Re: [PATCH v2] dma-buf: system_heap: avoid reclaim for order 4

[Jaewon Kim]

>Using order 4 pages would be helpful for IOMMUs mapping, but trying to get order 4 pages could spend quite much time in the page allocation.
>From the perspective of responsiveness, the deterministic memory allocation speed, I think, is quite important.
>
>The order 4 allocation with __GFP_RECLAIM may spend much time in reclaim and compation logic. __GFP_NORETRY also may affect. These cause unpredictable delay.
>
>To get reasonable allocation speed from dma-buf system heap, use HIGH_ORDER_GFP for order 4 to avoid reclaim. And let me remove meaningless __GFP_COMP for order 0.
>
>According to my tests, order 4 with MID_ORDER_GFP could get more number of order 4 pages but the elapsed times could be very slow.
>
>         time	order 8	order 4	order 0
>     584 usec	0	160	0
>  28,428 usec	0	160	0
> 100,701 usec	0	160	0
>  76,645 usec	0	160	0
>  25,522 usec	0	160	0
>  38,798 usec	0	160	0
>  89,012 usec	0	160	0
>  23,015 usec	0	160	0
>  73,360 usec	0	160	0
>  76,953 usec	0	160	0
>  31,492 usec	0	160	0
>  75,889 usec	0	160	0
>  84,551 usec	0	160	0
>  84,352 usec	0	160	0
>  57,103 usec	0	160	0
>  93,452 usec	0	160	0
>
>If HIGH_ORDER_GFP is used for order 4, the number of order 4 could be decreased but the elapsed time results were quite stable and fast enough.
>
>         time	order 8	order 4	order 0
>   1,356 usec	0	155	80
>   1,901 usec	0	11	2384
>   1,912 usec	0	0	2560
>   1,911 usec	0	0	2560
>   1,884 usec	0	0	2560
>   1,577 usec	0	0	2560
>   1,366 usec	0	0	2560
>   1,711 usec	0	0	2560
>   1,635 usec	0	28	2112
>     544 usec	10	0	0
>     633 usec	2	128	0
>     848 usec	0	160	0
>     729 usec	0	160	0
>   1,000 usec	0	160	0
>   1,358 usec	0	160	0
>   2,638 usec	0	31	2064
>
>Signed-off-by: Jaewon Kim <jaewon31.kim@xxxxxxxxxxx>
>---
> drivers/dma-buf/heaps/system_heap.c | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
>diff --git a/drivers/dma-buf/heaps/system_heap.c b/drivers/dma-buf/heaps/system_heap.c
>index e8bd10e60998..920db302a273 100644
>--- a/drivers/dma-buf/heaps/system_heap.c
>+++ b/drivers/dma-buf/heaps/system_heap.c
>@@ -41,12 +41,11 @@ struct dma_heap_attachment {
> 	bool mapped;
> };
> 
>-#define LOW_ORDER_GFP (GFP_HIGHUSER | __GFP_ZERO | __GFP_COMP) -#define MID_ORDER_GFP (LOW_ORDER_GFP | __GFP_NOWARN)
>+#define LOW_ORDER_GFP (GFP_HIGHUSER | __GFP_ZERO)
> #define HIGH_ORDER_GFP  (((GFP_HIGHUSER | __GFP_ZERO | __GFP_NOWARN \
> 				| __GFP_NORETRY) & ~__GFP_RECLAIM) \
> 				| __GFP_COMP)
>-static gfp_t order_flags[] = {HIGH_ORDER_GFP, MID_ORDER_GFP, LOW_ORDER_GFP};
>+static gfp_t order_flags[] = {HIGH_ORDER_GFP, HIGH_ORDER_GFP, 
>+LOW_ORDER_GFP};
> /*
>  * The selection of the orders used for allocation (1MB, 64K, 4K) is designed
>  * to match with the sizes often found in IOMMUs. Using order 4 pages instead
>--
>2.17.1
>
>

added John Stultz <jstultz@xxxxxxxxxx>