* Andi Kleen ([EMAIL PROTECTED]) wrote: > > Setting the thread flag being an atomic operation, I would expect > > setting/clearing it asynchronously from another thread to be a valid > > It could be a very short stop. Also do you start kernel tracing that often? >
It's not a matter of how often I start tracing, but more about what impact I want this operation to have on a running production system. If I start tracing on a server to try detecting particularly nasty race conditions, I prefer not to interfere with the normal execution too much. The same applies when we try to figure out the source of some unexpected latencies experienced in user-space : stopping the processes could be considered as having too much impact on the system studied. I was already reluctant about iterating on every thread to set a flag (this was proposed by Martin and Rebecca, in their Google ktrace implementation), but I accepted to go forward this solution because of the performance benefits. However, I would prefer not to go as far as stopping each process on the system upon trace start/stop to perform this unless it's the only solution left. > > Here is a modified version where I add my test only in the path where we > > know that we have work to do, therefore removing the supplementary test > > from the performance critical path. Would it be more acceptable ? > > It's better, but stopping would be even better. I wouldn't > be surprised if there are other problems with async thread flags changing. > Do you mean architectures other than x86_64 could also assume that the thread flags will stay unchanged between two consecutive reads ? If those thread flags were meant not to be asynchronously updated, why would they require an atomic update at all ? > Also I object to you calling this a bug. It's a new feature. > Agreed. ptrace seems to be correct as is. It would only be needed if we plan to use the flags as I described TIF_KERNEL_TRACE. Mathieu -- Mathieu Desnoyers Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
