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

Commit 763b218ddf ("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/20190124115648.9433-3-urezki@gmail.com
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Thomas Garnier <thgarnie@google.com>
Cc: Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Joel Fernandes <joelaf@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Tejun Heo <tj@kernel.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/vmalloc.c

@@ -662,23 +662,27 @@ 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) {
@@ -686,7 +690,9 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
 
 		__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;