Re: [PATCH v2 bpf-next 1/2] xdp: Add tracepoint for bulk XDP_TX

[Toshiaki Makita <toshiaki.makita1@gmail.com>]

On 2019/06/05 16:59, Jesper Dangaard Brouer wrote:
> On Wed,  5 Jun 2019 14:36:12 +0900
> Toshiaki Makita <toshiaki.makita1@gmail.com> wrote:
> 
>> This is introduced for admins to check what is happening on XDP_TX when
>> bulk XDP_TX is in use, which will be first introduced in veth in next
>> commit.
> 
> Is the plan that this tracepoint 'xdp:xdp_bulk_tx' should be used by
> all drivers?

I guess you mean all drivers that implement similar mechanism should use 
this? Then yes.
(I don't think all drivers needs bulk tx mechanism though)

> (more below)
> 
>> Signed-off-by: Toshiaki Makita <toshiaki.makita1@gmail.com>
>> ---
>>   include/trace/events/xdp.h | 25 +++++++++++++++++++++++++
>>   kernel/bpf/core.c          |  1 +
>>   2 files changed, 26 insertions(+)
>>
>> diff --git a/include/trace/events/xdp.h b/include/trace/events/xdp.h
>> index e95cb86..e06ea65 100644
>> --- a/include/trace/events/xdp.h
>> +++ b/include/trace/events/xdp.h
>> @@ -50,6 +50,31 @@
>>   		  __entry->ifindex)
>>   );
>>   
>> +TRACE_EVENT(xdp_bulk_tx,
>> +
>> +	TP_PROTO(const struct net_device *dev,
>> +		 int sent, int drops, int err),
>> +
>> +	TP_ARGS(dev, sent, drops, err),
>> +
>> +	TP_STRUCT__entry(
> 
> All other tracepoints in this file starts with:
> 
> 		__field(int, prog_id)
> 		__field(u32, act)
> or
> 		__field(int, map_id)
> 		__field(u32, act)
> 
> Could you please add those?

So... prog_id is the problem. The program can be changed while we are 
enqueueing packets to the bulk queue, so the prog_id at flush may be an 
unexpected one.

It can be fixed by disabling NAPI when changing XDP programs. This stops 
packet processing while changing XDP programs, but I guess it is an 
acceptable compromise. Having said that, I'm honestly not so eager to 
make this change, since this will require refurbishment of one of the 
most delicate part of veth XDP, NAPI disabling/enabling mechanism.

WDYT?

>> +		__field(int, ifindex)
>> +		__field(int, drops)
>> +		__field(int, sent)
>> +		__field(int, err)
>> +	),
> 
> The reason is that this make is easier to attach to multiple
> tracepoints, and extract the same value.
> 
> Example with bpftrace oneliner:
> 
> $ sudo bpftrace -e 'tracepoint:xdp:xdp_* { @action[args->act] = count(); }'
> Attaching 8 probes...
> ^C
> 
> @action[4]: 30259246
> @action[0]: 34489024
> 
> XDP_ABORTED = 0 	
> XDP_REDIRECT= 4
> 
> 
>> +
>> +	TP_fast_assign(
> 
> 		__entry->act		= XDP_TX;

OK

> 
>> +		__entry->ifindex	= dev->ifindex;
>> +		__entry->drops		= drops;
>> +		__entry->sent		= sent;
>> +		__entry->err		= err;
>> +	),
>> +
>> +	TP_printk("ifindex=%d sent=%d drops=%d err=%d",
>> +		  __entry->ifindex, __entry->sent, __entry->drops, __entry->err)
>> +);
>> +
> 
> Other fun bpftrace stuff:
> 
> sudo bpftrace -e 'tracepoint:xdp:xdp_*map* { @map_id[comm, args->map_id] = count(); }'
> Attaching 5 probes...
> ^C
> 
> @map_id[swapper/2, 113]: 1428
> @map_id[swapper/0, 113]: 2085
> @map_id[ksoftirqd/4, 113]: 2253491
> @map_id[ksoftirqd/2, 113]: 25677560
> @map_id[ksoftirqd/0, 113]: 29004338
> @map_id[ksoftirqd/3, 113]: 31034885
> 
> 
> $ bpftool map list id 113
> 113: devmap  name tx_port  flags 0x0
> 	key 4B  value 4B  max_entries 100  memlock 4096B
> 
> 
> p.s. People should look out for Brendan Gregg's upcoming book on BPF
> performance tools, from which I learned to use bpftrace :-)

Where can I get information on the book?

--
Toshiaki Makita