Hello, Joe. On Sun, Sep 28, 2014 at 12:39:34PM -0400, Joe Lawrence wrote: > On Sun, 28 Sep 2014, Tejun Heo wrote: > > Hello, Joe. > > > > On Fri, Sep 26, 2014 at 10:52:27AM -0400, Joe Lawrence wrote: > > ... > > > I was wondering if such behavior was expected on !PREEMPT kernels, > > > especially after b22ce2785d97 "workqueue: cond_resched() after > > > processing each work item". In the ftraces I've observed from the RCU > > > stall, cond_resched() never seems to schedule. Under what conditions > > > would it be expected to do so? > > > > Hmmm... it should yield if there's a higher priority task scheduled. > > Maybe the workqueue in question is a highpri one? > > I don't believe any high priority tasks or workqueues are involved here. > Consider a synthetic repro module like: ... > When I load this, "kworker/4:1" goes to 100% CPU and the RCU grace > periods reported by rcusched freeze on CPU4 for minutes at a time. > > > > Changing the worker function to reschedule with at least 1 jiffy avoids > > > this problem -- I was just curious if a fix at that end was appropriate. > > > > The fact that 0 delay means immediate execution is depended upon in > > multiple places and changing it will break other things. The > > workqueue user itself sounds buggy to me. Can't that be fixed? > > Certainly modifying the workqueue user should be sufficient here. I was > only wondering if the cond_resched in process_one_work was expected to > give the CPU some breathing room in this scenario.
It yields iff there are other higher priority tasks to run. I haven't dug the code but if sched RCU is using and sched RCU implements grace periods passively (e.g. something like if all tasks have switched since point X), cond_resched() is gonna be able stall RCU grace periods by running continuously in the absense of higher priority task to run on the same CPU. That said, RCU could have implemented a workaround for situations like that in the meantime so it could be worthwhile to test mainline. Thanks. -- tejun -- 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/
