On Mon, 20 Apr 2015 10:09:03 -0700 "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > On Mon, Apr 20, 2015 at 11:35:54AM -0500, Clark Williams wrote: > > On Sat, 18 Apr 2015 19:05:42 -0700 > > "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > > > > > Real-time priority to use for RCU worker threads (RCU_KTHREAD_PRIO) [0] (NEW) > > > > > > > > > > Indeed, Linus complained about this one. ;-) > > > > > > > > :-) Yes, it's an essentially unanswerable question. > > > > > > > > > This Kconfig parameter is a stopgap, and needs a real solution. > > > > > People with crazy-heavy workloads involving realtime cannot live > > > > > without it, but that means that most people don't have to care. I > > > > > have had solving this on my list, and this clearly increases its > > > > > priority. > > > > > > > > So what value do they use, prio 99? 98? It might be better to offer > > > > this option as a binary choice, and set a given priority. If -rt > > > > people complain then they might help us in solving it properly. > > > > > > I honestly do not remember what priority they were using, it is > > > not in email, and I don't keep IRC logs that far back. Adding > > > linux-rt-users@xxxxxxxxxxxxxxx on CC. > > > > As I recall, we started out using fifo:1, but when you get heavy > > workloads running at higher fifo priorities, we wanted to boost the rcu > > worker threads over those workloads. > > > > Currently the irq threads default to fifo:50, so maybe a good > > default choice for the rcu threads on RT is fifo:49. That of course > > presumes rational behavior on the part of application developers. > > > > I seem to recall that you and I had a discussion about making this > > value a runtime knob in /sys but that didn't go anywhere. Do we need to > > crank that up again and just use the config as a default/starting > > value? If so then we could just default to fifo:1 and let sysadmins > > tweak the value to match up with the workload. > > 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. Yeah, that will work. > > That said, if the lack of a sysfs knob has been causing real problems, > let's make that happen. I'll talk to the other RT-ers and get back to you on that. I suspect most folks would like it just to not have to reboot while tuning, but not sure it's worth the extra code. Clark
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