> On Tue, 3 Mar 2009 11:57:47 -0500
> Neil Horman <nhorman@tuxdriver.com> wrote:
>
> >
> > Create Network Drop Monitoring service in the kernel
> >
> > A few weeks ago I posted an RFC requesting some feedback on a proposal that I
> > had to enhance our ability to monitor the Linux network stack for dropped
> > packets. This patchset is the result of that RFC and its feedback.
> >
> > Overview:
> >
> > The Linux networking stack, from a users point of view suffers from four
> > shortcommings:
> >
> > 1) Consolidation: The ability to detect dropped network packets is spread out
> > over several proc file interfaces and various other utilities (tc,
> > /proc/net/dev, snmp, etc)
> >
> > 2) Clarity: The ability to discern which statistics reflect dropped packets is
> > not always clear
> >
> > 3) Ambiguity: The ability to understand the root cause of a lost packet is not
> > always clear (some stats are incremented at multiple points in the kernel for
> > subtly different reasons)
> >
> > 4) Performance: Interrogating all of these interface as they currently exist
> > requires a polling operation, and potentially requires the serialization of
> > various kernel operations, which can result in performance degradation.
> >
> > Proposed solution: dropwatch
> >
> > My proposed solution consists of 4 primary aspects:
> >
> > A) A hook into kfree_skb to detect dropped packets. Based on feedback from the
> > earlier RFC, there are relatively few places in the kernel where packets are
> > dropped because they have been successfully received or send (for lack of a
> > better term, end-of-line points). The remaining calls to kfree_skb are made
> > because there is something wrong and the packet must be discarded. I've split
> > kfree_skb into two calls: kfree_skb and kfree_skb_clean. The later is simply a
> > pass through to __kfree_skb, while the former adds a trace hook to capture a
> > pointer to the skb and the location of the call.
> >
> > B) A trace hook to monitor the trace point in (A). this records the locations
> > at which frames were dropped, and saves them for periodic reporting.
> >
> > C) A netlink protocol to both control the enabling/disabling of the trace hook
> > in (B) and to deliver information on drops to interested applications in user
> > space
> >
> > D) A user space application to listen for drop alerts from (C) and report them
> > to an adminstrator/save them for later analysis/etc. I've implmented the start
> > of this application, which relies on this patch set here:
> >
https://fedorahosted.org/dropwatch/
> >
> >
> > Implementation Notes:
> >
> > About the only out-of the ordinary aspects I'd like to call attention to at this
> > point are:
> >
> > 1) The trace point. I know that tracepoints are currently a controversial
> > subject, and that their need was discussed briefly during the RFC. I elected to
> > use a tracepoint here, simply because I felt like I was re-inventing the wheel
> > otherwise. In order to implement this feature, I needed an ability to record
> > when kfree_skb was called in certain places who's performance impact would be 0
> > when the feature wasn't configured into the kernel, and when it was configured,
> > but disabled. Given that anything else I used or wrote myself to hook into this
> > point in the kernel would be a partial approximation of what tracepoints already
> > offer, I think its preferable to go with a tracepoint here, simply because its
> > good use of existing function.
> >
> > 2) The configuration messages in the netlink protocol are just a placeholder
> > right now. I'm ok with that, given that the dropwatch user app doesn't have
> > code to configure anything yet anyway (it just turns the service off/on and
> > listens for drops right now). I figure I'll implment configuration messages in
> > the app and kernel in parallel.
> >
> > 3) Performance. I'm not sure of the best way to model the performance here, but
> > I disassembled the code in question, and the point at which we hook kfree_skb,
> > this patch set only adds a conditional branch to the path, which is optimized
> > for the not-taken case (the case in which the service is disabled), so adding
> > this feature is as close to a zero impact as it can be when the service is
> > disabled. Likewise, when tracepoints are not configured in the kernel, the
> > tracepoint (which is defined as a macro) is preprocessed away, making the
> > performance impact zero. That leave the case in which the service is enabled.
> > While I don't have specific numbers, I can say that the trace path is lockless
> > and per-cpu, and should run O(n) where n is the number of recordable drop points
> > (default is 64). Sendingi/allocation of frames to userspace is done in the
> > context of keventd, with a timer for hysteresis, to keep the number of sends
> > lower and consolidate drop information. So performance should be reasonably
> > good there. Again, no hard numbers, but I've monitored drops by passing udp
> > traffic through localhost with netcat and SIGSTOP-ing the receiver. Console and
> > ssh access remained very responsive
> >
> >
> > Ok, so thats it, hope it meets with everybodys approval!
> > Regards
> > Neil
> >
> > --
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>
> It would be good to have a way to mask off certain tracepoints.
> For example, if running performance test and after measuring number
> of packets dropped in TX queue overflow, only see others.
>
