mm: use sc->priority for slab shrink targets

Previously we were using the ratio of the number of lru pages scanned to
the number of eligible lru pages to determine the number of slab objects
to scan.  The problem with this is that these two things have nothing to
do with each other, so in slab heavy work loads where there is little to
no page cache we can end up with the pages scanned being a very low
number.  This means that we reclaim next to no slab pages and waste a
lot of time reclaiming small amounts of space.

Consider the following scenario, where we have the following values and
the rest of the memory usage is in slab

  Active:            58840 kB
  Inactive:          46860 kB

Every time we do a get_scan_count() we do this

  scan = size >> sc->priority

where sc->priority starts at DEF_PRIORITY, which is 12.  The first loop
through reclaim would result in a scan target of 2 pages to 11715 total
inactive pages, and 3 pages to 14710 total active pages.  This is a
really really small target for a system that is entirely slab pages.
And this is super optimistic, this assumes we even get to scan these
pages.  We don't increment sc->nr_scanned unless we 1) isolate the page,
which assumes it's not in use, and 2) can lock the page.  Under pressure
these numbers could probably go down, I'm sure there's some random pages
from daemons that aren't actually in use, so the targets get even
smaller.

Instead use sc->priority in the same way we use it to determine scan
amounts for the lru's.  This generally equates to pages.  Consider the
following

  slab_pages = (nr_objects * object_size) / PAGE_SIZE

What we would like to do is

  scan = slab_pages >> sc->priority

but we don't know the number of slab pages each shrinker controls, only
the objects.  However say that theoretically we knew how many pages a
shrinker controlled, we'd still have to convert this to objects, which
would look like the following

  scan = shrinker_pages >> sc->priority
  scan_objects = (PAGE_SIZE / object_size) * scan

or written another way

  scan_objects = (shrinker_pages >> sc->priority) *
		 (PAGE_SIZE / object_size)

which can thus be written

  scan_objects = ((shrinker_pages * PAGE_SIZE) / object_size) >>
		 sc->priority

which is just

  scan_objects = nr_objects >> sc->priority

We don't need to know exactly how many pages each shrinker represents,
it's objects are all the information we need.  Making this change allows
us to place an appropriate amount of pressure on the shrinker pools for
their relative size.

Link: http://lkml.kernel.org/r/1510780549-6812-1-git-send-email-josef@toxicpanda.com


Signed-off-by: Josef Bacik <jbacik@fb.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Dave Chinner <david@fromorbit.com>
Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>