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

Peter Zijlstra
2010-01-11T22:21:26Z

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();

?