Re: [PATCH] percpu: preemptless __per_cpu_counter_add

[Tejun Heo <tj@kernel.org>]

Hello, Shaohua.

On Wed, Apr 27, 2011 at 01:43:29PM +0800, Shaohua Li wrote:
> > That would be a pathelogical case but, even then, after the change the
> > number becomes much higher as it becomes a function of batch *
> > num_updaters, right?
>
> I don't understand the difference between batch * num_updaters and batch
> * num_cpus except preempt. So the only problem here is _add should have
> preempt disabled? I agree preempt can make deviation worse.
> except the preempt issue, are there other concerns against the atomic
> convert? in the preempt disabled case, before/after the atomic convert
> the deviation is the same (batch*num_cpus)

Yes, with preemption disabled, I think the patheological worst case
wouldn't be too different.

> > I don't really worry about _sum performance.  It's a quite slow path
> > and most of the cost is from causing cacheline bounces anyway.  That
> > said, I don't see how the above would help the deviation problem.
> > Let's say an updater reset per cpu counter but got preempted before
> > updating the global counter.  What differences does it make to check
> > fbc->counter before & after like above?
>
> yes, this is a problem. Again I don't mind to disable preempt in _add.

Okay, this communication failure isn't my fault.  Please re-read what
I wrote before, my concern wasn't primarily about pathological worst
case - if that many concurrent updates are happening && the counter
needs to be accurate, it can't even use atomic counter.  It should be
doing full exclusion around the counter and the associated operation
_together_.

I'm worried about sporadic erratic behavior happening regardless of
update frequency and preemption would contribute but isn't necessary
for that to happen.

Thanks.

-- 
tejun

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