On 07/06/2024 11:35, Ryan Roberts wrote:
> On 24/05/2024 18:17, Chris Li wrote:
>> Track the nonfull cluster as well as the empty cluster
>> on lists. Each order has one nonfull cluster list.
>>
>> The cluster will remember which order it was used during
>> new cluster allocation.
>>
>> When the cluster has free entry, add to the nonfull[order]
>> list. When the free cluster list is empty, also allocate
>> from the nonempty list of that order.
>>
>> This improves the mTHP swap allocation success rate.
>
> If I've understood correctly, the aim here is to link all the current per-cpu
> clusters for a given order together so that if a cpu can't allocate a new
> cluster for a given order, then it can steal another CPU's current cluster for
> that order?
>
> If that's the intent, couldn't that be done just by iterating over the per-cpu,
> per-order cluster pointers? Then you don't need all the linked list churn
> (althogh I like the linked list changes as a nice cleanup, I'm not sure the
> churn is neccessary for this change?). There would likely need to be some
> locking considerations, but it would also allow you to get access to the next
> entry within the cluster for allocation.
>
> However, fundamentally, I don't think this change solves the problem; it just
> takes a bit longer before the allocation fails. The real problem is
> fragmentation due to freeing individual pages from swap entries at different times.
>
> Wouldn't it be better to just extend scanning to support high order allocations?
> Then we can steal a high order block from any cluster, even clusters that were
> previously full, just like we currently do for order-0. Given we are already
> falling back to this path for order-0, I don't think it would be any more
> expensive; infact its less expensive because we only scan once for the high
> order block, rather than scan for every split order-0 page.
>
> Of course that still doesn't solve the proplem entirely; if swap is so
> fragmented that there is no contiguous block of the required order then you
> still have to fall back to splitting. As an extra optimization, you could store
> the largest contiguous free space available in each cluster to avoid scanning in
> case its too small?
>
>
>>
>> There are limitations if the distribution of numbers of
>> different orders of mTHP changes a lot. e.g. there are a lot
>> of nonfull cluster assign to order A while later time there
>> are a lot of order B allocation while very little allocation
>> in order A. Currently the cluster used by order A will not
>> reused by order B unless the cluster is 100% empty.
>>
>> This situation is best addressed by the longer term "swap
>> buddy allocator", in future patches.
>> ---
>> include/linux/swap.h | 4 ++++
>> mm/swapfile.c | 25 +++++++++++++++++++++++--
>> 2 files changed, 27 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>> index 0d3906eff3c9..1b7f0794b9bf 100644
>> --- a/include/linux/swap.h
>> +++ b/include/linux/swap.h
>> @@ -255,10 +255,12 @@ struct swap_cluster_info {
>> * cluster
>> */
>> unsigned int count:16;
>> + unsigned int order:8;
>> unsigned int flags:8;
>> struct list_head next;
>> };
>> #define CLUSTER_FLAG_FREE 1 /* This cluster is free */
>> +#define CLUSTER_FLAG_NONFULL 2 /* This cluster is on nonfull list */
>>
>>
>> /*
>> @@ -297,6 +299,8 @@ struct swap_info_struct {
>> unsigned char *swap_map; /* vmalloc'ed array of usage counts */
>> struct swap_cluster_info *cluster_info; /* cluster info. Only for SSD */
>> struct list_head free_clusters; /* free clusters list */
>> + struct list_head nonfull_clusters[SWAP_NR_ORDERS];
>> + /* list of cluster that contains at least one free slot */
>> unsigned int lowest_bit; /* index of first free in swap_map */
>> unsigned int highest_bit; /* index of last free in swap_map */
>> unsigned int pages; /* total of usable pages of swap */
>> diff --git a/mm/swapfile.c b/mm/swapfile.c
>> index 205a60c5f9cb..51923aba500e 100644
>> --- a/mm/swapfile.c
>> +++ b/mm/swapfile.c
>> @@ -363,8 +363,11 @@ static void swap_cluster_schedule_discard(struct swap_info_struct *si,
>>
>> static void __free_cluster(struct swap_info_struct *si, struct swap_cluster_info *ci)
>> {
>> + if (ci->flags & CLUSTER_FLAG_NONFULL)
>> + list_move_tail(&ci->next, &si->free_clusters);
>> + else
>> + list_add_tail(&ci->next, &si->free_clusters);
>> ci->flags = CLUSTER_FLAG_FREE;
>> - list_add_tail(&ci->next, &si->free_clusters);
>> }
>>
>> /*
>> @@ -486,7 +489,12 @@ static void dec_cluster_info_page(struct swap_info_struct *p, struct swap_cluste
>> ci->count--;
>>
>> if (!ci->count)
>> - free_cluster(p, ci);
>> + return free_cluster(p, ci);
>> +
>> + if (!(ci->flags & CLUSTER_FLAG_NONFULL)) {
>> + list_add_tail(&ci->next, &p->nonfull_clusters[ci->order]);
>> + ci->flags |= CLUSTER_FLAG_NONFULL;
>> + }
>> }
>>
>> /*
>> @@ -547,6 +555,14 @@ static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
>> ci = list_first_entry(&si->free_clusters, struct swap_cluster_info, next);
>> list_del(&ci->next);
>> spin_lock(&ci->lock);
>> + ci->order = order;
>> + ci->flags = 0;
>> + spin_unlock(&ci->lock);
>> + tmp = (ci - si->cluster_info) * SWAPFILE_CLUSTER;
>> + } else if (!list_empty(&si->nonfull_clusters[order])) {
>> + ci = list_first_entry(&si->nonfull_clusters[order], struct swap_cluster_info, next);
>> + list_del(&ci->next);
>> + spin_lock(&ci->lock);
>> ci->flags = 0;
>> spin_unlock(&ci->lock);
>> tmp = (ci - si->cluster_info) * SWAPFILE_CLUSTER;
>
> This looks wrong to me; if the cluster is on the nonfull list then it will have
> had some entries already allocated (by another cpu). So pointing tmp to the
> first block in the cluster will never yield a free block. The cpu from which you
> are stealing the cluster stores the next free block location in its per-cpu
> structure. So perhaps iterating over the other cpu's `struct percpu_cluster`s is
> a better approach than the nonfull list?
Ahh; of course the cluster scan below will move this along to a free block.
>
> Additionally, this cluster will be stored back to this cpu's current cluster at
> the bottom of the function. That may or may not be what you intended.
>
>> @@ -578,6 +594,7 @@ static bool scan_swap_map_try_ssd_cluster(struct swap_info_struct *si,
>> break;
>> tmp += nr_pages;
>> }
>> + WARN_ONCE(ci->order != order, "expecting order %d got %d", order, ci->order);
>> unlock_cluster(ci);
>> }
>> if (tmp >= max) {
>> @@ -956,6 +973,7 @@ static void swap_free_cluster(struct swap_info_struct *si, unsigned long idx)
>> ci = lock_cluster(si, offset);
>> memset(si->swap_map + offset, 0, SWAPFILE_CLUSTER);
>> ci->count = 0;
>> + ci->order = 0;
>> ci->flags = 0;
>> free_cluster(si, ci);
>> unlock_cluster(ci);
>> @@ -2882,6 +2900,9 @@ static int setup_swap_map_and_extents(struct swap_info_struct *p,
>> INIT_LIST_HEAD(&p->free_clusters);
>> INIT_LIST_HEAD(&p->discard_clusters);
>>
>> + for (i = 0; i < SWAP_NR_ORDERS; i++)
>> + INIT_LIST_HEAD(&p->nonfull_clusters[i]);
>> +
>> for (i = 0; i < swap_header->info.nr_badpages; i++) {
>> unsigned int page_nr = swap_header->info.badpages[i];
>> if (page_nr == 0 || page_nr > swap_header->info.last_page)
>>
>
