Re: Use case for TASKS_RCU

[Steven Rostedt]

On Tue, 23 May 2017 13:00:35 -0700
"Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:

 
> > > Unfortunately, it does not work, as I should have known ahead of
> > > time from the dyntick-idle experience.  Not all context switches
> > > go through context_switch().  :-/  
> > 
> > Wait. What context switch doesn't go through a context switch? Or do
> > you mean a user/kernel context switch?  
> 
> I mean that putting printk() before and after the call to
> context_switch() can show tasks switching out twice without switching
> in and vice versa. No sign of lost printk()s, and I also confirmed
> this behavior using a flag in task_struct.

I hope you meant trace_printk()s' as printk is a huge overhead and can
cause side effects.

> 
> One way that this can happen on some architectures is via the "helper"
> mechanism, where the task sleeps normally, but where a later interrupt
> or exception takes on its context "behind the scenes" in the arch
> code. This is what messed up my attempt to use a simple
> interrupt-nesting counter for RCU dynticks some years back.  What I
> counted on there was that the idle loop would never do that sort of
> thing, so I could zero the count when entering idle from process
> context.
> 
> But I have not yet found a similar trick for counting voluntary
> context switches.
> 
> I also tried making context_switch() look like a momentary quiescent
> state, but of course that means that tasks that block forever also
> block the grace period forever.  At which point, I need to scan the
> task list to find them.  And that pretty much brings me back to the
> current RCU-tasks implementation.  :-/
> 

Nothing should block in a preempted state forever, and if it does, that
means we want to wait forever. Because it could be preempted on the
trampoline.

-- Steve