[PATCH v4 for 4.14 1/3] membarrier: Provide register expedited private command

[Peter Zijlstra]

On Fri, Sep 29, 2017 at 02:27:57AM +1000, Nicholas Piggin wrote:

> The biggest power boxes are more tightly coupled than those big
> SGI systems, but even so just plodding along taking and releasing
> locks in turn would be fine on those SGI ones as well really. Not DoS
> level. This is not a single mega hot cache line or lock that is
> bouncing over the entire machine, but one process grabbing a line and
> lock from each of 1000 CPUs.
> 
> Slight disturbance sure, but each individual CPU will see it as 1/1000th
> of a disturbance, most of the cost will be concentrated in the syscall
> caller.

But once the:

	while (1)
		sys_membarrier()

thread has all those (lock) lines in M state locally, it will become
very hard for the remote CPUs to claim them back, because its constantly
touching them. Sure it will touch a 1000 other lines before its back to
this one, but if they're all local that's fairly quick.

But you're right, your big machines have far smaller NUMA factors.

> > Bouncing that lock across the machine is *painful*, I have vague
> > memories of cases where the lock ping-pong was most the time spend.
> > 
> > But only Power needs this, all the other architectures are fine with the
> > lockless approach for MEMBAR_EXPEDITED_PRIVATE.
> 
> Yes, we can add an iterator function that power can override in a few
> lines. Less arch specific code than this proposal.

A semi related issue; I suppose we can do a arch upcall to flush_tlb_mm
and reset the mm_cpumask when we change cpuset groups.