On Thu, Aug 8, 2024 at 4:16 AM Kairui Song <ryncsn@xxxxxxxxx> wrote:
>
> On Wed, Aug 7, 2024 at 4:25 PM Barry Song <21cnbao@xxxxxxxxx> 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.
>
> Hi Barry,
>
> Thanks for the patch, I also noticed this could be optimized when
> working on the batch freeing of mthp pages in the series you
> mentioned, a very nice improvement.
>
> >
> > [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 | 78 +++++++++++++++++++++++++++++++++++++++++++--------
> > 1 file changed, 67 insertions(+), 11 deletions(-)
> >
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 35cb58373493..25c3f98fa8d5 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;
>
> I haven't tried this yet, but looking at this if. If a mthp or thp was
> split, and part of the slots are "1", rest of the slots are "HAS_CACHE
> | 1", this will also return true, is this a problem?
>
> These slots with "1" don't have an entry in the swap cache, so the
> following reclaim in free_swap_and_cache_nr might not work as
> expected, could they be stuck in HAS_CACHE state?
good catch. Kairui, Thanks! In this case, I am leaking swap slots whose
count == 1.
The original non-batch code will still free those slots in non-batched mode
but my code will just write them to SWAP_HAS_CACHE.
this can be fixed by:
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;
unsigned char count = *map;
if (swap_count(count) != 1)
return false;
do {
if (*map != count)
return false;
} while (++map < map_end);
*has_cache = !!(count & SWAP_HAS_CACHE);
return true;
}
Thanks
Barry
