Date: Fri, 7 Jun 2024 13:53:41 -0700
From: Chris Li <chrisl@...nel.org>
To: Ryan Roberts <ryan.roberts@....com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Kairui Song <kasong@...cent.com>, 
	"Huang, Ying" <ying.huang@...el.com>, linux-kernel@...r.kernel.org, linux-mm@...ck.org, 
	Barry Song <baohua@...nel.org>
Subject: Re: [PATCH 2/2] mm: swap: mTHP allocate swap entries from nonfull list

On Fri, Jun 7, 2024 at 3:57 AM Ryan Roberts <ryan.roberts@....com> wrote:
>
> 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.

You mean the (offset < max) loop, right?

Agree.

Chris

>
> >
> > 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)
> >>
> >
>

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