Re: [PATCH] percpu: preemptless __per_cpu_counter_add

[Shaohua Li <shaohua.li@intel.com>]

Hi,
On Thu, 2011-04-28 at 18:09 +0800, Tejun Heo wrote:
> On Thu, Apr 28, 2011 at 11:28:04AM +0800, Shaohua Li wrote:
> > > 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.
> >
> > Ok, I misunderstood the mail you sent to Christoph, sorry. So you have
> > no problem about the atomic convert. I'll update the patch against base
> > tree, given the preemptless patch has problem.
> 
> Hmm... we're now more lost than ever. :-( Can you please re-read my
> message two replies ago?  The one where I talked about sporadic
> erratic behaviors in length and why I was worried about it.
> 
> In your last reply, you talked about preemption and that you didn't
> have problems with disabling preemption, which, unfortunately, doesn't
> have much to do with my concern with the sporadic erratic behaviors
> and that's what I pointed out in my previous reply.  So, it doesn't
> feel like anything is resolved.
ok, I got your point. I'd agree there is sporadic erratic behaviors, but
I expect there is no problem here. We all agree the worst case is the
same before/after the change. Any program should be able to handle the
worst case, otherwise the program itself is buggy. Discussing a buggy
program is meaningless. After the change, something behavior is changed,
but the worst case isn't. So I don't think this is a big problem.

Thanks,
Shaohua

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