On Sun, Jul 14, 2019 at 05:48:00PM +0300, Dmitry Vyukov wrote: > On Sun, Jul 7, 2019 at 4:17 AM Paul E. McKenney <paulmck@xxxxxxxxxxxxx> 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. > > Trying to digest this thread. > > Do I understand correctly that setting rcutree.kthread_prio=99 won't > help because the deadline priority is higher? > And there are no other existing mechanisms to either fix the stalls > nor make kernel reject the non well-behaving parameters? Kernel tries > to filter out non well-behaving parameters, but the check detects only > obvious misconfigurations, right? > This reminds of priority inversion/inheritance problem. I wonder if > there are other kernel subsystems that suffer from the same problem. > E.g. the background kernel thread that destroys net namespaces and any > other type of async work. A high prio user process can overload the > queue and make kernel eat all memory. May be relatively easy to do > even unintentionally. I suspect the problem is not specific to rcu and > plumbing just rcu may just make the next problem pop up. > Should user be able to starve basic kernel services? User should be > able to prioritize across user processes (potentially in radical > ways), but perhaps it should not be able to badly starve kernel > functions that just happened to be asynchronous? I guess it's not as > simple as setting the highest prio for all kernel threads because in > normal case we want to reduce latency of user work by making the work > async. But user must not be able to starve kernel threads > infinitely... sounds like something similar to the deadline scheduling > -- kernel threads need to get at least some time slice per unit of > time. As I understand it, there is provision for giving other threads slack time even in SCHED_DEADLINE, but the timing of that slack time depends on the other tasks' SCHED_DEADLINE settings. And RCU's kthreads do need some response time: Optimally a few milliseconds, preferably about a hundred milliseconds, but definitely a second. With the huge cycle time specified, RCU might not get that. And yes, I suspect that RCU is not the only thing in the system needing a little CPU time fairly frequently, for some ill-defined notion of "fairly frequently". > But short term I don't see any other solution than stop testing > sched_setattr because it does not check arguments enough to prevent > system misbehavior. Which is a pity because syzkaller has found some > bad misconfigurations that were oversight on checking side. > Any other suggestions? Keep the times down to a few seconds? Of course, that might also fail to find interesting bugs. Thanx, Paul
