> On Sun, Jun 22, 2008 at 02:27:05PM -0400, Mathieu Desnoyers wrote:
>> * Alexey Dobriyan (
adobriyan@gmail.com) wrote:
>>> On Sun, Jun 22, 2008 at 01:11:35PM -0400, Mathieu Desnoyers wrote:
>>>> Tracepoint proposal
>>>>
>>>> - Tracepoint infrastructure
>>>> - In-kernel users
>>>> - Complete typing, verified by the compiler
>>>> - Dynamically linked and activated
>>>>
>>>> - Marker infrastructure
>>>> - Exported API to userland
>>>> - Basic types only
>>>>
>>>> - Dynamic vs static
>>>> - In-kernel probes are dynamically linked, dynamically activated, connected to
>>>> tracepoints. Type verification is done at compile-time. Those in-kernel
>>>> probes can be a probe extracting the information to put in a marker or a
>>>> specific in-kernel tracer such as ftrace.
>>>> - Information sinks (LTTng, SystemTAP) are dynamically connected to the
>>>> markers inserted in the probes and are dynamically activated.
>>>>
>>>> - Near instrumentation site vs in a separate tracer module
>>>>
>>>> A probe module, only if provided with the kernel tree, could connect to internal
>>>> tracing sites. This argues for keeping the tracepoing probes near the
>>>> instrumentation site code. However, if a tracer is general purpose and exports
>>>> typing information to userspace through some mechanism, it should only export
>>>> the "basic type" information and could be therefore shipped outside of the
>>>> kernel tree.
>>>>
>>>> In-kernel probes should be integrated to the kernel tree. They would be close to
>>>> the instrumented kernel code and would translate between the in-kernel
>>>> instrumentation and the "basic type" exports. Other in-kernel probes could
>>>> provide a different output (statistics available through debugfs for instance).
>>>> ftrace falls into this category.
>>>>
>>>> Generic or specialized information "sinks" (LTTng, systemtap) could be connected
>>>> to the markers put in tracepoint probes to extract the information to userspace.
>>>> They would extract both typing information and the per-tracepoint execution
>>>> information to userspace.
>>>>
>>>> Therefore, the code would look like :
>>>>
>>>> kernel/sched.c:
>>>>
>>>> #include "sched-trace.h"
>>>>
>>>> schedule()
>>>> {
>>>> ...
>>>> trace_sched_switch(prev, next);
>>>> ...
>>>> }
>>> Once this is accepted you're going to add hundreds of such calls to every
>>> core subsystem, right?
>>>
>> The LTTng instrumentation has about 125 of such calls. Tests have
>> revealed that adding such dormant tracepoints to the kernel often
>> increase kernel performances rather than decreasing it (see the ia64
>> benchmarks posted on lkml a few weeks ago).
>
> We're not adding this for performance increase, you do realize this?
>
>> The core subsystem maintainers are being involved in the process.
>
> NAK this from proc if you about this.
