Re: [PATCH -mm -v3] mm, swap: Sort swap entries before free

[Minchan Kim]

On Wed, Apr 19, 2017 at 04:14:43PM +0800, Huang, Ying wrote:
> Minchan Kim <minchan@xxxxxxxxxx> writes:
> 
> > Hi Huang,
> >
> > On Fri, Apr 07, 2017 at 02:49:01PM +0800, Huang, Ying wrote:
> >> From: Huang Ying <ying.huang@xxxxxxxxx>
> >> 
> >> 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@xxxxxxxxx>
> >> Acked-by: Tim Chen <tim.c.chen@xxxxxxxxx>
> >> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> >> Cc: Shaohua Li <shli@xxxxxxxxxx>
> >> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> >> Cc: Rik van Riel <riel@xxxxxxxxxx>
> >> 
> >> 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.
> 
> Yes.  That should be better.  I just don't know whether the added
> complexity is necessary, given the array is short and sort is fast.

Huh?

1. swapon /dev/XXX1
2. swapon /dev/XXX2
3. swapoff /dev/XXX2
4. use only one swap
5. then, always pointless sort.

Do not add such bogus code.

Nacked.