Messages in this thread | | | Subject | Re: [2/3 PATCH] Kprobes: User space probes support- readpage hooks | From | Richard J Moore <> | Date | Tue, 21 Mar 2006 09:14:06 +0000 |
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Andrew, the need is probably better stated as one where system-wide probing or tracing is possible. There are times where one needs a global view. Of course one can use multiple tools to obtain such data e.g. probes in kernel space strace in user space and so on. The advantages of supporting user probes are as follows:
1) A single tool providing the data capture in a consistent manner eases the problems of correlation of events across multiple tools (for kernel and user space) 2) The dynamic aspect allows ad hoc probepoints to be inserted where no existing instrumentation is provided (emergency debug scenario for example). 3) The user probe distinguishes itself from all other externally managed tracing mechanisms in that probepoints are globally applied - i.e. without reference to PID. Compare this with ptrace breakpoints (hence strace and gdb) where tracepoints and breakpoints are localized to a specified set of processes. user-probes achieves this by design without the side effects (privatization of pages) that ptrace has. Again this supports the global view. 4) user-probes also supports the registering of the probepoints before an the probed code is loaded. The clearly has advantages for catching initialization problems.
A real life example of where this capability would have been very useful is with a performance problem I am currently investigating. It involves a GPFS + SAMBA + TCPIP + RDAC and some user-space video serving application. We are looking are where the latencies are accumulating in the system for the specific user application. It's a very hard problem. Having multiple tools serve up a partial view and having to coordinate these view from both data analysis and data gathering perspectives is a real nightmare.
- - Richard J Moore IBM Advanced Linux Response Team - Linux Technology Centre MOBEX: 264807; Mobile (+44) (0)7739-875237 Office: (+44) (0)1962-817072
Andrew Morton <akpm@osdl.org> To 21/03/2006 prasanna@in.ibm.com 02:12 cc ak@suse.de, davem@davemloft.net, suparna@in.ibm.com, Richard J Moore/UK/IBM@IBMGB, linux-kernel@vger.kernel.org bcc Subject Re: [2/3 PATCH] Kprobes: User space probes support- readpage hooks
Prasanna S Panchamukhi <prasanna@in.ibm.com> wrote: > > The basic idea is to insert probes on user applications which may or > may not be in memory, at the time of probe insertion.
umm yes, but what for?
What does this entire feature *do*? Why does Linux need it?
OK, so it allows kernel modules to set breakpoints (via debug traps) into user code. But why do we want to be able to do that? What are the use-cases?
This may sound like boringly obvious stuff to you, but without a complete problem statement from the designers, how are we to evaluate their proposed solution?
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