netdev - Re: [PATCH net-next] net: add network device notifier trace points

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Message-ID: <d1c3a477-f0fe-2c62-84c6-e29ad9be828c@iogearbox.net>
Date:   Wed, 19 Dec 2018 16:46:05 +0100
From:   Daniel Borkmann <borkmann@...earbox.net>
To:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Stephen Hemminger <stephen@...workplumber.org>
Cc:     netdev@...r.kernel.org, Stephen Hemminger <sthemmin@...rosoft.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH net-next] net: add network device notifier trace points

On 12/19/2018 08:36 AM, Jesper Dangaard Brouer wrote:
> On Tue, 18 Dec 2018 18:27:06 -0800
> Stephen Hemminger <stephen@...workplumber.org> wrote:
> 
>> This is the result of a conversation about monitoring of link
>> state changes with BPF.
> 
> If you want to use this from BPF then you are in for a surprise.  As
> tracepoints BPF cannot read these "__string" constructs, here the netdev
> name.  I tried a lot of different tricks that didn't work, see [1],
> until Alexei explained that it simply isn't supported.
> 
> I instead recommend adding the ifindex to the tracepoint.  The__string
> and __assign_str is also a performance concern as it does strcpy behind
> your back.
> 
> I have an year old TODO list item about improving this:
>  ** TODO Make perf-script plugin for ifindex to name translation
>     SCHEDULED: <2017-11-20 Mon>
> 
> Today, the existing network tracepoints using dev->name is not that
> usable by BPF, as BPF cannot identify the interface.  Thus IMHO it would
> make sense to convert the existing network tracepoints dev->name into
> dev->ifindex, and then let perf-script convert this to the interface
> name.  Either in userspace via if_indextoname(3), or (as ACME pointed
> out at the time) we might want to have a lookup table stored together
> with perf.data for later inspection (in-case ifindexes changed).

Hmm, why not just doing something as in your example below with napi_poll()
where you pass in the napi pointer, and then use bpf_probe_read_str() on
ctx->dev for fetching the name? At least there this should work and should
be okay given it's rather slow-path event.

Thanks,
Daniel

> [1] https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/samples/bpf/napi_monitor_kern.c#L34-L130
> 
>> Signed-off-by: Stephen Hemminger <stephen@...workplumber.org>
>> ---
>>  include/trace/events/net.h | 115 +++++++++++++++++++++++++++++++++++++
>>  net/core/dev.c             |   9 ++-
>>  2 files changed, 123 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/trace/events/net.h b/include/trace/events/net.h
>> index 1efd7d9b25fe..141310d24610 100644
>> --- a/include/trace/events/net.h
>> +++ b/include/trace/events/net.h
> [...]
>> +TRACE_EVENT(net_dev_notifier_entry,
>> +
>> +	TP_PROTO(const struct netdev_notifier_info *info, unsigned long val),
>> +
>> +	TP_ARGS(info, val),
>> +
>> +	TP_STRUCT__entry(
>> +		__string(	name,		 info->dev->name )
>> +		__field(	enum netdev_cmd, event	         )
>> +	),
>> +
>> +	TP_fast_assign(
>> +		__assign_str(name, info->dev->name);
>> +		__entry->event = val;
>> +       ),
> 
> These __string and __assign_str are costly and behind the scenes does a
> strcpy.
> 
>> +
>> +	TP_printk("dev=%s event=%s",
>> +		  __get_str(name), netdev_event_type(__entry->event))
>> +);
>> +
>> +TRACE_EVENT(net_dev_notifier,
>> +
>> +	TP_PROTO(const struct netdev_notifier_info *info, int rc, unsigned long val),
>> +
>> +	TP_ARGS(info, rc, val),
>> +
>> +	TP_STRUCT__entry(
>> +		__string(	name,		 info->dev->name   )
>> +		__field(	enum netdev_cmd, event	           )
>> +		__field(	int,		 rc		   )
>> +	),
>> +
>> +	TP_fast_assign(
>> +		__assign_str(name, info->dev->name);
>> +		__entry->event = val;
>> +		__entry->rc = rc;
>> +       ),
>> +
>> +	TP_printk("dev=%s event=%s ret=%d",
>> +		  __get_str(name), netdev_event_type(__entry->event),
>> +		  __entry->rc)
>> +);
>> +
> 
> The bare minimum change is to _also_ add (info->dev->)ifindex to the
> tracepoint, as this makes it usable from BPF.
>