Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry

[Frederic Weisbecker <fweisbec@gmail.com>]

On Thu, May 07, 2015 at 12:48:45PM +0200, Ingo Molnar wrote:
> 
> * Rik van Riel <riel@redhat.com> wrote:
> 
> > > If, on the other hand, you're just going to remotely sample the 
> > > in-memory context, that sounds good.
> > 
> > It's the latter.
> > 
> > If you look at /proc/<pid>/{stack,syscall,wchan} and other files, 
> > you will see we already have ways to determine, from in memory 
> > content, where a program is running at a certain point in time.
> > 
> > In fact, the timer interrupt based accounting does a similar thing. 
> > It has a task examine its own in-memory state to figure out what it 
> > was doing before the timer interrupt happened.
> > 
> > The kernel side stack pointer is probably enough to tell us whether 
> > a task is active in kernel space, on an irq stack, or (maybe) in 
> > user space. Not convinced about the latter, we may need to look at 
> > the same state the RCU code keeps track of to see what mode a task 
> > is in...
> > 
> > I am looking at the code to see what locks we need to grab.
> > 
> > I suspect the runqueue lock may be enough, to ensure that the task 
> > struct, and stack do not go away while we are looking at them.
> 
> That will be enough, especially if you get to the task reference via 
> rq->curr.
> 
> > We cannot take the lock_trace(task) from irq context, and we 
> > probably do not need to anyway, since we do not care about a precise 
> > stack trace for the task.
> 
> So one worry with this and similar approaches of statistically 
> detecting user mode would be the fact that on the way out to 
> user-space we don't really destroy the previous call trace - we just 
> pop off the stack (non-destructively), restore RIPs and are gone.
> 
> We'll need that percpu flag I suspect.

Note we have the context tracking state which tells where the current
task is: user/system/guest.

> 
> And once we have the flag, we can get rid of the per syscall RCU 
> callback as well, relatively easily: with CMPXCHG (in 
> synchronize_rcu()!) we can reliably sample whether a CPU is in user 
> mode right now, while the syscall entry/exit path does not use any 
> atomics, we can just use a simple MOV.
> 
> Once we observe 'user mode', then we have observed quiescent state and 
> synchronize_rcu() can continue. If we've observed kernel mode we can 
> frob the remote task's TIF_ flags to make it go into a quiescent state 
> publishing routine on syscall-return.
> 
> The only hard requirement of this scheme from the RCU synchronization 
> POV is that all kernel contexts that may touch RCU state need to flip 
> this flag reliably to 'kernel mode': i.e. all irq handlers, traps, 
> NMIs and all syscall variants need to do this.
> 
> But once it's there, it's really neat.
> 
> Thanks,
> 
> 	Ingo
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