Re: [PATCH 2/3] ring-buffer: try to discard unneeded timestamps

[Tim Bird <tim.bird@am.sony.com>]

Steven Rostedt wrote:
> On Wed, 3 Jun 2009, Tim Bird wrote:
>> Is this new routine only for discarding uncommitted events,
>> or can it be used on committed events?
>>
>> I assume the former, since I see nothing about adjusting the
>> commit position.
> 
> Only uncommitted events.
OK.

>> In the ring_buffer API I see that there's a function for
>> discarding events (committed ones), but not for free-ing them.
>> In function duration filtering, it is desirable to free the
>> last committed event, which for a function exit of short
>> duration will be it's entry event 99% of the time.
> 
> In filtering we deside before commiting if we want to discard or not. 
> (Note, this is only in tip right now.)  Once we commit it, there is no way 
> to safely remove it from the ring buffer. Additions of items are not under 
> a lock (only the moving from page to page is).
> 
> For the event tracer we check if we want to disard it or not before we 
> commit.

Yeah - that's what I thought.  I have duration filtering working
(well, the user interface is not done yet), but with the above
limitations, I can only free the exit from the trace, and mark
the entry event as discarded.  It would save a whole lot more
space to free the entry event as well.

I'm experimenting with free-ing only the last committed event,
when no other write has occurred in the buffer.  But I'm still
not sure I can make it safe.  Under normal conditions this
would be sufficient to catch 99% of the cases.  I did this
in KFT, but under locks, and I know you want to be lockless with
ftrace.

Are writes the only issue, or is it a problem with readers?
I was thinking of experimenting with allowing it when no readers
were active (or were on a different page).

>> P.S. I'm very sorry about the missing '>' on the Signed-off-by line.
>> I ran checkpatch and got a passing score, but missed this.
> 
> No prob, you only made me spam LKML (and others) with about 10 garbage 
> emails ;-)
LOL.  Sorry again. ;-)

=============================
Tim Bird
Architecture Group Chair, CE Linux Forum
Senior Staff Engineer, Sony Corporation of America
=============================