On Sun, Mar 27, 2016 at 10:45:59PM +0200, Peter Zijlstra wrote: > On Sun, Mar 27, 2016 at 08:40:18AM -0700, Paul E. McKenney wrote: > > Oh, and the patch I am running with is below. I am running x86, and so > > some other architectures would of course need the corresponding patch > > on that architecture. > > > -#define TIF_POLLING_NRFLAG 21 /* idle is polling for TIF_NEED_RESCHED > > */ > > +/* #define TIF_POLLING_NRFLAG 21 idle is polling for > > TIF_NEED_RESCHED */ > > x86 is the only arch that really uses this heavily IIRC. > > Most of the other archs need interrupts to wake up remote cores. > > So what we try to do is avoid sending IPIs when the CPU is idle, for the > remote wakeup case we use set_nr_if_polling() which sets > TIF_NEED_RESCHED if TIF_POLLING_NRFLAG was set. If it wasn't, we'll send > the IPI. Otherwise we rely on the idle loop to do sched_ttwu_pending() > when it breaks out of loop due to TIF_NEED_RESCHED. > > But, you need hotplug for this to happen, right?
I do, but Ross Green is seeing something that looks similar, and without CPU hotplug. > We should not be migrating towards, or waking on, CPUs no longer present > in cpu_active_map, and there is a rcu/sched_sync() after clearing that > bit. Furthermore, migration_call() does a sched_ttwu_pending() (waking > any remaining stragglers) before we migrate all runnable tasks off the > dying CPU. OK, so I should instrument migration_call() if I get the repro rate up? > The other interesting case would be resched_cpu(), which uses > set_nr_and_not_polling() to kick a remote cpu to call schedule(). It > atomically sets TIF_NEED_RESCHED and returns if TIF_POLLING_NRFLAG was > not set. If indeed not, it will send an IPI. > > This assumes the idle 'exit' path will do the same as the IPI does; and > if you look at cpu_idle_loop() it does indeed do both > preempt_fold_need_resched() and sched_ttwu_pending(). > > Note that one cannot rely on irq_enter()/irq_exit() being called for the > scheduler IPI. OK, thank you for the info! Any specific debug actions? Thanx, Paul