[ltt-dev] UST double-buffering won't always trigger consumer daemon

[Mathieu Desnoyers]

* Anders Carlsson (capricornalpha2 at gmail.com) wrote:
> Hello!
> 
> I working on a UST port, but I cannot _reliably_ get trace data trough to
> the consumer daemon. To me it seems the traced application fails to trigger
> the consumer daemon when a sub-buffer boundary is crossed.

Can you specify which UST version you are working on ?

This looks like one of the few design issues that I fixed in the generic
ring buffer that I introduce in userspace in UST 2.0.

Thanks,

Mathieu

> 
> From buffers.h:
> 
> static __inline__ void ltt_check_deliver(struct ust_channel *chan,
>         struct ust_buffer *buf,
>         long offset, long commit_count, long idx)
> {
>     long old_commit_count = commit_count - chan->subbuf_size;
> 
>     /* Check if all commits have been done */
>     if (unlikely((BUFFER_TRUNC(offset, chan)
>             >> chan->n_subbufs_order)
>             - (old_commit_count
>                & chan->commit_count_mask) == 0)) {
> ...
> 
> This condition becomes true only when commit_count equals a multiple of
> chan->subbuf_size, but when I add a printout of the values it appears that
> the buffer boundary is often crossed without commit_count ever becoming
> equal to the subbuffer size. In those cases, the trace ceases to operate.
> This seems to depend on the various tracepoint data sizes, and the order of
> commit etc.
> 
> I suppose this could be taken care of by aligning the reservations to the
> subbuffer boundaries. It seems to me there is already code to handle that
> (in ltt_relay_try_reserve_slow()) Am I correct?
> 
> Am I missing something (e.g. alignment differences between ARM and x86)?
> 
> Should I look somewhere specific?
> 
> Thank you for reading this far. Any help is appreciated.
> 
> Kind regards
> Anders

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-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com