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

[Jaewon Kim]

> > On Tue, Jan 17, 2023 at 10:54 PM John Stultz <jstultz@xxxxxxxxxx> wrote:
> > >
> > > On Tue, Jan 17, 2023 at 12:31 AM Jaewon Kim <jaewon31.kim@xxxxxxxxxxx> wrote:
> > > > > Using order 4 pages would be helpful for many IOMMUs, but it could spend
> > > > > quite much time in page allocation perspective.
> > > > >
> > > > > 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.
> > >
> > > Thanks for sharing this!
> > > The case where the allocation gets stuck behind reclaim under pressure
> > > does sound undesirable, but I'd be a bit hesitant to tweak numbers
> > > that have been used for a long while (going back to ion) without a bit
> > > more data.
> > >
> > > It might be good to also better understand the tradeoff of potential
> > > on-going impact to performance from using low order pages when the
> > > buffer is used.  Do you have any details like or tests that you could
> > > share to help ensure this won't impact other users?
> > >
> > > TJ: Do you have any additional thoughts on this?
> > >
> > I don't have any data on how often we hit reclaim for mid order
> > allocations. That would be interesting to know. However the 70th
> > percentile of system-wide buffer sizes while running the camera on my
> > phone is still only 1 page, so it looks like this change would affect
> > a subset of use-cases.
> > 
> > Wouldn't this change make it less likely to get an order 4 allocation
> > (under memory pressure)? The commit message makes me think the goal of
> > the change is to get more of them.
> 
> Hello John Stultz
> 
> I've been waiting for your next reply.
> 
> With my commit, we may gather less number of order 4 pages and fill the 
> requested size with more number of order 0 pages. I think, howerver, stable
> allocation speed is quite important so that corresponding user space
> context can move on within a specific time.
> 
> Not only compaction but reclaim also, I think, would be invoked more if the 
> __GFP_RECLAIM is added on order 4. I expect the reclaim could be decreased
> if we move to order 0.
> 

Additionally I'd like to say the old legacy ion system heap also used the
__GFP_RECLAIM only for order 8, not for order 4.

drivers/staging/android/ion/ion_system_heap.c

static gfp_t high_order_gfp_flags = (GFP_HIGHUSER | __GFP_ZERO | __GFP_NOWARN |
                                    __GFP_NORETRY) & ~__GFP_RECLAIM;
static gfp_t low_order_gfp_flags  = GFP_HIGHUSER | __GFP_ZERO;
static const unsigned int orders[] = {8, 4, 0};

static int ion_system_heap_create_pools(struct ion_page_pool **pools)
{
       int i;

       for (i = 0; i < NUM_ORDERS; i++) {
               struct ion_page_pool *pool;
               gfp_t gfp_flags = low_order_gfp_flags;

               if (orders[i] > 4)
                       gfp_flags = high_order_gfp_flags;


> Thank you
> Jaewon Kim
> 
> > 
> > Actually with the low order being 0, I don't think __GFP_COMP makes
> > sense in LOW_ORDER_GFP. But I guess that flag isn't harmful there.
> > 
> > > thanks
> > > -john