Messages in this thread | | | Subject | Re: [PATCH net] xsk: remove cheap_dma optimization | From | Daniel Borkmann <> | Date | Sat, 27 Jun 2020 01:00:19 +0200 |
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On 6/26/20 3:43 PM, Björn Töpel wrote: > From: Björn Töpel <bjorn.topel@intel.com> > > When the AF_XDP buffer allocation API was introduced it had an > optimization, "cheap_dma". The idea was that when the umem was DMA > mapped, the pool also checked whether the mapping required a > synchronization (CPU to device, and vice versa). If not, it would be > marked as "cheap_dma" and the synchronization would be elided. > > In [1] Christoph points out that the optimization above breaks the DMA > API abstraction, and should be removed. Further, Christoph points out > that optimizations like this should be done within the DMA mapping > core, and not elsewhere. > > Unfortunately this has implications for the packet rate > performance. The AF_XDP rxdrop scenario shows a 9% decrease in packets > per second. > > [1] https://lore.kernel.org/netdev/20200626074725.GA21790@lst.de/ > > Cc: Christoph Hellwig <hch@lst.de> > Fixes: 2b43470add8c ("xsk: Introduce AF_XDP buffer allocation API") > Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Given there is roughly a ~5 weeks window at max where this removal could still be applied in the worst case, could we come up with a fix / proposal first that moves this into the DMA mapping core? If there is something that can be agreed upon by all parties, then we could avoid re-adding the 9% slowdown. :/
Thanks, Daniel
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