Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v3a)

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

On Mon, Jan 11, 2010 at 11:20:16PM +0100, Peter Zijlstra wrote:
> On Mon, 2010-01-11 at 17:04 -0500, Mathieu Desnoyers wrote:
> > * Peter Zijlstra (peterz@infradead.org) wrote:
> > > On Mon, 2010-01-11 at 15:52 -0500, Mathieu Desnoyers wrote:
> > > > 
> > > > So the clear bit can occur far, far away in the future, we don't care.
> > > > We'll just send extra IPIs when unneeded in this time-frame.
> > > 
> > > I think we should try harder not to disturb CPUs, particularly in the
> > > face of RT tasks and DoS scenarios. Therefore I don't think we should
> > > just wildly send to mm_cpumask(), but verify (although speculatively)
> > > that the remote tasks' mm matches ours.
> > 
> > Well, my point of view is that if IPI TLB shootdown does not care about
> > disturbing CPUs running other processes in the time window of the lazy
> > removal, why should we ?
> 
> while (1)
>  sys_membarrier();
> 
> is a very good reason, TLB shootdown doesn't have that problem.

You can get a similar effect by doing mmap() to a fixed virtual address
in a tight loop, right?  Of course, mmap() has quite a bit more overhead
than sys_membarrier(), so the resulting IPIs probably won't hit the
other CPUs quite as hard, but it will hit them repeatedly.

> >  We're adding an overhead very close to that of
> > an unrequired IPI shootdown which returns immediately without doing
> > anything.
> 
> Except we don't clear the mask.
> 
> > The tradeoff here seems to be:
> > - more overhead within switch_mm() for more precise mm_cpumask.
> > vs
> > - lazy removal of the cpumask, which implies that some processors
> >   running a different process can receive the IPI for nothing.
> > 
> > I really doubt we could create an IPI DoS based on such a small
> > time window.
> 
> What small window? When there's less runnable tasks than available mm
> contexts some architectures can go quite a long while without
> invalidating TLBs.
> 
> So what again is wrong with:
> 
>  int cpu, this_cpu = get_cpu();
> 
>  smp_mb(); 
> 
>  for_each_cpu(cpu, mm_cpumask(current->mm)) {
>    if (cpu == this_cpu)
>      continue;
>    if (cpu_curr(cpu)->mm != current->mm)
>      continue;
>    smp_send_call_function_single(cpu, do_mb, NULL, 1);
>  }
> 
>  put_cpu();
> 
> ?

Well, if you have lots of CPUs, you will have disabled preemption for
quite some time.  Not that there aren't already numerous similar
problems throughout the Linux kernel...

							Thanx, Paul