On 14/06/16 21:44, Peter Zijlstra wrote: > On Tue, Jun 14, 2016 at 06:39:08PM +0100, Juri Lelli wrote: > > On 07/06/16 21:56, Peter Zijlstra wrote: > > > rt_mutex_waiter::prio is a copy of task_struct::prio which is updated > > > during the PI chain walk, such that the PI chain order isn't messed up > > > by (asynchronous) task state updates. > > > > > > Currently rt_mutex_waiter_less() uses task state for deadline tasks; > > > this is broken, since the task state can, as said above, change > > > asynchronously, causing the RB tree order to change without actual > > > tree update -> FAIL. > > > > > > Fix this by also copying the deadline into the rt_mutex_waiter state > > > and updating it along with its prio field. > > > > > > Ideally we would also force PI chain updates whenever DL tasks update > > > their deadline parameter, but for first approximation this is less > > > broken than it was. > > > > > > > The patch looks OK to me. However, I'm failing to see when we can update > > dl.deadline of a waiter asynchronously. Since a waiter is blocked, we > > can't really change his dl.deadline by calling setscheduler on him, as > > the update would operate on dl.dl_deadline. The new values will start to > > be used as soon as it gets unblocked. The situation seems different for > > RT tasks, for which priority change takes effect immediately. > > > > What am I missing? :-) > > Ah, I missed the dl_deadline vs deadline thing. Still, with optimistic > spinning the waiter could hit its throttle/refresh path, right? And then > that would update deadline. >
I guess it's not that likely, but yes it could potentially happen that a waiter is optimistically spinning, depletes its runtime, gets throttled and then replenished when still spinning. Maybe it doesn't really make sense continuing spinning in this situation, but I guess things get really complicated. :-/ Anyway, as said, I think this patch is OK. Maybe we want to add a comment just to remember what situation can cause an issue if we don't do this? Patch changelog would be OK as well for such a comment IMHO. Thanks, - Juri
