Re: [RFC PATCH v2] membarrier: expedited private command

[Paul E. McKenney]

On Tue, Aug 01, 2017 at 04:16:54PM +0200, Peter Zijlstra wrote:
> On Tue, Aug 01, 2017 at 06:23:09AM -0700, Paul E. McKenney wrote:
> > On Tue, Aug 01, 2017 at 12:22:03PM +0200, Peter Zijlstra wrote:
> > 
> > [ . . . ]
> > 
> > > As to scheduler IPIs, those are limited to the CPUs the user is limited
> > > to and are rate limited by the wakeup-latency of the tasks. After all,
> > > all the time a task is runnable but not running, wakeups are no-ops.
> > 
> > Can't that wakeup-latency limitation be overcome by a normal user simply
> > by having lots of tasks to wake up, which then go back to sleep almost
> > immediately?  Coupled with very a low-priority CPU-bound task on each CPU?
> 
> Let me put it like this; there is no way to cause more interference
> using IPIs then there is simply running while(1) loops ;-)

Very good, that does give us some guidance, give or take context switches
happening during the IPI latency window.  ;-)

							Thanx, Paul