Re: Unified tracing buffer

["K.Prasad" <prasad@linux.vnet.ibm.com>]

On Sat, Sep 20, 2008 at 02:07:58AM +0200, Peter Zijlstra wrote:
> Oddly whitespace damaged mail..
> 
> On Fri, 2008-09-19 at 14:33 -0700, Martin Bligh wrote:
> > During kernel summit and Plumbers conference, Linus and others
> > expressed a desire for a unified
> > tracing buffer system for multiple tracing applications (eg ftrace,
> > lttng, systemtap, blktrace, etc) to use.
> > This provides several advantages, including the ability to interleave
> > data from multiple sources,
> > not having to learn 200 different tools, duplicated code/effort, etc.
> > 
> > Several of us got together last night and tried to cut this down to
> > the simplest usable system
> > we could agree on (and nobody got hurt!). This will form version 1.
> > I've sketched out a few
> > enhancements we know that we want, but have agreed to leave these
> > until version 2.
> > The answer to most questions about the below is "yes we know, we'll
> > fix that in version 2"
> > (or 3). Simplicity was the rule ...
> > 
> > Sketch of design.  Enjoy flaming me. Code will follow shortly.
> > 
> > 
> > STORAGE
> > -------
> > 
> > We will support multiple buffers for different tracing systems, with
> > separate names, event id spaces.
> > Event ids are 16 bit, dynamically allocated.
> > A "one line of text" print function will be provided for each event,
> > or use the default (probably hex printf)
> > Will provide a "flight data recorder" mode, and a "spool to disk" mode.
> > 
> > Circular buffer per cpu, protected by per-cpu spinlock_irq
> > Word aligned records.
> > Variable record length, header will start with length record.
> > Timestamps in fixed timebase, monotonically increasing (across all CPUs)
> > 
> > 
> > INPUT_FUNCTIONS
> > ---------------
> > 
> > allocate_buffer (name, size)
> >         return buffer_handle
> > 
> > register_event (buffer_handle, event_id, print_function)
> >         You can pass in a requested event_id from a fixed set, and
> > will be given it, or an error
> >         0 means allocate me one dynamically
> >         returns event_id     (or -E_ERROR)
> > 
> > record_event (buffer_handle, event_id, length, *buf)
> 
> I'd hoped for an interface like:
> 
> struct ringbuffer *ringbuffer_alloc(const char *name, size_t size);
> void ringbuffer_free(struct ringbuffer *buffer);
> int ringbuffer_write(struct ringbuffer *buffer, const char *buf, size_t size);
> int ringbuffer_read(struct ringbuffer *buffer, int cpu, char *buf, size_t size);
> 
> On top of which you'd do the event thing, the register event with a
> callback idea makes sense, except I'd split the consumption into two:
>  - one method to pull the binary event out, which knows how long it
> ought to be etc..
>  - one method to convert the binary event to ASCII
>
In conjunction with the previous email on this thread
(http://lkml.org/lkml/2008/9/22/160), may I suggest
the equivalent interfaces in -mm tree (2.6.27-rc5-mm1) to be:

relay_printk(<some struct with default filenames/pathnames>, <string>,
...) ;
relay_dump(<some struct with default filenames/pathnames>, <binary
data>);
and
relay_cleanup_all(<the struct name>); - Single interface that cleans up
all files/directories/output data created under a logical entity.

Thanks,
K.Prasad