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Subject: Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory
 barrier (v3a)
From: Peter Zijlstra <peterz@infradead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
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Date: Mon, 11 Jan 2010 23:20:16 +0100
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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.

>  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();

?