Re: [PATCH 03/34] lib: percpu counter add unless less than functionality

Dave Chinner
2010-12-22T03:45:06Z

On Tue, Dec 21, 2010 at 08:20:14PM -0600, Alex Elder wrote:
> On Tue, 2010-12-21 at 18:28 +1100, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@redhat.com>
> >
> > To use the generic percpu counter infrastructure for counters that
> > require conditional addition based on a threshold value we need
> > special handling of the counter. Further, the caller needs to know
> > the status of the conditional addition to determine what action to
> > take depending on whether the addition occurred or not. Examples of
> > this sort of usage are resource counters that cannot go below zero
> > (e.g. filesystem free blocks).
> >
> > To allow XFS to replace it's complex roll-your-own per-cpu
> > superblock counters, a single generic conditional function is
> > required: percpu_counter_add_unless_lt(). This will add the amount
> > to the counter unless the result would be less than the given
> > threshold. A caller supplied threshold is required because XFS does
> > not necessarily use the same threshold for every counter.
> >
> > percpu_counter_add_unless_lt() attempts to minimise counter lock
> > traversals by only taking the counter lock when the threshold is
> > within the error range of the current counter value. Hence when the
> > threshold is not within the counter error range, the counter will
> > still have the same scalability characteristics as the normal
> > percpu_counter_add() function.
> >
> > Adding this functionality to the generic percpu counters allows us
> > to remove the much more complex and less efficient XFS percpu
> > counter code (~700 lines of code) and replace it with generic
> > percpu counters.
> >
> > Signed-off-by: Dave Chinner <dchinner@redhat.com>
>
> I want to look at this one more closely again in the
> morning, but for now I'll just mention one nit, and
> one easily fixed problem.
>
> -Alex
>
> . . .
>
> > + * Add @amount to @fdc if and only if result of addition is greater than or
> ^^^ should be fbc
>
> > +EXPORT_SYMBOL(percpu_counter_add_unless_lt);
> > +
>
> This has to be:
> EXPORT_SYMBOL(__percpu_counter_add_unless_lt);
> (with leading underscores).

Yup, saw you comment about that on IRC overnight. Already fixed. ;)

Cheers,,

Dave.
--
Dave Chinner
david@fromorbit.com