si->flags += SWP_SCANNING;
> /*
> * Use percpu scan base for SSD to reduce lock contention on
> @@ -831,8 +880,11 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
>
> /* SSD algorithm */
> if (si->cluster_info) {
> - if (!scan_swap_map_try_ssd_cluster(si, &offset, &scan_base))
> + if (!scan_swap_map_try_ssd_cluster(si, &offset, &scan_base, order)) {
> + if (order > 0)
> + goto no_page;
> goto scan;
> + }
> } else if (unlikely(!si->cluster_nr--)) {
> if (si->pages - si->inuse_pages < SWAPFILE_CLUSTER) {
> si->cluster_nr = SWAPFILE_CLUSTER - 1;
> @@ -874,26 +926,30 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
>
> checks:
> if (si->cluster_info) {
> - while (scan_swap_map_ssd_cluster_conflict(si, offset)) {
> + while (scan_swap_map_ssd_cluster_conflict(si, offset, order)) {
> /* take a break if we already got some slots */
> if (n_ret)
> goto done;
> if (!scan_swap_map_try_ssd_cluster(si, &offset,
> - &scan_base))
> + &scan_base, order)) {
> + if (order > 0)
> + goto no_page;
> goto scan;
> + }
> }
> }
> if (!(si->flags & SWP_WRITEOK))
> goto no_page;
> if (!si->highest_bit)
> goto no_page;
> - if (offset > si->highest_bit)
> + if (order == 0 && offset > si->highest_bit)

I don't think that we need to check "order == 0" here. The original
condition will always be false for "order != 0".

> scan_base = offset = si->lowest_bit;
>
> ci = lock_cluster(si, offset);
> /* reuse swap entry of cache-only swap if not busy. */
> if (vm_swap_full() && si->swap_map[offset] == SWAP_HAS_CACHE) {
> int swap_was_freed;
> + VM_WARN_ON(order > 0);

Instead of add WARN here, I think that it's better to add WARN at the
beginning of "scan" label. We should never scan if "order > 0", it can
capture even more abnormal status.

> unlock_cluster(ci);
> spin_unlock(&si->lock);
> swap_was_freed = __try_to_reclaim_swap(si, offset, TTRS_ANYWAY);
> @@ -905,17 +961,18 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
> }
>
> if (si->swap_map[offset]) {
> + VM_WARN_ON(order > 0);
> unlock_cluster(ci);
> if (!n_ret)
> goto scan;
> else
> goto done;
> }
> - WRITE_ONCE(si->swap_map[offset], usage);
> - inc_cluster_info_page(si, si->cluster_info, offset);
> + memset(si->swap_map + offset, usage, nr_pages);

Add barrier() here corresponds to original WRITE_ONCE()?
unlock_cluster(ci) may be NOP for some swap devices.

> + add_cluster_info_page(si, si->cluster_info, offset, nr_pages);
> unlock_cluster(ci);
>
> - swap_range_alloc(si, offset, 1);
> + swap_range_alloc(si, offset, nr_pages);
> slots[n_ret++] = swp_entry(si->type, offset);
>
> /* got enough slots or reach max slots? */

If "order > 0", "nr" must be 1. So, we will "goto done" in the
following code.

/* got enough slots or reach max slots? */
if ((n_ret == nr) || (offset >= si->highest_bit))
goto done;

We can add VM_WARN_ON() here to capture some abnormal status.

> @@ -936,8 +993,10 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
>
> /* try to get more slots in cluster */
> if (si->cluster_info) {
> - if (scan_swap_map_try_ssd_cluster(si, &offset, &scan_base))
> + if (scan_swap_map_try_ssd_cluster(si, &offset, &scan_base, order))
> goto checks;
> + if (order > 0)
> + goto done;

Don't need to add this, if "order > 0", we will never go here.

> } else if (si->cluster_nr && !si->swap_map[++offset]) {
> /* non-ssd case, still more slots in cluster? */
> --si->cluster_nr;
> @@ -964,7 +1023,8 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
> }
>
> done:
> - set_cluster_next(si, offset + 1);
> + if (order == 0)
> + set_cluster_next(si, offset + 1);
> si->flags -= SWP_SCANNING;
> return n_ret;
>
> @@ -997,38 +1057,6 @@ 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)
> -{
> - unsigned long idx;
> - struct swap_cluster_info *ci;
> - unsigned long offset;
> -
> - /*
> - * Should not even be attempting cluster allocations when huge
> - * page swap is disabled. Warn and fail the allocation.
> - */
> - if (!IS_ENABLED(CONFIG_THP_SWAP)) {
> - VM_WARN_ON_ONCE(1);
> - return 0;
> - }
> -
> - if (cluster_list_empty(&si->free_clusters))
> - return 0;
> -
> - idx = cluster_list_first(&si->free_clusters);
> - offset = idx * SWAPFILE_CLUSTER;
> - ci = lock_cluster(si, offset);
> - alloc_cluster(si, idx);
> - cluster_set_count(ci, SWAPFILE_CLUSTER);
> -
> - memset(si->swap_map + offset, SWAP_HAS_CACHE, SWAPFILE_CLUSTER);
> - unlock_cluster(ci);
> - swap_range_alloc(si, offset, SWAPFILE_CLUSTER);
> - *slot = swp_entry(si->type, offset);
> -
> - return 1;
> -}
> -
> static void swap_free_cluster(struct swap_info_struct *si, unsigned long idx)
> {
> unsigned long offset = idx * SWAPFILE_CLUSTER;
> @@ -1050,8 +1078,8 @@ int get_swap_pages(int n_goal, swp_entry_t swp_entries[], int entry_size)
> int n_ret = 0;
> int node;
>
> - /* Only single cluster request supported */
> - WARN_ON_ONCE(n_goal > 1 && size == SWAPFILE_CLUSTER);
> + /* Only single THP request supported */
> + WARN_ON_ONCE(n_goal > 1 && size > 1);
>
> spin_lock(&swap_avail_lock);
>
> @@ -1088,14 +1116,10 @@ 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 (si->flags & SWP_BLKDEV)
> - n_ret = swap_alloc_cluster(si, swp_entries);
> - } else
> - n_ret = scan_swap_map_slots(si, SWAP_HAS_CACHE,
> - n_goal, swp_entries);
> + n_ret = scan_swap_map_slots(si, SWAP_HAS_CACHE,
> + n_goal, swp_entries, size);
> spin_unlock(&si->lock);
> - if (n_ret || size == SWAPFILE_CLUSTER)
> + if (n_ret || size > 1)
> goto check_out;
> cond_resched();
>
> @@ -1647,7 +1671,7 @@ swp_entry_t get_swap_page_of_type(int type)
>
> /* This is called for allocating swap entry, not cache */
> spin_lock(&si->lock);
> - if ((si->flags & SWP_WRITEOK) && scan_swap_map_slots(si, 1, 1, &entry))
> + if ((si->flags & SWP_WRITEOK) && scan_swap_map_slots(si, 1, 1, &entry, 1))
> atomic_long_dec(&nr_swap_pages);
> spin_unlock(&si->lock);
> fail:
> @@ -3101,7 +3125,7 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
> p->flags |= SWP_SYNCHRONOUS_IO;
>
> if (p->bdev && bdev_nonrot(p->bdev)) {
> - int cpu;
> + int cpu, i;
> unsigned long ci, nr_cluster;
>
> p->flags |= SWP_SOLIDSTATE;
> @@ -3139,7 +3163,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags)
> struct percpu_cluster *cluster;
>
> cluster = per_cpu_ptr(p->percpu_cluster, cpu);
> - cluster->next = SWAP_NEXT_INVALID;
> + for (i = 0; i < SWAP_NR_ORDERS; i++)
> + cluster->next[i] = SWAP_NEXT_INVALID;
> }
> } else {
> atomic_inc(&nr_rotate_swap);

You also need to check whether we should add swap_entry_size() for some
functions to optimize for small system. We may need to add swap_order()
too.

--
Best Regards,
Huang, Ying