Re: the XSK buffer pool needs be to reverted

Jonathan Lemon
2020-07-06T19:59:34Z

On Wed, Jul 01, 2020 at 10:46:40AM +0100, Robin Murphy wrote:
> On 2020-06-30 20:08, Jonathan Lemon wrote:
> > On Mon, Jun 29, 2020 at 02:15:16PM +0100, Robin Murphy wrote:
> > > On 2020-06-27 08:02, Christoph Hellwig wrote:
> > > > On Fri, Jun 26, 2020 at 01:54:12PM -0700, Jonathan Lemon wrote:
> > > > > On Fri, Jun 26, 2020 at 09:47:25AM +0200, Christoph Hellwig wrote:
> > > > > >
> > > > > > Note that this is somewhat urgent, as various of the APIs that the code
> > > > > > is abusing are slated to go away for Linux 5.9, so this addition comes
> > > > > > at a really bad time.
> > > > >
> > > > > Could you elaborate on what is upcoming here?
> > > >
> > > > Moving all these calls out of line, and adding a bypass flag to avoid
> > > > the indirect function call for IOMMUs in direct mapped mode.
> > > >
> > > > > Also, on a semi-related note, are there limitations on how many pages
> > > > > can be left mapped by the iommu? Some of the page pool work involves
> > > > > leaving the pages mapped instead of constantly mapping/unmapping them.
> > > >
> > > > There are, but I think for all modern IOMMUs they are so big that they
> > > > don't matter. Maintaines of the individual IOMMU drivers might know
> > > > more.
> > >
> > > Right - I don't know too much about older and more esoteric stuff like POWER
> > > TCE, but for modern pagetable-based stuff like Intel VT-d, AMD-Vi, and Arm
> > > SMMU, the only "limits" are such that legitimate DMA API use should never
> > > get anywhere near them (you'd run out of RAM for actual buffers long
> > > beforehand). The most vaguely-realistic concern might be a pathological
> > > system topology where some old 32-bit PCI device doesn't have ACS isolation
> > > from your high-performance NIC such that they have to share an address
> > > space, where the NIC might happen to steal all the low addresses and prevent
> > > the soundcard or whatever from being able to map a usable buffer.
> > >
> > > With an IOMMU, you typically really *want* to keep a full working set's
> > > worth of pages mapped, since dma_map/unmap are expensive while dma_sync is
> > > somewhere between relatively cheap and free. With no IOMMU it makes no real
> > > difference from the DMA API perspective since map/unmap are effectively no
> > > more than the equivalent sync operations anyway (I'm assuming we're not
> > > talking about the kind of constrained hardware that might need SWIOTLB).
> > >
> > > > > On a heavily loaded box with iommu enabled, it seems that quite often
> > > > > there is contention on the iova_lock. Are there known issues in this
> > > > > area?
> > > >
> > > > I'll have to defer to the IOMMU maintainers, and for that you'll need
> > > > to say what code you are using. Current mainlaine doesn't even have
> > > > an iova_lock anywhere.
> > >
> > > Again I can't speak for non-mainstream stuff outside drivers/iommu, but it's
> > > been over 4 years now since merging the initial scalability work for the
> > > generic IOVA allocator there that focused on minimising lock contention, and
> > > it's had considerable evaluation and tweaking since. But if we can achieve
> > > the goal of efficiently recycling mapped buffers then we shouldn't need to
> > > go anywhere near IOVA allocation either way except when expanding the pool.
> >
> >
> > I'm running a set of patches which uses the page pool to try and keep
> > all the RX buffers mapped as the skb goes up the stack, returning the
> > pages to the pool when the skb is freed.
> >
> > On a dual-socket 12-core Intel machine (48 processors), and 256G of
> > memory, when iommu is enabled, I see the following from 'perf top -U',
> > as the hottest function being run:
> >
> > - 43.42% worker [k] queued_spin_lock_slowpath
> > - 43.42% queued_spin_lock_slowpath
> > - 41.69% _raw_spin_lock_irqsave
> > + 41.39% alloc_iova
> > + 0.28% iova_magazine_free_pfns
> > + 1.07% lock_sock_nested
> >
> > Which likely is heavy contention on the iovad->iova_rbtree_lock.
> > (This is on a 5.6 based system, BTW). More scripts and data are below.
> > Is there a way to reduce the contention here?
>
> Hmm, how big are your DMA mappings? If you're still hitting the rbtree a
> lot, that most likely implies that either you're making giant IOVA
> allocations that are too big to be cached, or you're allocating/freeing
> IOVAs in a pathological pattern that defeats the whole magazine cache
> mechanism (It's optimised for relatively-balanced allocation and freeing of
> sizes up order 6). On a further hunch, does the "intel_iommu=forcedac"
> option make any difference at all?

The allocations are only 4K in size (packet memory) but there are a lot
of them. I tried running with "forcedac" over the weekend, and that
seems to have made a huge difference. Why the name of 'forcedac'? It
would seem this should be the default for a system with a sane 64-bit PCI
device.

> Either way if this persists after some initial warm-up period, it further
> implies that the page pool is not doing its job properly (or at least in the
> way I would have expected). The alloc_iova() call is part of the dma_map_*()
> overhead, and if the aim is to keep pages mapped then that should only be
> called relatively infrequently. The optimal behaviour would be to dma_map()
> new clean pages as they are added to the pool, use dma_sync() when they are
> claimed and returned by the driver, and only dma_unmap() if they're actually
> freed back to the page allocator. And if you're still seeing a lot of
> dma_map/unmap time after that, then the pool itself is churning pages and
> clearly needs its size/thresholds tuning.

Most of the dma_map() overhead is coming from the TX path, which isn't
using the page_pool.

With the RX datapath I'm using, pages are returned to the pool in a
non-napi context (so they enter the ring), then the driver refills the
cache from the ring. I'm seeing a 1% overflow (ring_full), so the
default size seems appropriate. The single ring lock is likely a
problem, but can be addressed now that things aren't blowing up in
alloc_iova().

From a box with the mlx4 driver + persistent page pool:

rx_alloc_pages: 35239524767
cache_hit: 34381377143
cache_full: 19935
cache_empty: 859266494
ring_consume: 31665993135
ring_produce: 31666119491
ring_full: 353805844

--
Jonathan