On Sat, 18 Apr 2015 19:05:42 -0700 "Paul E. McKenney" <paul...@linux.vnet.ibm.com> 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-us...@vger.kernel.org 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. Clark
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