Re: [PATCH v6 4/6] mm: swap: Allow storage of all mTHP orders

[Ryan Roberts <ryan.roberts@arm.com>]

On 07/04/2024 07:02, Huang, Ying wrote:
> David Hildenbrand <david@redhat.com> writes:
> 
>> On 03.04.24 13:40, Ryan Roberts 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@intel.com>
>>> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
>>> ---
>>>   include/linux/swap.h |  10 ++-
>>>   mm/swap_slots.c      |   6 +-
>>>   mm/swapfile.c        | 175 ++++++++++++++++++++++++-------------------
>>>   3 files changed, 109 insertions(+), 82 deletions(-)
>>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>>> index 5e1e4f5bf0cb..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 {
>>> @@ -471,7 +477,7 @@ swp_entry_t folio_alloc_swap(struct folio *folio);
>>>   bool folio_free_swap(struct folio *folio);
>>>   void put_swap_folio(struct folio *folio, swp_entry_t entry);
>>>   extern swp_entry_t get_swap_page_of_type(int);
>>> -extern int get_swap_pages(int n, swp_entry_t swp_entries[], int entry_size);
>>> +extern int get_swap_pages(int n, swp_entry_t swp_entries[], int order);
>>>   extern int add_swap_count_continuation(swp_entry_t, gfp_t);
>>>   extern void swap_shmem_alloc(swp_entry_t);
>>>   extern int swap_duplicate(swp_entry_t);
>>> diff --git a/mm/swap_slots.c b/mm/swap_slots.c
>>> index 53abeaf1371d..13ab3b771409 100644
>>> --- a/mm/swap_slots.c
>>> +++ b/mm/swap_slots.c
>>> @@ -264,7 +264,7 @@ static int refill_swap_slots_cache(struct swap_slots_cache *cache)
>>>   	cache->cur = 0;
>>>   	if (swap_slot_cache_active)
>>>   		cache->nr = get_swap_pages(SWAP_SLOTS_CACHE_SIZE,
>>> -					   cache->slots, 1);
>>> +					   cache->slots, 0);
>>>     	return cache->nr;
>>>   }
>>> @@ -311,7 +311,7 @@ swp_entry_t folio_alloc_swap(struct folio *folio)
>>>     	if (folio_test_large(folio)) {
>>>   		if (IS_ENABLED(CONFIG_THP_SWAP))
>>> -			get_swap_pages(1, &entry, folio_nr_pages(folio));
>>> +			get_swap_pages(1, &entry, folio_order(folio));
>>
>> The only comment I have is that this nr_pages -> order conversion adds
>> a bit of noise to this patch.
>>
>> AFAIKS, it's primarily only required for "cluster->next[order]",
>> everything else doesn't really require the order.
>>
>> I'd just have split that out into a separate patch, or simply
>> converted nr_pages -> order where required.
>>
>> Nothing jumped at me, but I'm not an expert on that code, so I'm
>> mostly trusting the others ;)
> 
> The nr_pages -> order conversion replaces ilog2(nr_pages) with
> (1<<order).  IIUC, "<<" is a little faster than "ilog2()".  And, we
> don't need to worry about whether nr_pages is a power of 2.  Do you
> think that this makes sense?

I think that David's point was that I should just split out that change to its
own patch to aid readability? I'm happy to do that if no one objects.

> 
> --
> Best Regards,
> Huang, Ying