On Wed, Feb 17, 2021 at 07:54:47AM -0800, Paul E. McKenney wrote: > On Wed, Feb 17, 2021 at 04:32:53PM +0100, Sebastian Andrzej Siewior wrote: > > On 2021-02-16 10:36:09 [-0800], Paul E. McKenney wrote: > > > Hello, Sebastian, > > > > Hi Paul, > > > > > I punted on this for the moment by making RCU priority boosting testing > > > depend on CONFIG_PREEMPT_RT, but longer term I am wondering if RCU's > > > various timed delays and timeouts should use hrtimers rather than normal > > > timers in kernels built with CONFIG_RCU_BOOST. As it is, RCU priority > > > boosting can be defeated if any of the RCU grace-period kthread's timeouts > > > are serviced by the non-realtime ksoftirqd. > > > > I though boosting is accomplished by acquiring a rt_mutex in a > > rcu_read() section. Do you have some code to point me to, to see how a > > timer is involved here? Or is it the timer saying that *now* boosting is > > needed. > > Yes, this last, which is in the grace-period kthread code, for example, > in rcu_gp_fqs_loop(). > > > If your hrtimer is a "normal" hrtimer then it will be served by > > ksoftirqd, too. You would additionally need one of the > > HRTIMER_MODE_*_HARD to make it work. > > Good to know. Anything I should worry about for this mode? > > Also, the current test expects callbacks to be invoked, which involves a > number of additional kthreads and timers, for example, in nocb_gp_wait(). > I suppose I could instead look at grace-period sequence numbers, but I > believe that real-life use cases needing RCU priority boosting also need > the callbacks to be invoked reasonably quickly (as in within hundreds > of milliseconds up through very small numbers of seconds). > > Thoughts? Hmmm... Unless there are current use cases where callbacks are being prevented from being invoked, I will modify rcutorture's testing of RCU priority boosting to look only at grace-period progress on the theory that most real-time uses offload callbacks, and in that case it is the sysadm's job to make sure that they get the CPU time they needs. Thanx, Paul > > > This might require things like swait_event_idle_hrtimeout_exclusive(), > > > either as primitives or just open coded. > > > > > > Thoughts? > > > > > > Thanx, Paul > > > > Sebastian
