Re: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option

[Paul E. McKenney]

On Thu, Jul 27, 2017 at 10:53:12AM +0200, Peter Zijlstra wrote:
> On Wed, Jul 26, 2017 at 06:01:15PM +0000, Mathieu Desnoyers wrote:
> 
> > Another alternative for a MEMBARRIER_CMD_SHARED_EXPEDITED would be rate-limiting
> > per thread. For instance, we could add a new "ulimit" that would bound the
> > number of expedited membarrier per thread that can be done per millisecond,
> > and switch to synchronize_sched() whenever a thread goes beyond that limit
> > for the rest of the time-slot.
> 
> You forgot to ask yourself how you could abuse this.. just spawn more
> threads.
> 
> Per-thread limits are nearly useless, because spawning new threads is
> cheap.

Agreed -- any per-thread limit has to be some portion/fraction of a
global limit.

							Thanx, Paul