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

[Mathieu Desnoyers]

* Mathieu Desnoyers (compudj at krystal.dyndns.org) wrote:
> * 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.

For everyone's benefit, I received a private reply stating that the
problem was not in UST upstream, but rather in the specific adaptation
Anders is working on.

Thanks,

Mathieu

> 
> 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
> 
> > _______________________________________________
> > ltt-dev mailing list
> > ltt-dev at lists.casi.polymtl.ca
> > http://lists.casi.polymtl.ca/cgi-bin/mailman/listinfo/ltt-dev
> 
> 
> -- 
> Mathieu Desnoyers
> Operating System Efficiency R&D Consultant
> EfficiOS Inc.
> http://www.efficios.com
> 
> _______________________________________________
> ltt-dev mailing list
> ltt-dev at lists.casi.polymtl.ca
> http://lists.casi.polymtl.ca/cgi-bin/mailman/listinfo/ltt-dev
> 

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