mm/vmalloc.c: add priority threshold to __purge_vmap_area_lazy()

Commit 763b218 ("mm: add preempt points into __purge_vmap_area_lazy()")
introduced some preempt points, one of those is making an allocation
more prioritized over lazy free of vmap areas.

Prioritizing an allocation over freeing does not work well all the time,
i.e.  it should be rather a compromise.

1) Number of lazy pages directly influences the busy list length thus
   on operations like: allocation, lookup, unmap, remove, etc.

2) Under heavy stress of vmalloc subsystem I run into a situation when
   memory usage gets increased hitting out_of_memory -> panic state due to
   completely blocking of logic that frees vmap areas in the
   __purge_vmap_area_lazy() function.

Establish a threshold passing which the freeing is prioritized back over
allocation creating a balance between each other.

Using vmalloc test driver in "stress mode", i.e.  When all available
test cases are run simultaneously on all online CPUs applying a
pressure on the vmalloc subsystem, my HiKey 960 board runs out of
memory due to the fact that __purge_vmap_area_lazy() logic simply is
not able to free pages in time.

How I run it:

1) You should build your kernel with CONFIG_TEST_VMALLOC=m
2) ./tools/testing/selftests/vm/test_vmalloc.sh stress

During this test "vmap_lazy_nr" pages will go far beyond acceptable
lazy_max_pages() threshold, that will lead to enormous busy list size
and other problems including allocation time and so on.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Uladzislau Rezki (Sony) <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Thomas Garnier <[email protected]>
Cc: Oleksiy Avramchenko <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Joel Fernandes <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Joel Fernandes <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/vmalloc.c

@@ -662,31 +662,37 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
 	struct llist_node *valist;
 	struct vmap_area *va;
 	struct vmap_area *n_va;
-	bool do_free = false;
+	int resched_threshold;
 
 	lockdep_assert_held(&vmap_purge_lock);
 
 	valist = llist_del_all(&vmap_purge_list);
+	if (unlikely(valist == NULL))
+		return false;
+
+	/*
+	 * TODO: to calculate a flush range without looping.
+	 * The list can be up to lazy_max_pages() elements.
+	 */
 	llist_for_each_entry(va, valist, purge_list) {
 		if (va->va_start < start)
 			start = va->va_start;
 		if (va->va_end > end)
 			end = va->va_end;
-		do_free = true;
 	}
 
-	if (!do_free)
-		return false;
-
 	flush_tlb_kernel_range(start, end);
+	resched_threshold = (int) lazy_max_pages() << 1;
 
 	spin_lock(&vmap_area_lock);
 	llist_for_each_entry_safe(va, n_va, valist, purge_list) {
 		int nr = (va->va_end - va->va_start) >> PAGE_SHIFT;
 
 		__free_vmap_area(va);
 		atomic_sub(nr, &vmap_lazy_nr);
-		cond_resched_lock(&vmap_area_lock);
+
+		if (atomic_read(&vmap_lazy_nr) < resched_threshold)
+			cond_resched_lock(&vmap_area_lock);
 	}
 	spin_unlock(&vmap_area_lock);
 	return true;