kselftests: cgroup: update kmem test precision tolerance

1813e51 ("memcg: increase MEMCG_CHARGE_BATCH to 64") has changed
the batch size while this test case has been left behind. This has led
to a test failure reported by test bot:
not ok 2 selftests: cgroup: test_kmem # exit=1

Update the tolerance for the pcp charges to reflect the
MEMCG_CHARGE_BATCH change to fix this.

[[email protected]: update comments, per Roman]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 1813e51 ("memcg: increase MEMCG_CHARGE_BATCH to 64")
Signed-off-by: Michal Hocko <[email protected]>
Reported-by: kernel test robot <[email protected]>
  Link: https://lore.kernel.org/oe-lkp/[email protected]
Acked-by: Shakeel Butt <[email protected]>
Acked-by: Roman Gushchin <[email protected]>
Tested-by: Yujie Liu <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Feng Tang <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: "Michal Koutný" <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Soheil Hassas Yeganeh <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Michal Hocko

tools/testing/selftests/cgroup/test_kmem.c

@@ -19,12 +19,12 @@
 
 
 /*
- * Memory cgroup charging is performed using percpu batches 32 pages
+ * Memory cgroup charging is performed using percpu batches 64 pages
  * big (look at MEMCG_CHARGE_BATCH), whereas memory.stat is exact. So
  * the maximum discrepancy between charge and vmstat entries is number
- * of cpus multiplied by 32 pages.
+ * of cpus multiplied by 64 pages.
  */
-#define MAX_VMSTAT_ERROR (4096 * 32 * get_nprocs())
+#define MAX_VMSTAT_ERROR (4096 * 64 * get_nprocs())
 
 
 static int alloc_dcache(const char *cgroup, void *arg)