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Date: Thu, 06 May 2010 17:37:56 +0200
From: Eric Dumazet <eric.dumazet@...il.com>
To: Tom Herbert <therbert@...gle.com>
Cc: David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next-2.6] net: Consistent skb timestamping
Le jeudi 06 mai 2010 à 08:12 -0700, Tom Herbert a écrit :
> On Thu, May 6, 2010 at 5:01 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > With RPS inclusion, skb timestamping is not consistent in RX path.
> >
> > If netif_receive_skb() is used, its deferred after RPS dispatch.
> >
> > If netif_rx() is used, its done before RPS dispatch.
> >
> > This can give strange tcpdump timestamps results.
> >
> > I think timestamping should be done as soon as possible in the receive
> > path, to get meaningful values (ie timestamps taken at the time packet
> > was delivered by NIC driver to our stack), even if NAPI already can
> > defer timestamping a bit (RPS can help to reduce the gap)
> >
> The counter argument to this is that it moves another thing into the
> serialized path for networking which slows everyone down. I'm not
> concerned about when tcpdump is running since performance will suck
> anyway, but what is bad is if any single socket in the system turns on
> SO_TIMESTAMP, overhead is incurred on *every* packet. This happens
> regardless of whether the application ever actually gets a timestamp,
> or even whether timestamps are supported by the protocol (try setting
> SO_TIMESTAMP on a TCP socket ;-) ). I'm contemplating changing
> SO_TIMESTAMP to not enable global timestamps, but only take the
> timestamp for a packet once the socket is identified and the timestamp
> flag is set (this is the technique done in FreeBSD and Solaris, so I
> believe the external semantics would still be valid).
I agree with you, thanks for this excellent argument.
Right now, timestamping is not meant for userland pleasure, but for
sniffers and network diagnostics. (I mean with current API, not with a
new one we could add later)
Once we settle a per socket timestamping, not global, we can reconsider
the thing (or not reconsider it, since socket timestamping will be done
after RPS dispatch)
Its true our global variable to enable/disable timestamp sucks, but its
a separate issue ;)
We probably could have a sysctl to let admin chose the moment timestamp
takes place (before or after RPS)
If TSC is available, here is the "perf top" of the cpu handling
1.200.000 packets per second, while timestamping is requested :
You can hardly see something about time services :
--------------------------------------------------------------------------------------------------------------------------
PerfTop: 983 irqs/sec kernel:99.5% [1000Hz cycles], (all, cpu: 10)
--------------------------------------------------------------------------------------------------------------------------
samples pcnt function DSO
_______ _____ ___________________________________ _______
1568.00 14.9% bnx2x_rx_int vmlinux
1133.00 10.7% eth_type_trans vmlinux
798.00 7.6% kmem_cache_alloc_node vmlinux
720.00 6.8% _raw_spin_lock vmlinux
709.00 6.7% __kmalloc_node_track_caller vmlinux
547.00 5.2% __memset vmlinux
540.00 5.1% __slab_alloc vmlinux
453.00 4.3% get_rps_cpu vmlinux
402.00 3.8% _raw_spin_lock_irqsave vmlinux
295.00 2.8% enqueue_to_backlog vmlinux
271.00 2.6% default_send_IPI_mask_sequence_phys vmlinux
259.00 2.5% get_partial_node vmlinux
235.00 2.2% __alloc_skb vmlinux
227.00 2.2% vlan_gro_common vmlinux
206.00 2.0% swiotlb_dma_mapping_error vmlinux
201.00 1.9% skb_put vmlinux
118.00 1.1% getnstimeofday vmlinux
97.00 0.9% csd_lock vmlinux
96.00 0.9% swiotlb_map_page vmlinux
85.00 0.8% read_tsc vmlinux
76.00 0.7% dev_gro_receive vmlinux
75.00 0.7% __napi_complete vmlinux
74.00 0.7% bnx2x_poll vmlinux
73.00 0.7% unmap_single vmlinux
72.00 0.7% netif_receive_skb vmlinux
66.00 0.6% irq_entries_start vmlinux
65.00 0.6% net_rps_action_and_irq_enable vmlinux
62.00 0.6% __phys_addr vmlinux
If HPET or acpi_pm is used, then you can cry :)
(820.000 pps, or 570.000 pps max)
--------------------------------------------------------------------------------------------------------------------------
PerfTop: 1001 irqs/sec kernel:100.0% [1000Hz cycles], (all, cpu: 10)
--------------------------------------------------------------------------------------------------------------------------
samples pcnt function DSO
_______ _____ ___________________________________ _______
6488.00 48.4% read_hpet vmlinux
1214.00 9.1% bnx2x_rx_int vmlinux
820.00 6.1% eth_type_trans vmlinux
679.00 5.1% _raw_spin_lock vmlinux
678.00 5.1% kmem_cache_alloc_node vmlinux
607.00 4.5% __slab_alloc vmlinux
478.00 3.6% __kmalloc_node_track_caller vmlinux
404.00 3.0% __memset vmlinux
246.00 1.8% get_partial_node vmlinux
213.00 1.6% get_rps_cpu vmlinux
195.00 1.5% enqueue_to_backlog vmlinux
171.00 1.3% __alloc_skb vmlinux
163.00 1.2% vlan_gro_common vmlinux
135.00 1.0% swiotlb_dma_mapping_error vmlinux
118.00 0.9% skb_put vmlinux
88.00 0.7% getnstimeofday vmlinux
60.00 0.4% swiotlb_map_page vmlinux
59.00 0.4% dev_gro_receive vmlinux
--------------------------------------------------------------------------------------------------------------------------
PerfTop: 1001 irqs/sec kernel:100.0% [1000Hz cycles], (all, cpu: 10)
--------------------------------------------------------------------------------------------------------------------------
samples pcnt function DSO
_______ _____ ___________________________________ _______
2573.00 68.3% acpi_pm_read vmlinux
237.00 6.3% bnx2x_rx_int vmlinux
153.00 4.1% eth_type_trans vmlinux
101.00 2.7% kmem_cache_alloc_node vmlinux
99.00 2.6% __kmalloc_node_track_caller vmlinux
79.00 2.1% get_rps_cpu vmlinux
75.00 2.0% __memset vmlinux
72.00 1.9% _raw_spin_lock vmlinux
68.00 1.8% __slab_alloc vmlinux
40.00 1.1% enqueue_to_backlog vmlinux
39.00 1.0% __alloc_skb vmlinux
27.00 0.7% get_partial_node vmlinux
23.00 0.6% swiotlb_dma_mapping_error vmlinux
22.00 0.6% vlan_gro_common vmlinux
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