On Tue, Aug 6, 2024 at 8:56 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 06.08.24 03:24, Barry Song wrote:
> > From: Barry Song <v-songbaohua@xxxxxxxx>
> >
> > 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@xxxxxxxx/
> > [2] https://lore.kernel.org/linux-mm/20240730-swap-allocator-v5-0-cb9c148b9297@xxxxxxxxxx/
> >
> > Cc: Kairui Song <kasong@xxxxxxxxxxx>
> > Cc: Chris Li <chrisl@xxxxxxxxxx>
> > Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
> > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> > Cc: Kalesh Singh <kaleshsingh@xxxxxxxxxx>
> > Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
> > Cc: David Hildenbrand <david@xxxxxxxxxx>
> > Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
> > ---
> > 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?
that is because I found other _free_ functions are all using "p":
static unsigned char __swap_entry_free(struct swap_info_struct *p,
swp_entry_t entry)
{
...
}
static void swap_entry_free(struct swap_info_struct *p, swp_entry_t entry)
{
...
}
For sure I can move from "p" to "si".
>
> > + 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;
> }
>
>
good suggestion. will do this in v2.
> > /*
> > * 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
>
Thanks
Barry
