On Sat, Jul 06, 2019 at 11:03:11AM -0700, Paul E. McKenney wrote: > On Sat, Jul 06, 2019 at 11:02:26AM -0400, Theodore Ts'o wrote: > > On Fri, Jul 05, 2019 at 11:16:31PM -0700, Paul E. McKenney wrote: > > > I suppose RCU could take the dueling-banjos approach and use increasingly > > > aggressive scheduler policies itself, up to and including SCHED_DEADLINE, > > > until it started getting decent forward progress. However, that > > > sounds like the something that just might have unintended consequences, > > > particularly if other kernel subsystems were to also play similar > > > games of dueling banjos. > > > > So long as the RCU threads are well-behaved, using SCHED_DEADLINE > > shouldn't have much of an impact on the system --- and the scheduling > > parameters that you can specify on SCHED_DEADLINE allows you to > > specify the worst-case impact on the system while also guaranteeing > > that the SCHED_DEADLINE tasks will urn in the first place. After all, > > that's the whole point of SCHED_DEADLINE. > > > > So I wonder if the right approach is during the the first userspace > > system call to shced_setattr to enable a (any) real-time priority > > scheduler (SCHED_DEADLINE, SCHED_FIFO or SCHED_RR) on a userspace > > thread, before that's allowed to proceed, the RCU kernel threads are > > promoted to be SCHED_DEADLINE with appropriately set deadline > > parameters. That way, a root user won't be able to shoot the system > > in the foot, and since the vast majority of the time, there shouldn't > > be any processes running with real-time priorities, we won't be > > changing the behavior of a normal server system. > > It might well be. However, running the RCU kthreads at real-time > priority does not come for free. For example, it tends to crank up the > context-switch rate. > > Plus I have taken several runs at computing SCHED_DEADLINE parameters, > but things like the rcuo callback-offload threads have computational > requirements that are controlled not by RCU, and not just by the rest of > the kernel, but also by userspace (keeping in mind the example of opening > and closing a file in a tight loop, each pass of which queues a callback). > I suspect that RCU is not the only kernel subsystem whose computational > requirements are set not by the subsystem, but rather by external code. > > OK, OK, I suppose I could just set insanely large SCHED_DEADLINE > parameters, following syzkaller's example, and then trust my ability to > keep the RCU code from abusing the resulting awesome power. But wouldn't > a much nicer approach be to put SCHED_DEADLINE between SCHED_RR/SCHED_FIFO > priorities 98 and 99 or some such? Then the same (admittedly somewhat > scary) result could be obtained much more simply via SCHED_FIFO or > SCHED_RR priority 99. > > Some might argue that this is one of those situations where simplicity > is not necessarily an advantage, but then again, you can find someone > who will complain about almost anything. ;-) > > > (I suspect there might be some audio applications that might try to > > set real-time priorities, but for desktop systems, it's probably more > > important that the system not tie its self into knots since the > > average desktop user isn't going to be well equipped to debug the > > problem.) > > Not only that, but if core counts continue to increase, and if reliance > on cloud computing continues to grow, there are going to be an increasing > variety of mixed workloads in increasingly less-controlled environments. > > So, yes, it would be good to solve this problem in some reasonable way. > > I don't see this as urgent just yet, but I am sure you all will let > me know if I am mistaken on that point. > > > > Alternatively, is it possible to provide stricter admission control? > > > > I think that's an orthogonal issue; better admission control would be > > nice, but it looks to me that it's going to be fundamentally an issue > > of tweaking hueristics, and a fool-proof solution that will protect > > against all malicious userspace applications (including syzkaller) is > > going to require solving the halting problem. So while it would be > > nice to improve the admission control, I don't think that's a going to > > be a general solution. > > Agreed, and my earlier point about the need to trust the coding abilities > of those writing ultimate-priority code is all too consistent with your > point about needing to solve the halting problem. Nevertheless, I believe > that we could make something that worked reasonably well in practice. > > Here are a few components of a possible solution, in practice, but > of course not in theory: > > 1. We set limits to SCHED_DEADLINE parameters, perhaps novel ones. > For one example, insist on (say) 10 milliseconds of idle time > every second on each CPU. Yes, you can configure beyond that > given sufficient permissions, but if you do so, you just voided > your warranty. > > 2. Only allow SCHED_DEADLINE on nohz_full CPUs. (Partial solution, > given that such a CPU might be running in the kernel or have > more than one runnable task. Just for fun, I will suggest the > option of disabling SCHED_DEADLINE during such times.) > > 3. RCU detects slowdowns, and does something TBD to increase its > priority, but only while the slowdown persists. This likely > relies on scheduling-clock interrupts to detect the slowdowns, > so there might be additional challenges on a fully nohz_full > system. 4. SCHED_DEADLINE treats the other three scheduling classes as each having a period, deadline, and a modest CPU consumption budget for the members of the class in aggregate. But this has to have been discussed before. How did that go? > 5. Your idea here. Thanx, Paul
