On Mon, 20 Apr 2015 18:22:58 -0700 "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > On Mon, Apr 20, 2015 at 04:50:07PM -0500, Clark Williams wrote: > > On Mon, 20 Apr 2015 14:15:04 -0700 > > "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > > > > On Mon, Apr 20, 2015 at 04:40:49PM -0400, Steven Rostedt wrote: > > > > On Mon, Apr 20, 2015 at 10:09:03AM -0700, Paul E. McKenney wrote: > > > > > > > > > > The sysfs knob might be nice, but as far as I know nobody has been > > > > > complaining about it. > > > > > > > > > > Besides, we already have the rcutree.kthread_prio= kernel-boot parameter. > > > > > So how about if the Kconfig parameter selects either SCHED_OTHER > > > > > (the default) or SCHED_FIFO:1, and then the boot parameter can be used > > > > > to select other values. > > > > > > > > Hmm, what priority is this for anyway. To change the priority of the boost > > > > value at run time, do we only need to change the priority of the rcub threads? > > > > > > > > And the priority of the other rcu threads can change as well with a simple > > > > chrt? > > > > > > > > If that's the case, then we don't need a sysctl knob at all. > > > > > > For the grace-period kthreads and the boost kthread, that is the case. > > > It is also the case for the per-CPU kthreads that invoke RCU callbacks > > > for the non-offloaded RCU_BOOST configuration (and that replace all > > > softirq RCU work in -rt). > > > > > > So, should I just ditch all of the priority-setting within RCU and tell > > > users to just use chrt? > > > > Looks to me like all we need to do is tell people if they need a boost > > higher than the compiled in default (RCU_KTHREAD_PRIO), then chrt the > > priority of the rcub thread to the desired priority. > > There's the rub. They also need to chrt the RCU grace-period kthreads > as well as the per-CPU kthreads (rcuc). Which is a pain and easy to > get wrong. > > So at this point, I am leaning towards keeping RCU_KTHREAD_PRIO, but > hiding it behind RCU_EXPERT. Someone in an emergency situation can use > chrt to get RCU going, at least assuming that they had the foresight to > leave a prio-99 shell running somewhere and assuming that they do the > chrt before the system hits OOM. But they have to do all that anyway > if they were to use a sysfs or similar interface. And it is easy to > tell when you have boosted all the necessary kthreads because RCU > grace periods start advancing once again. You don't get that feedback > when you set things up at boot time. ;-) > > So again, at least for the moment, I believe that RCU need not provide > a run-time interface for changing RCU kthread priorities, that the > RCU_KTHREAD_PRIO Kconfig parameter should remain, except that it needs > to be hidden behind RCU_EXPERT, and that the rcutree.kthread_prio= > kernel-boot parameter should also remain. > > Seem reasonable? > Does chrt override the kthread_prio at run time? If so, then great. Otherwise, the sysadmin should still have a way to control their priorities of kernel threads (with few exceptions like the migration thread). -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
