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    SubjectRe: [PATCH -mm -v3] mm, swap: Sort swap entries before free
    Hi Huang,

    On Fri, Apr 07, 2017 at 02:49:01PM +0800, Huang, Ying wrote:
    > From: Huang Ying <ying.huang@intel.com>
    >
    > To reduce the lock contention of swap_info_struct->lock when freeing
    > swap entry. The freed swap entries will be collected in a per-CPU
    > buffer firstly, and be really freed later in batch. During the batch
    > freeing, if the consecutive swap entries in the per-CPU buffer belongs
    > to same swap device, the swap_info_struct->lock needs to be
    > acquired/released only once, so that the lock contention could be
    > reduced greatly. But if there are multiple swap devices, it is
    > possible that the lock may be unnecessarily released/acquired because
    > the swap entries belong to the same swap device are non-consecutive in
    > the per-CPU buffer.
    >
    > To solve the issue, the per-CPU buffer is sorted according to the swap
    > device before freeing the swap entries. Test shows that the time
    > spent by swapcache_free_entries() could be reduced after the patch.
    >
    > Test the patch via measuring the run time of swap_cache_free_entries()
    > during the exit phase of the applications use much swap space. The
    > results shows that the average run time of swap_cache_free_entries()
    > reduced about 20% after applying the patch.
    >
    > Signed-off-by: Huang Ying <ying.huang@intel.com>
    > Acked-by: Tim Chen <tim.c.chen@intel.com>
    > Cc: Hugh Dickins <hughd@google.com>
    > Cc: Shaohua Li <shli@kernel.org>
    > Cc: Minchan Kim <minchan@kernel.org>
    > Cc: Rik van Riel <riel@redhat.com>
    >
    > v3:
    >
    > - Add some comments in code per Rik's suggestion.
    >
    > v2:
    >
    > - Avoid sort swap entries if there is only one swap device.
    > ---
    > mm/swapfile.c | 12 ++++++++++++
    > 1 file changed, 12 insertions(+)
    >
    > diff --git a/mm/swapfile.c b/mm/swapfile.c
    > index 90054f3c2cdc..f23c56e9be39 100644
    > --- a/mm/swapfile.c
    > +++ b/mm/swapfile.c
    > @@ -37,6 +37,7 @@
    > #include <linux/swapfile.h>
    > #include <linux/export.h>
    > #include <linux/swap_slots.h>
    > +#include <linux/sort.h>
    >
    > #include <asm/pgtable.h>
    > #include <asm/tlbflush.h>
    > @@ -1065,6 +1066,13 @@ void swapcache_free(swp_entry_t entry)
    > }
    > }
    >
    > +static int swp_entry_cmp(const void *ent1, const void *ent2)
    > +{
    > + const swp_entry_t *e1 = ent1, *e2 = ent2;
    > +
    > + return (long)(swp_type(*e1) - swp_type(*e2));
    > +}
    > +
    > void swapcache_free_entries(swp_entry_t *entries, int n)
    > {
    > struct swap_info_struct *p, *prev;
    > @@ -1075,6 +1083,10 @@ void swapcache_free_entries(swp_entry_t *entries, int n)
    >
    > prev = NULL;
    > p = NULL;
    > +
    > + /* Sort swap entries by swap device, so each lock is only taken once. */
    > + if (nr_swapfiles > 1)
    > + sort(entries, n, sizeof(entries[0]), swp_entry_cmp, NULL);

    Let's think on other cases.

    There are two swaps and they are configured by priority so a swap's usage
    would be zero unless other swap used up. In case of that, this sorting
    is pointless.

    As well, nr_swapfiles is never decreased so if we enable multiple
    swaps and then disable until a swap is remained, this sorting is
    pointelss, too.

    How about lazy sorting approach? IOW, if we found prev != p and,
    then we can sort it.

    Thanks.

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