Re: RFC for a new Scheduling policy/class in the Linux-kernel

[Peter Zijlstra]

On Thu, 2009-07-16 at 08:59 -0400, James H. Anderson wrote:
> 
> Raistlin wrote:
> > Also, I'm not sure I can find in the FMLP paper information about the
> > possibility of a task to suspend itself (e.g., I/O completion) while
> > holding a short lock... I assume this is not recommended, but may be
> > wrong, and, in that case, I hope Prof. Anderson and Bjorn will excuse
> > and correct me. :-)
> >
> >   
> This is a really excellent point and something I probably should have 
> mentioned.  We developed the FMLP strictly for real-time (only) 
> workloads.  We were specifically looking at protecting memory-resident 
> resources (not I/O).  The FMLP would have to be significantly extended 
> to work in settings where these assumptions don't hold.

One thing I've been thinking about is extending lockdep to help verify
things like this.

If we were to annotate a syscall/trap with something like:

  lockdep_assume_rt()

And teach lockdep about non-RT blocking objects, we could validate that
the callchain down from lockdep_assume_rt() would not indeed contain a
non-RT resource, but also that we don't take locks which might in other
another code path.

That is, suppose:

  sys_foo()
    lockdep_assume_rt()
    mutex_lock(&my_lock)

vs

  sys_bar()
    mutex_lock(&my_lock)
      down_read(&mm->mmap_sem)

vs

  page-fault
    down_read(&mm->mmap_sem)
      lock_page(page)

Would indeed generate a warning because mmap_sem is known to block on
IO, and there is a dependency (through sys_bar()) between my_lock and
mmap_sem, therefore sys_foo()'s assumption is invalid.



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