Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier

[Steven Rostedt <rostedt@goodmis.org>]

On Sat, 2010-01-09 at 16:03 -0800, Paul E. McKenney wrote:
> On Sat, Jan 09, 2010 at 06:16:40PM -0500, Steven Rostedt wrote:
> > On Sat, 2010-01-09 at 18:05 -0500, Steven Rostedt wrote:
> > 
> > > Then we should have O(tasks) for spinlocks taken, and 
> > > O(min(tasks, CPUS)) for IPIs.
> > 
> > And for nr tasks >> CPUS, this may help too:
> > 
> > > cpumask = 0;
> > > foreach task {
> > 
> > 	if (cpumask == online_cpus)
> > 		break;
> > 
> > > 	spin_lock(task_rq(task)->rq->lock);
> > > 	if (task_rq(task)->curr == task)
> > > 		cpu_set(task_cpu(task), cpumask);
> > > 	spin_unlock(task_rq(task)->rq->lock);
> > > }
> > > send_ipi(cpumask);
> 
> Good point, erring on the side of sending too many IPIs is safe.  One
> might even be able to just send the full set if enough of the CPUs were
> running the current process and none of the remainder were running
> real-time threads.  And yes, it would then be necessary to throttle
> calls to sys_membarrier().
> 

If you need to throttle calls to sys_membarrier(), than why bother
optimizing it? Again, this is like calling synchronize_sched() in the
kernel, which is a very heavy operation, and should only be called by
those that are not performance critical.

Why are we struggling so much with optimizing the slow path?

Here's how I take it. This method is much better that sending signals to
all threads. The advantage the sys_membarrier gives us, is also a way to
keep user rcu_read_locks barrier free, which means that rcu_read_locks
are quick and scale well.

So what if we have a linear decrease in performance with the number of
threads on the write side?

-- Steve