Subject: Re: INFO: rcu detected stall in ext4_write_checks

From: "Theodore Ts'o" <[email-masked]>
Date: Sat, 6 Jul 2019 11:02:26 -0400

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.

(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.)

> 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.

					- Ted