On Thu, 19 Feb 2015, Josh Poimboeuf wrote: > So I've looked at kgr_needs_lazy_migration(), but I still have no idea > how it works. > > First of all, I think reading the stack while its being written to could > give you some garbage values, and a completely wrong nr_entries value > from save_stack_trace_tsk().
I believe we've already been discussing this some time ago ... I agree that this is a very crude optimization that should probably be either removed (which would only cause slower convergence in the presence of CPU-bound tasks), or rewritten to perform IPI-based stack dumping (probably on a voluntarily-configurable basis). Reading garbage values could only happen if the task would be running in kernelspace. nr_entries would then be at least 2. But I agree that relying on this very specific behavior is not really safe in general in case someone changes the stack dumping implementation in the future in an unpredictable way. > But also, how would you walk a stack without knowing its stack pointer? > That function relies on the saved stack pointer in > task_struct.thread.sp, which, AFAICT, was last saved during the last > call to schedule(). Since then, the stack could have been completely > rewritten, with different size stack frames, before the task exited the > kernel. Same argument holds here as well, I believe. Thanks, -- Jiri Kosina SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
