Re: [RFC PATCH v3 4/5] skb: add tracepoints to freeing skb

[Neil Horman]

On Wed, Jul 21, 2010 at 04:02:57PM +0900, Koki Sanagi wrote:
> (2010/07/20 20:50), Neil Horman wrote:
> > On Tue, Jul 20, 2010 at 09:49:10AM +0900, Koki Sanagi wrote:
> >> [RFC PATCH v3 4/5] skb: add tracepoints to freeing skb
> >> This patch adds tracepoint to consume_skb, dev_kfree_skb_irq and
> >> skb_free_datagram_locked. Combinating with tracepoint on dev_hard_start_xmit,
> >> we can check how long it takes to free transmited packets. And using it, we can
> >> calculate how many packets driver had at that time. It is useful when a drop of
> >> transmited packet is a problem.
> >>
> >>           <idle>-0     [001] 241409.218333: consume_skb: skbaddr=dd6b2fb8
> >>           <idle>-0     [001] 241409.490555: dev_kfree_skb_irq: skbaddr=f5e29840
> >>
> >>         udp-recv-302   [001] 515031.206008: skb_free_datagram_locked: skbaddr=f5b1d900
> >>
> >>
> >> Signed-off-by: Koki Sanagi <sanagi.koki@xxxxxxxxxxxxxx>
> >> ---
> >>  include/trace/events/skb.h |   42 ++++++++++++++++++++++++++++++++++++++++++
> >>  net/core/datagram.c        |    1 +
> >>  net/core/dev.c             |    2 ++
> >>  net/core/skbuff.c          |    1 +
> >>  4 files changed, 46 insertions(+), 0 deletions(-)
> >>
> >> diff --git a/include/trace/events/skb.h b/include/trace/events/skb.h
> >> index 4b2be6d..84c9041 100644
> >> --- a/include/trace/events/skb.h
> >> +++ b/include/trace/events/skb.h
> >> @@ -35,6 +35,48 @@ TRACE_EVENT(kfree_skb,
> >>  		__entry->skbaddr, __entry->protocol, __entry->location)
> >>  );
> >>  
> >> +DECLARE_EVENT_CLASS(free_skb,
> >> +
> >> +	TP_PROTO(struct sk_buff *skb),
> >> +
> >> +	TP_ARGS(skb),
> >> +
> >> +	TP_STRUCT__entry(
> >> +		__field(	void *,	skbaddr	)
> >> +	),
> >> +
> >> +	TP_fast_assign(
> >> +		__entry->skbaddr = skb;
> >> +	),
> >> +
> >> +	TP_printk("skbaddr=%p", __entry->skbaddr)
> >> +
> >> +);
> >> +
> >> +DEFINE_EVENT(free_skb, consume_skb,
> >> +
> >> +	TP_PROTO(struct sk_buff *skb),
> >> +
> >> +	TP_ARGS(skb)
> >> +
> >> +);
> >> +
> >> +DEFINE_EVENT(free_skb, dev_kfree_skb_irq,
> >> +
> >> +	TP_PROTO(struct sk_buff *skb),
> >> +
> >> +	TP_ARGS(skb)
> >> +
> >> +);
> >> +
> >> +DEFINE_EVENT(free_skb, skb_free_datagram_locked,
> >> +
> >> +	TP_PROTO(struct sk_buff *skb),
> >> +
> >> +	TP_ARGS(skb)
> >> +
> >> +);
> >> +
> > 
> > Why create these last two tracepoints at all?  dev_kfree_skb_irq will eventually
> > pass through kfree_skb anyway, getting picked up by the tracepoint there, the
> > while the latter won't (since it uses __kfree_skb instead), I think that could
> > be fixed up by add a call to trace_kfree_skb there directly, saving you two
> > tracepoints.
> > 
> > Neil
> > 
> I think dev_kfree_skb_irq isn't chased by trace_kfree_skb or trace_consume_skb
> completely. Because net_tx_action frees skb by __kfree_skb. So it is better to
> add trace_kfree_skb before it. skb_free_datagram_locked is same.
> 
It isn't, you're right, but that was the point I made above.  Those missed areas
could be easily handled by adding calls to trace_kfree_skb which already exists,
to the missed areas.  Then you don't need to create those new tracepoints.  The
way your doing this, if someone wants to trace all skb frees in debugfs, they
would have to enable three tracepoints, not just one.  Not that thats the point
of your patch, but its something to consider, and it simplifies your code.
Neil

> Thanks,
> Koki Sanagi.
> >>
> > 
> > 
> 
> 
> 
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