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

Huang, Ying
2024-03-13T01:35:13Z

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

> On 12/03/2024 07:51, Huang, Ying wrote:
>> 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
>
> Sure, will fix in next version.
>
>>
>>> + * 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.
>
> I agree that this will make the interface clearer/self documenting. I'll do it
> in the next version.
>
>>
>>> {
>>> 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".
>
> I spent ages looking at this and couldn't quite convince myself that this is
> definitely safe. Certainly it would be catastrophic if we modified the returned
> offset for a non-order-0 case (the code below assumes order-0 when checking). So
> I decided in the end to be safe and add this condition. Looking again, I agree
> with you. Will fix in next version.
>
>>
>>> 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.
>
> OK, will do.
>
>>
>>> 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);
>
> And remove this one too? (relying on the one in scan instead)

Yes. I think so.

>>> 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.
>
> Yep, good spot!
>
>>
>>> + 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.
>
> I've deliberately implemented scan_swap_map_slots() so that it allows nr > 1 for
> order > 0. And leave it to the higher layers to decide on policy.
>
>>
>> /* 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.
>
> That was actually how I implemented initially. But decided that it doesn't cost
> anything to allow nr > 1 for order > 0, and IMHO makes the function easier to
> understand because we remove this uneccessary constraint.

This sounds reasonable to me.

>>
>>> @@ -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.
>
> As per above.
>
>>
>>> } 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.
>
> I was planning to convert swap_entry_size() to swap_entry_order() as part of
> switching to pass order instead of nr_pages. There is one other site that uses
> swap_entry_size() and needs a size, so was going to just change it to 1 <<
> swap_entry_order(). Does that work for you?

Yes.

> I'll do an audit for places to use swap_entry_order() but quick scan just now
> suggests that the constant should propagate to all the static functions from
> get_swap_pages().
>

--
Best Regards,
Huang, Ying