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

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Date:   Tue, 17 Jan 2023 22:54:22 -0800
From:   John Stultz <jstultz@...gle.com>
To:     jaewon31.kim@...sung.com
Cc:     "sumit.semwal@...aro.org" <sumit.semwal@...aro.org>,
        "daniel.vetter@...ll.ch" <daniel.vetter@...ll.ch>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "hannes@...xchg.org" <hannes@...xchg.org>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jaewon31.kim@...il.com" <jaewon31.kim@...il.com>,
        "T.J. Mercier" <tjmercier@...gle.com>
Subject: Re: [PATCH] dma-buf: system_heap: avoid reclaim for order 4

On Tue, Jan 17, 2023 at 12:31 AM Jaewon Kim <jaewon31.kim@...sung.com> 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?

thanks
-john