Re: [PATCH v2] membarrier: provide register sync core cmd

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Aug 29, 2017 at 10:01:56PM -0700, Andy Lutomirski wrote:
> > On Aug 27, 2017, at 8:05 PM, Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:
> >
> > ----- On Aug 27, 2017, at 3:53 PM, Andy Lutomirski luto@amacapital.net wrote:
> >
> >>> On Aug 27, 2017, at 1:50 PM, Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> >>> wrote:
> >>>
> >>> Add a new MEMBARRIER_CMD_REGISTER_SYNC_CORE command to the membarrier
> >>> system call. It allows processes to register their intent to have their
> >>> threads issue core serializing barriers in addition to memory barriers
> >>> whenever a membarrier command is performed.
> >>>
> >>
> >> Why is this stateful?  That is, why not just have a new membarrier command to
> >> sync every thread's icache?
> >
> > If we'd do it on every CPU icache, it would be as trivial as you say. The
> > concern here is sending IPIs only to CPUs running threads that belong to the
> > same process, so we don't disturb unrelated processes.
> >
> > If we could just grab each CPU's runqueue lock, it would be fairly simple
> > to do. But we want to avoid hitting each runqueue with exclusive atomic
> > access associated with grabbing the lock. (cache-line bouncing)
> 
> Hmm.  Are there really arches where there is no clean implementation
> without this hacker?  It seems rather unfortunate that munmap() can be
> done efficiently but this barrier can't be.
> 
> At the very least, could there be a register command *and* a special
> sync command?  I dislike the idea that the sync command does something
> different depending on some other state.  Even better (IMO) would be a
> design where you ask for an isync and, if the arch can do it
> efficiently (x86), you get an efficient isync and, if the arch can't
> (arm64?) you take all the rq locks?

In some cases I suspect that IPIs might be required.  Regardless of
that, we might well need to provide a way for architectures to do
special things.

But I must defer to Mathieu on this.

							Thanx, Paul