On Tue, Apr 9, 2024 at 6:40 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> 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.
>
> Reviewed-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> ---
> include/linux/swap.h | 8 ++-
> mm/swapfile.c | 162 ++++++++++++++++++++++++-------------------
> 2 files changed, 98 insertions(+), 72 deletions(-)
>
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index b888e1080a94..11c53692f65f 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 d2e3d3cd439f..148ef08f19dd 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 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[], int order)
> {
> struct swap_cluster_info *ci;
> unsigned long offset;
> unsigned long scan_base;
> unsigned long last_in_cluster = 0;
> int latency_ration = LATENCY_LIMIT;
> + unsigned int nr_pages = 1 << order;
> int n_ret = 0;
> bool scanned_many = false;
>
> @@ -817,6 +846,25 @@ static int scan_swap_map_slots(struct swap_info_struct *si,
> * And we let swap pages go all over an SSD partition. Hugh
> */
>
> + if (order > 0) {
> + /*
> + * 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) {
> + 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 +879,11 @@ 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)) {
Hi Ryan,
Sorry for bringing up an old thread.
During the initial hour of utilizing an Android phone with 64KiB mTHP,
we noticed that the
anon_swpout_fallback rate was less than 10%. However, after several
hours of phone
usage, we observed a significant increase in the anon_swpout_fallback
rate, reaching
100%.
As I checked the code of scan_swap_map_try_ssd_cluster(),
static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
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[order];
if (tmp == SWAP_NEXT_INVALID) {
if (!cluster_list_empty(&si->free_clusters)) {
tmp = cluster_next(&si->free_clusters.head) *
SWAPFILE_CLUSTER;
} else if (!cluster_list_empty(&si->discard_clusters)) {
/*
* we don't have free cluster but have some clusters in
* discarding, do discard now and reclaim them, then
* reread cluster_next_cpu since we dropped si->lock
*/
swap_do_scheduled_discard(si);
*scan_base = this_cpu_read(*si->cluster_next_cpu);
*offset = *scan_base;
goto new_cluster;
} else
return false;
}
...
}
Considering the cluster_list_empty() checks, is it necessary to have
free_cluster to
ensure a continuous allocation of swap slots for large folio swap out?
For instance,
if numerous clusters still possess ample free swap slots, could we
potentially miss
out on them due to a lack of execution of a slow scan?
I'm not saying your patchset has problems, just that I have some questions.
Thanks
Barry
