On Fri 2016-04-08 09:31:31, Josh Poimboeuf wrote: > On Fri, Apr 08, 2016 at 10:03:04AM +0200, Petr Mladek wrote: > > The big advantage about checking the stack is that it does not add > > any overhead to the scheduler code, does not eat any TIF flag or > > memory. The overhead is only when we are migrating a task and it is > > charged to a separate process. > > My biggest concern about checking the stack for preempt_schedule_irq() > is that it's kind of brittle: > > - What if the preemption code changes such that it's no longer a > reliable indicator? For example, what if preempt_schedule_irq() is > only called in some places, and a new __preempt_schedule_irq() is > called elsewhere? > > - Or due to some obscure gcc optimization like partial inlining or > sibling tail calls, preempt_schedule_irq() doesn't show up on the > stack? > > - Or the code could silently break if there were another static > preempt_schedule_irq symbol somewhere (though we could prevent this by > searching all symbols to ensure there are no duplicates). > > These scenarios are unlikely, but they could conceivably happen...
You are right. > Anyway, we really wouldn't have to eat a TIF flag. We could instead add > something to task_struct. I can at least propose it. If anybody > doesn't like it, maybe they'll suggest something else, or maybe then we > can go with checking the stack. Yup, we should give the extra task struct stuff a try. Another advantage is that it would be easier to port for another architectures. Best Regards, Petr
