[PATCH] um: add RCU syscall hack for time-travel

[Benjamin Berg]

Hi,

On Thu, 2024-09-12 at 21:02 +0200, Richard Weinberger wrote:
> On Fri, Aug 30, 2024 at 5:38 PM Benjamin Berg
> <benjamin at sipsolutions.net> wrote:
> > 
> > From: Benjamin Berg <benjamin.berg at intel.com>
> > 
> > In time-travel mode userspace can do a lot of work without any time
> > passing. Unfortunately, this can result in OOM situations as the
> > RCU
> > core code will never be run.
> > 
> > Work around that by kicking the RCU using rcu_sched_clock_irq. So
> > behave to the RCU code as if a clock tick happened every syscall.
> > 
> > Signed-off-by: Benjamin Berg <benjamin.berg at intel.com>
> > 
> > [SNIP]
> 
> While I acknowledge that time-travel itself is a beautiful hack, I'd
> like to keep the hacks
> to keep it working minimal.
> So, the problem here is that RCU callbacks never run and just pile up?

Yes. A simple example of this is doing a "find /". This will allocate a
lot of inode information which is only free'ed at a later point.

> I wonder why such a situation does not happen in a nohz_full setup on
> regular systems.

Had to search for a bit. But, I think the boot CPU will still have a
tick even on a NOHZ_FULL setup. see the nohz_full= boot parameter.

It does look like the RCU code might try to force scheduling (tiny RCU)
or wake up a worker (tree RCU) in these situations. But neither of
these attempts is going to fix the situation as there will be no call
to rcu_sched_clock_irq with time-travel.

Benjamin