Minor ULE changes and optimizations

[John Baldwin]

On Monday, March 02, 2015 02:02:53 PM Harrison Grundy wrote:
> > But why would a driver want to do that?  This code:
> > 	sched_pin(td);
> > 	
> > 	/* do something */
> > 	
> > 	thread_lock(td);
> > 	sched_unpin(td);
> > 	sched_bind(td, PCPU_GET(cpuid));
> > 	thread_unlock(td);
> > 	
> > 	/* do something else */
> > 	
> > 	thread_lock(td);
> > 	sched_unbind(td);
> > 	thread_unlock(td);
> > 
> > Is equivalent to:
> > 	sched_pin(td);
> > 	
> > 	/* do something */
> > 	
> > 	/* do something else */
> > 	
> > 	sched_unpin(td);
> > 
> > But the latter form is lighter weight and easier to read / understand.
> > 
> > Letting you sched_bind() to the current CPU while you are pinned doesn't
> > enable any new functionality than you can already achieve by just using
> > sched_pin() and sched_unpin().
> 
> The difference between the two is that TSF_BOUND is set for "do
> something else" in the former case.
> 
> As I understand the difference, sched_pin is designed for temporarily
> assigning to a CPU, while sched_bind is intended for longer-term affinity.

sched_bind() calls sched_pin() internally, so that isn't the difference.  The 
flag only exists to know which type of pinning is in force.  The differences 
are that sched_pin() assumes the current CPU rather than a specific CPU and 
that it can nest, whereas sched_bind() is used to move a thread to a specific
CPU and it cannot nest.  It is true that one cannot bind a pinned thread, but 
that is because the semantics conflict, not because one is longer term than 
the other.  You can't return from a system call using either sched_bind or 
sched_pin for example.  For a longer term binding you need to set the thread's 
cpuset instead.  sched_pin() and sched_bind() are both "short term" in that 
regard.

> The patch would allow you to set the bound flag without unpinning,
> basically. It seems easier to do this here, than add a "set bound flag"
> function that allows drivers to "promote" themselves from pinned to
> bound, though that would also be an option to get to the same place.

I don't see a use case for why a driver would want to do this.  Do you have a 
specific real world use case in mind?

-- 
John Baldwin