Re: [PATCH RFC net-next 2/4] net: page_pool: avoid calling no-op externals when possible

[Alexander Duyck <alexander.duyck@gmail.com>]

On Fri, Jun 30, 2023 at 5:30 AM Alexander Lobakin
<aleksander.lobakin@intel.com> wrote:
>
> From: Alexander H Duyck <alexander.duyck@gmail.com>
> Date: Thu, 29 Jun 2023 09:45:26 -0700
>
> > On Thu, 2023-06-29 at 17:23 +0200, Alexander Lobakin wrote:
> >> Turned out page_pool_put{,_full}_page() can burn quite a bunch of cycles
> >> even when on DMA-coherent platforms (like x86) with no active IOMMU or
> >> swiotlb, just for the call ladder.
> >> Indeed, it's
>
> [...]
>
> >> @@ -341,6 +345,12 @@ static bool page_pool_dma_map(struct page_pool *pool, struct page *page)
> >>
> >>      page_pool_set_dma_addr(page, dma);
> >>
> >> +    if ((pool->p.flags & PP_FLAG_DMA_MAYBE_SYNC) &&
> >> +        dma_need_sync(pool->p.dev, dma)) {
> >> +            pool->p.flags |= PP_FLAG_DMA_SYNC_DEV;
> >> +            pool->p.flags &= ~PP_FLAG_DMA_MAYBE_SYNC;
> >> +    }
> >> +
> >>      if (pool->p.flags & PP_FLAG_DMA_SYNC_DEV)
> >>              page_pool_dma_sync_for_device(pool, page, pool->p.max_len);
> >>
> >
> > I am pretty sure the logic is flawed here. The problem is
> > dma_needs_sync depends on the DMA address being used. In the worst case
> > scenario we could have a device that has something like a 32b DMA
> > address space on a system with over 4GB of memory. In such a case the
> > higher addresses would need to be synced because they will go off to a
> > swiotlb bounce buffer while the lower addresses wouldn't.
> >
> > If you were to store a flag like this it would have to be generated per
> > page.
>
> I know when DMA might need syncing :D That's the point of this shortcut:
> if at least one page needs syncing, I disable it for the whole pool.
> It's a "better safe than sorry".
> Using a per-page flag involves more changes and might hurt some
> scenarios/setups. For example, non-coherent systems, where you always
> need to do syncs. The idea was to give some improvement when possible,
> otherwise just fallback to what we have today.

I am not a fan of having the page pool force the syncing either. Last
I knew I thought the PP_FLAG_DMA_SYNC_DEV was meant to be set by the
driver, not by the page pool API itself. The big reason for that being
that the driver in many cases will have to take care of the DMA sync
itself instead of letting the allocator take care of it.

Basically we are just trading off the dma_need_sync cost versus the
page_pool_dma_sync_for_device cost. If we think it is a win to call
dma_need_sync for every frame then maybe we should look at folding it
into page_pool_dma_sync_for_device itself since that is the only
consumer of it it or just fold it into the PP_FLAG_DMA_SYNC_DEV if
statement after the flag check rather than adding yet another flag
that will likely always be true for most devices. Otherwise you are
just adding overhead for the non-exception case and devices that don't
bother setting PP_FLAG_DMA_SYNC_DEV.