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

[Richard Weinberger]

----- Ursprüngliche Mail -----
> Von: "Benjamin Berg" <benjamin at sipsolutions.net>
>> 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.

Agreed. I think having a house keeping CPU (thread) will not work in
time-travel mode.
Kicking RCU whenever a syscall is executed is okay, the question is,
are there other scenarios where RCU work can pile up and no syscall is
run for a long time? Maybe we need to kick it at other places (page fault handler?)
too.

Thanks,
//richard