Re: [RFC patch] introduce sys_membarrier(): process-wide memory barrier (v9)

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 2010-02-26 at 16:08 +1100, Nick Piggin wrote:
> On Thu, Feb 25, 2010 at 11:53:01AM -0500, Mathieu Desnoyers wrote:

> > This is actually what I did in v1 of the patch, but this implementation met
> > resistance from the RT people, who were concerned about the impact on RT tasks
> > of a lower priority process doing lots of sys_membarrier() calls. So if we want
> > to do other-process-aware sys_membarrier(), we would have to iterate on all
> > cpus, for every running process shared memory maps and see if there is something
> > shared with all shm of the current process. This is clearly not as trivial as
> > just broadcasting the IPI to all cpus.
> 
> I don't see how this is fundamentally worse than your existing approach,
> because on some architectures with asids, the mm_cpumask isn't cleared
> when a process is scheduled off the CPU then you could effectively just
> cause IPIs to lots of CPUs anyway.

That's why checking the mm_cpumask isn't the only check. That just
limits what CPUs we check, but before a IPI is sent, that cpu has its rq
lock held and a check against cpu_curr(cpu)->mm vs the current->mm. If
that fails, then that CPU does not have an IPI sent to it.

-- Steve