Re: [PATCH] percpu: preemptless __per_cpu_counter_add

[Christoph Lameter <cl@linux.com>]

On Thu, 28 Apr 2011, Tejun Heo wrote:

> On Thu, Apr 28, 2011 at 09:11:20AM -0500, Christoph Lameter wrote:
> > Sporadic erratic behavior exists today since any thread can add an
> > abitrary number to its local counter while you are adding up all the per
> > cpu differentials. If this happens just after you picked up the value then
> > a single cpu can cause a high deviation. If multiple cpus do this then a
> > high degree of deviation can even be had with todays implementation.
>
> Yeah, but that's still something which is expected.  If the user is
> adding/subtracing large number concurrently, sure.  What I'm concerned
> about is adding unexpected deviation.  Currently, the _sum interface
> basically provides basically the same level of expectedness (is this
> even a word?) as atomic_t - the deviations are solely caused and
> limited by concurrent updates.  After the proposed changes, one
> concurrent updater doing +1 can cause @batch deviation.

As I explained before _sum does not provide the accuracy that you think.

> > Can you show in some tests how the chance of deviations is increased? If
> > at all then in some special sitations. Maybe others get better?
>
> It's kinda obvious, isn't it?  Do relatively low freq (say, every
> 10ms) +1's and continuously do _sum().  Before, _sum() would never
> deviate much from the real count.  After, there will be @batch jumps.
> If you still need proof code, I would write it but please note that
> I'm pretty backed up.

"Obvious" could mean that you are drawing conclusions without a proper
reasoning chain. Here you assume certain things about the users of the
counters. The same assumptions were made when we had the vm counter
issues. The behavior of counter increments is typically not a regular
stream but occurs in spurts.

> > Looping over all differentials to get more accuracy is something that may
> > not work as we have seen recently with the VM counters issues that caused
> > bad behavior during reclaim.
>
> Such users then shouldn't use _sum() - maybe rename it to
> _very_slow_sum() if you're concerned about misusage.  percpu_counter()
> is already used in filesystems to count free blocks and there are
> times where atomic_t type accuracy is needed and _sum() achieves that.
> The proposed changes break that.  Why do I need to say this over and
> over again?

Because you are making strange assumptions about "accuracy" of the
counters? There is no atomic_t type accuracy with per cpu counters as we
have shown multiple times. Why are you repeating the same nonsense again
and again? If you want atomic_t accuracy then you need to use atomic_t and
take the performance penalty. Per cpu counters involve fuzziness and
through that fuzziness we gain a performance advantage.



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