[PATCH] staging: android: ion: Make ion_free asynchronous more actively.

[ming.ling]

From: Ming Ling <ming.ling@xxxxxxxxxxxxxx>

So far some ion heaps such as carveout_heap, chunk_heap,
system_heap have freed buffers asynchrounously. Freed buffers
are placed on a free list and freed from a low priority background
thread. If allocations from a particular heap fail, the free list
is drained.

But that mechanism does not work well to free those buffers when
cpu is very busy:
	1.That low priority background thread has no chance to run.
	2.And with helps of kinds of memory reclaim mechanisms,
	allocations from a system_heap never fail, so the free list
	can't be drained on that condition.
	3.Ion_heap_shrink_scan can shrink the free list, but it is
	too slow.

Too many freed buffers in heap's free list waiting to be freed will
lead to low memory killer over-killing innocent processes.

For example:
On my Android Mobile phone(arm32, 2G ram)
lowmemorykiller: Killing 'viders.calendar' (2271), adj 15,
to free 45060kB on behalf of 'kswapd0' (60) because
cache 17208kB is below limit 46080kB for oom_score_adj 0.
Free memory is 3424kB above reserved.

Memory footprint is as below:
--------------------------------------------
MemTotal:        1972164 kB
--------------------------------------------
Normal free:43624kB min:3228kB low:5308kB high:6116kB active_anon:
656kB inactive_anon:1308kB active_file:10600kB inactive_file:6628kB
HighMem free:13208kB min:512kB low:4720kB high:6352kB active_anon:
126188kB inactive_anon:74364kB active_file:325720kB inactive_file:
585832k
--------------------------------------------
ion system heap:
--------------------
	total orphaned		9961472
	total			58986496
	deferred free		630575104
--------------------------------------------

There are about 630575104=601MB memory on ion system heap's free
list waiting to be freed. And it kills more processes, until ion low
priority background thread gets a chance to run as below:
lowmemorykiller: Killing 'd.process.media' (1402), adj 11,
lowmemorykiller: Killing 'dsrt.srtmemtest' (2552), adj 9,
lowmemorykiller: Killing 'rd.engineermode' (2167), adj 8,
lowmemorykiller: Killing 'rd.fileexplorer' (2431), adj 7,
lowmemorykiller: Killing 'd.process.acore' (1499), adj 5,
...

Limit ion_heap_deferred_free thread as a low priority thread doesn't
make any sense. Remove that restriction makes ion_free asynchronous
more actively, and more background applications can survive.

Signed-off-by: Ming Ling <ming.ling@xxxxxxxxxxxxxx>
---
 drivers/staging/android/ion/ion_heap.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/drivers/staging/android/ion/ion_heap.c b/drivers/staging/android/ion/ion_heap.c
index 4e5c0f1..d68a80c 100644
--- a/drivers/staging/android/ion/ion_heap.c
+++ b/drivers/staging/android/ion/ion_heap.c
@@ -250,8 +250,6 @@ static int ion_heap_deferred_free(void *data)
 
 int ion_heap_init_deferred_free(struct ion_heap *heap)
 {
-	struct sched_param param = { .sched_priority = 0 };
-
 	INIT_LIST_HEAD(&heap->free_list);
 	init_waitqueue_head(&heap->waitqueue);
 	heap->task = kthread_run(ion_heap_deferred_free, heap,
@@ -261,7 +259,6 @@ int ion_heap_init_deferred_free(struct ion_heap *heap)
 		       __func__);
 		return PTR_ERR_OR_ZERO(heap->task);
 	}
-	sched_setscheduler(heap->task, SCHED_IDLE, &param);
 	return 0;
 }
 
-- 
1.9.1