Re: [PATCH] mm: attempt to batch free swap entries for zap_pte_range()

[David Hildenbrand <david@redhat.com>]

On 06.08.24 03:24, Barry Song wrote:
> From: Barry Song <v-songbaohua@oppo.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@tencent.com>
> Cc: Chris Li <chrisl@kernel.org>
> Cc: "Huang, Ying" <ying.huang@intel.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Kalesh Singh <kaleshsingh@google.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: David Hildenbrand <david@redhat.com>
> Signed-off-by: Barry Song <v-songbaohua@oppo.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,

Why call it "p" here and not "si" like in the other code you are touching?

> +		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;
> +	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;
> +	}
> +

I'm wondering if we could find a way to clean this up to achieve here:


if (WARN_ON(end_offset > si->max))
	goto out;

/*
  * First free all entries in the range.$
  */
any_only_cache = __free_swap_entries(si, entry, nr);

/*
  * Short-circuit the below loop if none of the entries had their
  * reference drop to zero.
  */
if (!any_only_cache)
	goto out;




Whereby move the fallback loop in that new function

static bool __free_swap_entries(struct swap_info_struct *si,
		swp_entry_t entry, int nr)
{
	const unsigned long start_offset = swp_offset(entry);
	const unsigned long end_offset = start_offset + nr;
	bool any_only_cache = false;

	if (nr > 1 && swap_count(data_race(si->swap_map[start_offset]) == 1)) {
		[... what try_batch_swap_entries_free() would do ...]
	}

fallback:
	for (offset = start_offset; offset < end_offset; offset++) {
		if (data_race(si->swap_map[offset])) {
		[... what the fallback code would do ...]
	}
	return any_only_cache;
}


>   	/*
>   	 * 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.

-- 
Cheers,

David / dhildenb