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Message-ID: <CAGsJ_4yKEGmXKd=rCavt6WGokjGg+w2Eky=PzP0b2562NKhbRA@mail.gmail.com>
Date: Tue, 6 Aug 2024 14:38:13 +0800
From: Barry Song <21cnbao@...il.com>
To: 20240806012409.61962-1-21cnbao@...il.com
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, Barry Song <v-songbaohua@...o.com>,
Kairui Song <kasong@...cent.com>, Chris Li <chrisl@...nel.org>,
"Huang, Ying" <ying.huang@...el.com>, Hugh Dickins <hughd@...gle.com>,
Kalesh Singh <kaleshsingh@...gle.com>, Ryan Roberts <ryan.roberts@....com>,
David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH] mm: attempt to batch free swap entries for zap_pte_range()
Next time, please use "> " and ">> " etc to reply to emails.
On Tue, Aug 6, 2024 at 3:23 PM zhiguojiang <justinjiang@...o.com> wrote:
>
> From: Barry Song <v-songbaohua@...o.com>
>
> Zhiguo reported that swap release could be a serious bottleneck
> during process exits[1]. With mTHP, we have the opportunity to
> batch free swaps.
> Thanks to the work of Chris and Kairui[2], I was able to achieve
> this optimization with minimal code changes by building on their
> efforts.
> If swap_count is 1, which is likely true as most anon memory are
> private, we can free all contiguous swap slots all together.
>
> Ran the below test program for measuring the bandwidth of munmap
> using zRAM and 64KiB mTHP:
>
> #include <sys/mman.h>
> #include <sys/time.h>
> #include <stdlib.h>
>
> unsigned long long tv_to_ms(struct timeval tv)
> {
> return tv.tv_sec * 1000 + tv.tv_usec / 1000;
> }
>
> main()
> {
> struct timeval tv_b, tv_e;
> int i;
> #define SIZE 1024*1024*1024
> void *p = mmap(NULL, SIZE, PROT_READ | PROT_WRITE,
> MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> if (!p) {
> perror("fail to get memory");
> exit(-1);
> }
>
> madvise(p, SIZE, MADV_HUGEPAGE);
> memset(p, 0x11, SIZE); /* write to get mem */
>
> madvise(p, SIZE, MADV_PAGEOUT);
>
> gettimeofday(&tv_b, NULL);
> munmap(p, SIZE);
> gettimeofday(&tv_e, NULL);
>
> printf("munmap in bandwidth: %ld bytes/ms\n",
> SIZE/(tv_to_ms(tv_e) - tv_to_ms(tv_b)));
> }
>
> The result is as below (munmap bandwidth):
> mm-unstable mm-unstable-with-patch
> round1 21053761 63161283
> round2 21053761 63161283
> round3 21053761 63161283
> round4 20648881 67108864
> round5 20648881 67108864
>
> munmap bandwidth becomes 3X faster.
>
> [1] https://lore.kernel.org/linux-mm/20240731133318.527-1-justinjiang@vivo.com/
> [2] https://lore.kernel.org/linux-mm/20240730-swap-allocator-v5-0-cb9c148b9297@kernel.org/
>
> Cc: Kairui Song <kasong@...cent.com>
> Cc: Chris Li <chrisl@...nel.org>
> Cc: "Huang, Ying" <ying.huang@...el.com>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Kalesh Singh <kaleshsingh@...gle.com>
> Cc: Ryan Roberts <ryan.roberts@....com>
> Cc: David Hildenbrand <david@...hat.com>
> Signed-off-by: Barry Song <v-songbaohua@...o.com>
> ---
> mm/swapfile.c | 61 +++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 61 insertions(+)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index ea023fc25d08..ed872a186e81 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -156,6 +156,25 @@ static bool swap_is_has_cache(struct swap_info_struct *si,
> return true;
> }
>
> +static bool swap_is_last_map(struct swap_info_struct *si,
> + unsigned long offset, int nr_pages,
> + bool *has_cache)
> +{
> + unsigned char *map = si->swap_map + offset;
> + unsigned char *map_end = map + nr_pages;
> + bool cached = false;
> +
> + do {
> + if ((*map & ~SWAP_HAS_CACHE) != 1)
> + return false;
> + if (*map & SWAP_HAS_CACHE)
> + cached = true;
> + } while (++map < map_end);
> +
> + *has_cache = cached;
> + return true;
> +}
> +
> /*
> * returns number of pages in the folio that backs the swap entry. If positive,
> * the folio was reclaimed. If negative, the folio was not reclaimed. If 0, no
> @@ -1469,6 +1488,39 @@ static unsigned char __swap_entry_free(struct swap_info_struct *p,
> return usage;
> }
>
> +static bool try_batch_swap_entries_free(struct swap_info_struct *p,
> + swp_entry_t entry, int nr, bool *any_only_cache)
> +{
> + unsigned long offset = swp_offset(entry);
> + struct swap_cluster_info *ci;
> + bool has_cache = false;
> + bool can_batch;
> + int i;
> +
> + /* cross into another cluster */
> + if (nr > SWAPFILE_CLUSTER - offset % SWAPFILE_CLUSTER)
> + return false;
> My understand of mTHP swap entries alloced by by cluster_alloc_swap()
> is that they belong to the same cluster in the same swapinfo , so
> theoretically it will not appear for
> (nr > SWAPFILE_CLUSTER - offset % SWAPFILE_CLUSTER)?
> Can you help confirm?
zap_pte_range() has no concept of folios (mTHP) as folios could have
gone. you could have the case:
folio1: last 16 slots of cluster1
folio2: first 16 slots of cluster2.
folio1 and folio2 are within the same PMD and virtually contiguous
before they are unmapped.
when both folio1 and folio2 have gone, zap_pte_range() 's
nr = swap_pte_batch(pte, max_nr, ptent);
nr will be 32. "mTHP swap entries alloced by by cluster_alloc_swap() belong
to the same cluster" is correct, but when you zap_pte_range(), your mTHPs
could have gone.
>
> + ci = lock_cluster_or_swap_info(p, offset);
> + can_batch = swap_is_last_map(p, offset, nr, &has_cache);
> + if (can_batch) {
> + for (i = 0; i < nr; i++)
> + WRITE_ONCE(p->swap_map[offset + i], SWAP_HAS_CACHE);
> + }
> + unlock_cluster_or_swap_info(p, ci);
> +
> + /* all swap_maps have count==1 and have no swapcache */
> + if (!can_batch)
> + goto out;
> + if (!has_cache) {
> + spin_lock(&p->lock);
> + swap_entry_range_free(p, entry, nr);
> + spin_unlock(&p->lock);
> + }
> + *any_only_cache = has_cache;
> +out:
> + return can_batch;
> +}
> +
> /*
> * Drop the last HAS_CACHE flag of swap entries, caller have to
> * ensure all entries belong to the same cgroup.
> @@ -1797,6 +1849,7 @@ void free_swap_and_cache_nr(swp_entry_t entry, int nr)
> bool any_only_cache = false;
> unsigned long offset;
> unsigned char count;
> + bool batched;
>
> if (non_swap_entry(entry))
> return;
> @@ -1808,6 +1861,13 @@ void free_swap_and_cache_nr(swp_entry_t entry, int nr)
> if (WARN_ON(end_offset > si->max))
> goto out;
>
> + if (nr > 1 && swap_count(data_race(si->swap_map[start_offset]) == 1)) {
> + batched = try_batch_swap_entries_free(si, entry, nr,
> + &any_only_cache);
> + if (batched)
> + goto reclaim;
> + }
> The mTHP swap entries are batch freed as a whole directly by skipping
> percpu swp_slots caches, instead of freeing every swap entry separately,
> which can accelerate the mTHP swap entries release. I think it is
> valuable.
yes. I have seen 3X performance improvement.
>
> +
> /*
> * First free all entries in the range.
> */
> @@ -1821,6 +1881,7 @@ void free_swap_and_cache_nr(swp_entry_t entry, int nr)
> }
> }
>
> +reclaim:
> /*
> * Short-circuit the below loop if none of the entries had their
> * reference drop to zero.
> --
> 2.34.1
>
> Thanks
> Zhiguo
>
>
>
Thanks
Barry
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