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
