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

[Will Deacon]

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.
> 
> > A RT system that really cares about not having userspace sending IPIs
> > to all cpus could set the ulimit value to 0, which would always use
> > synchronize_sched().
> > 
> > Thoughts ?
> 
> So I really don't like SHARED_EXPEDITED, and your use-cases (from later
> emails) makes me think sys_membarrier() should have a pointer argument
> to identify the shared mapping.
> 
> But even then, iterating the rmap for something that has 1000+ maps
> isn't going to be nice or fast, even in kernel space.
> 
> Another crazy idea is using madvise() for this. The new MADV_MEMBAR
> could revoke PROT_WRITE and PROT_READ for all extant PTEs. Then the
> tasks attempting access will fault and the fault handler can figure out
> if it still needs to issue a MB or not before reinstating the PTE.

If you did that, wouldn't you need to leave the faulting entries intact
until all CPUs with the mm scheduled have either faulted or
context-switched? We don't have per-cpu permission bits in the page tables,
unfortunately.

Will