netdev - RE: [RFC][PATCH] add tracepoint to __sk_mem

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Date:	Wed, 15 Jun 2011 16:15:27 -0400
From:	Satoru Moriya <satoru.moriya@....com>
To:	Neil Horman <nhorman@...driver.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"dle-develop@...ts.sourceforge.net" 
	<dle-develop@...ts.sourceforge.net>,
	Seiji Aguchi <seiji.aguchi@....com>
Subject: RE: [RFC][PATCH] add tracepoint to __sk_mem_schedule

On 06/15/2011 04:04 PM, Neil Horman wrote:
> On Wed, Jun 15, 2011 at 03:18:09PM -0400, Satoru Moriya wrote:
>
>> Right. We can catch packet drop events via dropwatch/kfree_skb tracepoint.
>> OTOH, what I'd like to know is why kernel dropped packets. Basically,
>> we're able to know the reason because kfree_skb tracepoint records its
>> return address. But in this case it is difficult to know a detailed reason
>> because there are some possibilities. I'll try to explain UDP case.
> Ok, but you're patch only gets us incrmentally closer to that goal.  You can
> tell if theres too much memory pressue to accept another packet on a socket, but
> it doesn't tell you if for instance if sk_filter indicated an error, or
> something else failed.  In short it sounds to me like you're trying to debug a
> specific problem, rather than add something generally useful.

You're right.

>> As you said, we're able to catch the packet drop in __udp_queue_rcv_skb
>> and it means ip_queue_rcv_skb/sock_queue_rcv_skb returned negative value.
>> In sock_queue_rcv_skb there are 3 possibilities where it returns nagative
>> val but we can't separate them from the kfree_skb tracepoint. Moreover
> This is true, but you're patch doesn't do that either (not that either feature
> holds that as a specific goal).

Right.

> 
>> sock_queue_rcv_skb calls __sk_mem_schedule and there are several
>> if-statement to decide whether kernel should drop a packet in it. I'd like
>> to know the condition when it returned error because some of them are
>> related to sysctl knob e.g. /proc/sys/net/ipv4/udp_mem, udp_rmem_min,
>> udp_wmem_min for UDP case and we can tune kernel behavior easily.
>> Also they are needed to show root cause of packet drop to our customers.
>>
>> Does it make sense?
>>
> Yes, it makes sense, but I think the patch could be made generally more useful.

OK. I'll update my patch to make it more useful.

Thanks,
Satoru--
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