On Thu, Apr 07, 2016 at 11:37:19PM +0200, Jiri Kosina wrote: > On Thu, 7 Apr 2016, Jessica Yu wrote: > > > Been sort of rattling my head over the scheduler code :-) Just following > > the calls in and out of __schedule() it doesn't look like there is a > > current flag/mechanism to tell whether or not a task has been > > preempted.. > > Performing the complete stack unwind just to determine whether task has > been preempted non-volutarily is a slight overkill indeed :/ > > > Is there any reason why you didn't just create a new task flag, > > something like TIF_PREEMPTED_IRQ, which would be set once > > preempt_schedule_irq() is entered and unset after __schedule() returns > > (for that task)? This would roughly correspond to setting the task flag > > when the frame for preempt_schedule_irq() is pushed and unsetting it > > just before the frame preempt_schedule_irq() is popped for that task. > > This seems simpler than walking through all the frames just to see if > > in_preempt_schedule_irq() had been called. Would that work? > > Alternatively, without eating up a TIF_ space, it'd be possible to push a > magic contents on top of the stack in preempt_schedule_irq() (and pop it > once we are returning from there), and if such magic value is detected, we > just don't bother and claim unreliability. > > That has advantages of both aproaches combined, i.e. it's relatively > low-cost in terms of performance penalty, and it's reliable (in a sense > that you don't have false positives). > > The small disadvantage is that you can (very rarely, depending on the > chosen magic) have false negatives. That probably doesn't hurt too much, > given the high inprobability and non-lethal consequences. > > How does that sound? To do that from C code, I guess we'd still need some arch-specific code in an asm() statement to do the actual push? I think I'd prefer just updating some field in the task_struct. That way it would be simple and arch-independent. And the stack walker wouldn't have to scan for some special value on the stack. -- Josh -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
