Re: [RFC PATCH v1 2/2] mm: swap: Swap-out small-sized THP without splitting

[Huang, Ying]

Ryan Roberts <ryan.roberts@xxxxxxx> writes:

> On 11/10/2023 11:36, Ryan Roberts wrote:
>> On 11/10/2023 09:25, Huang, Ying wrote:
>>> Ryan Roberts <ryan.roberts@xxxxxxx> writes:
>>>
>>>> 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@xxxxxxx>
>>>> ---
>>>>  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]; /*
>>>> +					     * 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.
>>>> +					     */
>>>
>>> I think that it is better to make this per-CPU.  That is, extend the
>>> percpu_cluster mechanism.  Otherwise, we may have scalability issue.
>> 
>> Is your concern that the swap_info spinlock will get too contended as its
>> currently written? From briefly looking at percpu_cluster, it looks like that
>> spinlock is always held when accessing the per-cpu structures - presumably
>> that's what's disabling preemption and making sure the thread is not migrated?
>> So I'm not sure what the benefit is currently? Surely you want to just disable
>> preemption but not hold the lock? I'm sure I've missed something crucial...
>
> I looked a bit further at how to implement what you are suggesting.
> get_swap_pages() is currently taking the swap_info lock which it needs to check
> and update some other parts of the swap_info - I'm not sure that part can be
> removed. swap_alloc_large() (my new function) is not doing an awful lot of work,
> so I'm not convinced that you would save too much by releasing the lock for that
> part. In contrast there is a lot more going on in scan_swap_map_slots() so there
> is more benefit to releasing the lock and using the percpu stuff - correct me if
> I've missunderstood.
>
> As an alternative approach, perhaps it makes more sense to beef up the caching
> layer in swap_slots.c to handle large folios too? Then you avoid taking the
> swap_info lock at all most of the time, like you currently do for single entry
> allocations.
>
> What do you think?

Sorry for late reply.

percpu_cluster is introduced in commit ebc2a1a69111 ("swap: make cluster
allocation per-cpu").  Please check the changelog for why it's
introduced.  Sorry about my incorrect memory about scalability.
percpu_cluster is introduced for disk performance mainly instead of
scalability.

--
Best Regards,
Huang, Ying

>> 
>>>
>>> And this should be enclosed in CONFIG_THP_SWAP.
>> 
>> Yes, I'll fix this in the next version.
>> 
>> Thanks for the review!
>> 
>>>
>>>>  	struct plist_node avail_lists[]; /*
>>>>  					   * entries in swap_avail_heads, one
>>>>  					   * entry per node.