Re: the XSK buffer pool needs be to reverted

[Robin Murphy <robin.murphy@arm.com>]

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?

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.

Robin.

> 
> 
> 
> The following quick and dirty [and possibly wrong] .bpf script was used
> to try and find the time spent in __alloc_and_insert_iova_range():
> 
> kprobe:alloc_iova_fast
> {
>          @fast = count();
> }
> 
> kprobe:alloc_iova
> {
>          @iova_start[tid] = nsecs;
>          @iova = count();
> }
> 
> kretprobe:alloc_iova / @iova_start[tid] /
> {
>          @alloc_h = hist(nsecs - @iova_start[tid] - @mem_delta[tid]);
>          delete(@iova_start[tid]);
>          delete(@mem_delta[tid]);
> }
> 
> kprobe:alloc_iova_mem / @iova_start[tid] /
> {
>          @mem_start[tid] = nsecs;
> }
> 
> kretprobe:alloc_iova_mem / @mem_start[tid] /
> {
>          @mem_delta[tid] = nsecs - @mem_start[tid];
>          delete(@mem_start[tid]);
> }
> 
> kprobe:iova_insert_rbtree / @iova_start[tid] /
> {
>          @rb_start[tid] = nsecs;
>          @rbtree = count();
> }
> 
> kretprobe:iova_insert_rbtree / @rb_start[tid] /
> {
>          @insert_h = hist(nsecs - @rb_start[tid]);
>          delete(@rb_start[tid]);
> }
> 
> interval:s:2
> {
>          print(@fast);
>          print(@iova);
>          print(@rbtree);
>          print(@alloc_h);
>          print(@insert_h);
>          printf("--------\n");
> }
> 
> I see the following results.
> 
> @fast: 1989223
> @iova: 725269
> @rbtree: 689306
> 
> @alloc_h:
> [64, 128)              2 |                                                    |
> [128, 256)           118 |                                                    |
> [256, 512)           983 |                                                    |
> [512, 1K)           3816 |@@                                                  |
> [1K, 2K)           10557 |@@@@@@                                              |
> [2K, 4K)           19540 |@@@@@@@@@@@@                                        |
> [4K, 8K)           31294 |@@@@@@@@@@@@@@@@@@@                                 |
> [8K, 16K)          38112 |@@@@@@@@@@@@@@@@@@@@@@@                             |
> [16K, 32K)         46948 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@                        |
> [32K, 64K)         69728 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@         |
> [64K, 128K)        83797 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ |
> [128K, 256K)       84317 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
> [256K, 512K)       82962 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ |
> [512K, 1M)         72751 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@        |
> [1M, 2M)           49191 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@                      |
> [2M, 4M)           26591 |@@@@@@@@@@@@@@@@                                    |
> [4M, 8M)           15559 |@@@@@@@@@                                           |
> [8M, 16M)          12283 |@@@@@@@                                             |
> [16M, 32M)         18266 |@@@@@@@@@@@                                         |
> [32M, 64M)         22539 |@@@@@@@@@@@@@                                       |
> [64M, 128M)         3005 |@                                                   |
> [128M, 256M)          41 |                                                    |
> [256M, 512M)           0 |                                                    |
> [512M, 1G)             0 |                                                    |
> [1G, 2G)               0 |                                                    |
> [2G, 4G)             101 |                                                    |
> 
> @insert_h:
> [128, 256)          2380 |                                                    |
> [256, 512)         70043 |@@@@@@@@                                            |
> [512, 1K)         431263 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
> [1K, 2K)          182804 |@@@@@@@@@@@@@@@@@@@@@@                              |
> [2K, 4K)            2742 |                                                    |
> [4K, 8K)              43 |                                                    |
> [8K, 16K)             25 |                                                    |
> [16K, 32K)             0 |                                                    |
> [32K, 64K)             0 |                                                    |
> [64K, 128K)            0 |                                                    |
> [128K, 256K)           6 |                                                    |
> 
> 
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