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

[Paul E. McKenney]

On Wed, Jul 26, 2017 at 02:11:46PM -0700, Paul E. McKenney wrote:
> On Wed, Jul 26, 2017 at 08:37:23PM +0000, Mathieu Desnoyers wrote:
> > ----- On Jul 26, 2017, at 2:30 PM, Paul E. McKenney paulmck@xxxxxxxxxxxxxxxxxx wrote:
> > 
> > > On Wed, Jul 26, 2017 at 06:01:15PM +0000, Mathieu Desnoyers wrote:
> > >> ----- On Jul 26, 2017, at 11:42 AM, Paul E. McKenney paulmck@xxxxxxxxxxxxxxxxxx
> > >> wrote:
> > >> 
> > >> > On Wed, Jul 26, 2017 at 09:46:56AM +0200, Peter Zijlstra wrote:
> > >> >> On Tue, Jul 25, 2017 at 10:50:13PM +0000, Mathieu Desnoyers wrote:
> > >> >> > This would implement a MEMBARRIER_CMD_PRIVATE_EXPEDITED (or such) flag
> > >> >> > for expedited process-local effect. This differs from the "SHARED" flag,
> > >> >> > since the SHARED flag affects threads accessing memory mappings shared
> > >> >> > across processes as well.
> > >> >> > 
> > >> >> > I wonder if we could create a MEMBARRIER_CMD_SHARED_EXPEDITED behavior
> > >> >> > by iterating on all memory mappings mapped into the current process,
> > >> >> > and build a cpumask based on the union of all mm masks encountered ?
> > >> >> > Then we could send the IPI to all cpus belonging to that cpumask. Or
> > >> >> > am I missing something obvious ?
> > >> >> 
> > >> >> I would readily object to such a beast. You far too quickly end up
> > >> >> having to IPI everybody because of some stupid shared map or something
> > >> >> (yes I know, normal DSOs are mapped private).
> > >> > 
> > >> > Agreed, we should keep things simple to start with.  The user can always
> > >> > invoke sys_membarrier() from each process.
> > >> 
> > >> 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.
> > >> 
> > >> 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 ?
> > > 
> > > The patch I posted reverts to synchronize_sched() in kernels booted with
> > > rcupdate.rcu_normal=1.  ;-)
> > > 
> > > But who is pushing for multiple-process sys_membarrier()?  Everyone I
> > > have talked to is OK with it being local to the current process.
> > 
> > I guess I'm probably the guilty one intending to do weird stuff in userspace ;)
> > 
> > Here are my two use-cases: 
> > 
> > * a new multi-process liburcu flavor, useful if e.g. a set of processes are
> >   responsible for updating a shared memory data structure, and a separate set
> >   of processes read that data structure. The readers can be killed without ill
> >   effect on the other processes. The synchronization could be done by one
> >   multi-process liburcu flavor per reader process "group".
> > 
> > * lttng-ust user-space ring buffers (shared across processes).
> > 
> > Both rely on a shared memory mapping for communication between processes, and
> > I would like to be able to issue a sys_membarrier targeting all CPUs that may
> > currently touch the shared memory mapping.
> > 
> > I don't really need a system-wide effect, but I would like to be able to target
> > a shared memory mapping and efficiently do an expedited sys_membarrier on all
> > cpus involved.
> > 
> > With lttng-ust, the shared buffers can spawn across 1000+ processes, so
> > asking each process to issue sys_membarrier would add lots of unneeded overhead,
> > because this would issue lots of needless memory barriers.
> > 
> > Thoughts ?
> 
> Dealing explicitly with 1000+ processes sounds like no picnic.  It instead
> sounds like a job for synchronize_sched_expedited().  ;-)

Actually...

Mathieu, does your use case require unprivileged access to sys_membarrier()?

							Thanx, Paul