On Mon, Oct 14, 2013 at 02:07:48PM -0700, Eric Dumazet wrote: > On Mon, 2013-10-14 at 09:49 +0200, Ingo Molnar wrote: > > * Andi Kleen <a...@firstfloor.org> wrote: > > > > > Neil Horman <nhor...@tuxdriver.com> writes: > > > > > > > Sébastien Dugué reported to me that devices implementing ipoib (which > > > > don't have checksum offload hardware were spending a significant > > > > amount of time computing > > > > > > Must be an odd workload, most TCP/UDP workloads do copy-checksum > > > anyways. I would rather investigate why that doesn't work. > > > > There's a fair amount of csum_partial()-only workloads, a packet does not > > need to hit user-space to be a significant portion of the system's > > workload. > > > > That said, it would indeed be nice to hear which particular code path was > > hit in this case, if nothing else then for education purposes. > > Many NIC do not provide a CHECKSUM_COMPLETE information for encapsulated > frames, meaning we have to fallback to software csum to validate > TCP frames, once tunnel header is pulled. > > So to reproduce the issue, all you need is to setup a GRE tunnel between > two hosts, and use any tcp stream workload. > > Then receiver profile looks like : > > 11.45% [kernel] [k] csum_partial > 3.08% [kernel] [k] _raw_spin_lock > 3.04% [kernel] [k] intel_idle > 2.73% [kernel] [k] ipt_do_table > 2.57% [kernel] [k] __netif_receive_skb_core > 2.15% [kernel] [k] copy_user_generic_string > 2.05% [kernel] [k] __hrtimer_start_range_ns > 1.42% [kernel] [k] ip_rcv > 1.39% [kernel] [k] kmem_cache_free > 1.36% [kernel] [k] _raw_spin_unlock_irqrestore > 1.24% [kernel] [k] __schedule > 1.13% [bnx2x] [k] bnx2x_rx_int > 1.12% [bnx2x] [k] bnx2x_start_xmit > 1.11% [kernel] [k] fib_table_lookup > 0.99% [ip_tunnel] [k] ip_tunnel_lookup > 0.91% [ip_tunnel] [k] ip_tunnel_rcv > 0.90% [kernel] [k] check_leaf.isra.7 > 0.89% [kernel] [k] nf_iterate > As I noted previously the workload that this got reported on was ipoib, which has a simmilar profile, since infiniband cards tend to not be able to do checksum offload for ip frames.
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